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INCUBATION
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INCUBATION and INCUBATORS. The subject of " incubation " (See also:Lat. incubare, to brood; in-cumbere, to See also:lie on), a See also:term which, while strictly signifying the See also:action of a See also:hen in sitting on her eggs to See also:hatch them, is also used in See also:pathology for the development within the See also:body of the germs of disease, is especially associated with the artificial means, or " incubators," devised for hatching eggs, or for analogous purposes of an artificial See also:foster-See also:mother nature, or for use in bacteriological laboratories. See also:Life is dependent, alike for its awakening and its See also:maintenance, upon the See also:influence of certain See also:physical and chemical factors, among which See also:heat and moisture may be regarded as the See also:chief. It is therefore obvious that any method of incubation must provide for a due degree of temperature and moisture. And this degree must be one within limits, for while all organisms are plastic and can attune themselves to a greater cr less range of variation in their physical environment, there is a given degree at which the processes of life in each See also:species proceed most favourably. It is this particular degree, which differs for different species, which must be attained, if artificial incubation is to be successfully conducted. In other words, the degree of temperature and moisture within the incubation drawer must remain See also:uniform throughout the See also:period of incubation if the best results are to be reached. It is not easy to attain these conditions, for there are many disturbing factors. We may therefore next consider the more important of them. The chief causes which operate to make the temperature within the incubator drawer variable are the changes of 'the temperature of the See also:outer See also:air, fluctuations in the pressure of the See also:gas when that is used as the source of heat, or the See also:gradual diminution of the oxidizing See also:power of the See also:flame and See also:wick when an oil See also:lamp is substituted for gas. Also, the necessary opening of the incubator drawer, either for airing or for sprinkling the eggs with See also:water when that is necessary, tends to reduce the temperature. But there is another equally important though less obvious source of disturbance, and this resides within the organism undergoing incubation. In the See also:case of the chick, at about the ninth or tenth days of incubation important Changes are occurring. Between this period and the fourteenth See also:day the chick becomes relatively large and bulky, and the temporary See also:respiratory See also:organ, the allantois, together with its See also:veins, increases greatly in See also:size and extent. As a consequence, the respiratory processes are enabled to proceed with greater activity, and the chemical processes of oxidation thus enhanced necessarily largely increase the amount of heat which the chick itself produces. Thus an incubator, to be successful, must be capable of automatically adjusting itself to this heightened temperature. The drawer of an incubator is a confined space and is usually packed as closely as possible with the contained eggs. The eggs are living structures and consequently need air. This necessitates some method of See also:direct See also:ventilation, and this in its turn necessarily increases the evaporation of water vapour from the See also:surface of the See also:egg. Unless, therefore, this evaporation is checked, the eggs will be too dry at the period-from the tenth day onwards— when moisture is more than ever an important See also:factor. There is, according to some poultry authorities, See also:reason to believe that the sitting hen secretes some oily substance which, becoming diffused over the surface of the egg, prevents or retards evaporation from within; presumably, this oil is permeable to See also:oxygen. In nature, with the sitting hen, and in the " Mamal " artificial incubating establishments of the Egyptians, direct air currents do not exist, owing to the large size of the See also:chambers, and consequently incubation can be successfully achieved without any See also:special See also:provision for the See also:supply of moisture. Artificial incubation has been known to the Egyptians and the See also:Chinese from almost See also:time immemorial. In See also:Egypt, at Berme on the See also:Delta, the See also:trade of artificial hatching is traditionally transmitted from See also:father to son, and is consequently confined to particular families. The secrets of the See also:process are guarded with a religious zeal, and the individuals who practise it are held under plighted word not to divulge them. It is highly probable that the process of artificial incubation as practised by the Egyptians is not so See also:simple as it is believed to be. But as far as the structures and processes involved have been ascertained by travellers, it appears that the " Mamal " is a See also:brick See also:building, consisting of four large ovens, each of such a size that several men could be contained within it. These ovens are in pairs, in each pair one See also:oven being above the other, on each See also:side of a See also:long passage, into which they open by a circular See also:aperture, just large enough for a See also:man to obtain See also:access to each. The eggs are placed in the See also:middle of the See also:floor of the oven, and in the gutters See also:round the sides the See also:fire is lighted. The material for this latter, according to one See also:account, consists of camels' dung and chopped See also:hay, and according to another of horses' dung. The attainment of the right degree of heat is apparently reached wholly by the skill of the persons employed. • When this has been attained, they plug the entrance hole with coarse See also:tow. On the tenth to twelfth days they cease to See also:light the fires. Each " Mamal " may contain from 40,000 to 8o,000 eggs. There are 386 " Mamals " in the See also:country, which are only worked for six months of the See also:year, and produce in that time eight broods. Many more than two-thirds of the eggs put in are successfully hatched. It is estimated that 90,000,000 eggs are annually hatched by the Bermeans. A method of incubating that appears to have been altogether overlooked in See also:England—or at least never to have been practised —is that carried on by the Couveurs or professional hatchers in See also:France. They make use of hen-turkeys for the purpose, and each See also:bird can be made to sit continuously for from three to six months. The modus operandi is as follows: a dark See also:room which is kept at a See also:constant temperature throughout the year contains a number of boxes, just large enough to accommodate a See also:turkey. The bottom of the See also:box is filled with some See also:vegetable material, bracken, hay, heather, See also:straw or See also:cocoa-See also:fibres. Each box is covered in with lattice-See also:work See also:wire, so arranged that the freedom of the sitting bird is limited and its See also:escape prevented. Dummy eggs, made by emptying addled ones and filling with See also:plaster of See also:Paris, are then placed in the See also:nest and a bird put in. At first it endeavours to escape, but after an See also:interval of a few days it becomes quiet, and the dummy eggs being then removed, fresh ones are inserted. As soon as the chickens are hatched, they are withdrawn and fresh eggs substituted. The hen turkeys are also used successfully as foster-mothers. Each bird can adequately See also:cover about two dozen eggs. Incubation as an See also:industry in See also:Europe and See also:America is of See also:recent development. The growing scarcity of See also:game birds of all kinds, coincident with the increase of See also:population, and the introduction of the See also:breech-loading See also:gun, together with the marked revival of See also:interest in See also:fancy poultry about the year 1870, led, however, to the See also:production of a See also:great variety of appliances designed to render artificial incubation successful. Previously to this, several interesting attempts had been made. As long ago as 1824, Walthew constructed an incubator designed to be used by farmers' wives with the aid of no more than See also:ordinary See also:household conditions. It consisted of a See also:double-walled See also:metal box, with several pipes opening into the walled space round thesides, bottom and See also:top of the incubator. These pipes were connected with an ordinary See also:kitchen See also:boiler. Walthew, however, constructed a fire See also:grate, with a special boiler adapted to the requirements of the incubator. Into the walled space of the incubator, See also:steam from the kitchen boiler passed; the excess steam escaped from an aperture in the roof, and the condensed steam through one in the floor. Ventilating holes and also plugs, into which thermometers were placed, pierced the See also:door of the incubator. . In 1827, J. H. See also:Barlow successfully reared hens and other birds by means of steam at See also:Drayton See also:Green, See also:Ealing. He constructed very large rooms and rearing houses, expending many thousands of pounds upon the work. He reared some 64,000 game birds annually. The celebrated physician See also:Harvey, and the famous anatomist See also:Hunter were much interested in his results.
To See also: In the latter case we may classify them according as their thermostat or temperature-regulator is actuated by a liquid expanding with rising temperature, or by solids, usually metals.
In America incubators of the hot-air type with solid and metallic thermostats are most used, while in Europe the " tank " type, with a thermostat of expansible liquid, prevails.
For the purpose of more adequately considering the various forms which have been in use, or are still used, we shall here divide them into the " hot-air " and " hot-water " (or " tank ") classes.
In the hot-air types the incubator chamber is heated by columns of hot air, while in the tank system this chamber is heated by a tank of warmed water.
(a) Hot-Water Incubators.—In 1866 See also:Colonel See also:Stuart Wortley de-scribed in The See also: It is claimed for this See also:instrument that since the temperature of boiling water at any particular locality remains practically constant, the disadvantages due to fluctuations in the activity of a lamp flame or the size of a gas flame are obviated. But it has the serious &S.-See also:advantage that there is no automatic adjustment to compensate for fluctuations of atmospheric temperature. And experiments by C. Hearson have shown that even if the temperature of the tank or source of heat be constant, that of the incubator drawer will nevertheless vary with fluctuations of See also:external temperature. Probably if the See also:mechanical difficulties of providing a self-regulator were over-come, it would prove an efficient and reliable incubator. The difficulties do not seem to be insuperable, and it appears possible that a thermostatic See also:bar could be so arranged as to automatically increase or decrease the length of hot-water pipes within the incubator, and therefore the incubator temperature. Another See also:early See also:form of incubator is See also:Brindley's, which was first in use about 1845, and in his hands it appeared occasionally to See also:act successfully, but it never became generally used. The egg chamber was lined with See also:felt, and was placed beneath a heated air chamber, the floor and roof of which were composed of glass. The air chamber was heated by a number of hot-water pipes which were connected with a See also:copper boiler. This latter was heated by means of a lamp so constructed as to See also:burn steadily. The temperature of the air chamber was regulated within certain limits by means of a balanced See also:valve, which could be so adjusted that it would open at any desired temperature. In Colonel Stuart Wortley's incubator the hot-water tubes passed directly into the egg chamber, and in Brindley's into a chamber above it. But in other forms of incubators in which the principle of an external boiler connected with water tubes is adopted, the latter pass not into the egg chamber nor into an air chamber, but open into and from a tank of water. The floor of this tank forms the roof of the egg chamber, so that the eggs are heated from above. This See also:device of warming the eggs from above was adopted in See also:imitation of the processes that presumably occur with the sitting hen; for it is generally assumed that the surface of the eggs in contact with the hen is warmer than that in contact with the See also:damp See also:soil or with the material of the nest. One of the earliest of this form of incubator is that invented by F. See also:Schroder, manager of the now See also:extinct See also:British See also:National Poultry See also:Company. In this incubator the form is circular, and there are four egg drawers, so that each one occupied the quadrant of a circle, and the inner corner of each drawer meets in the middle of the incubator. Front -the centre of the incubator a See also:vertical See also:chimney passes upwards and opens out from the inner corners of the four egg drawers. This chimney acts as a ventilator to the incubating chambers. These latter are open above, but their floors are made of perforated See also:zinc, and when in use they are partially filled with See also:chaff or similar material. Under them is a tank containing See also:cold water and See also:common to all four drawers; the slight vapour rising from the surface of the water diffuses through the egg drawers and thus insures a sufficient degree of humidity to the air within. Above the egg drawers is a circular tank containing warm water. The floor of this tank constitutes the roof of the egg drawers, while the roof forms the floor of a circular chamber above it, the side See also:wall of which is composed of perforated zinc. This upper chamber is used to dry the chicks when they are just hatched and to See also:rear them until they are strong enough for removal. It is partially filled with See also:sand, which serves the double purpose of retaining the heat in the warm-water tank beneath and of forming a See also:bed for the chicks. The water in the warm-water tank is heated by means of a boiler which is external to the incubator, and in communication with the tank by means of an inlet and an outlet See also:pipe. There is no valve to regulate the temperature, and the latter is measured by means of a thermometer, the bulb of which is situated not in the incubator drawers, but in the warm-water tank. This is a wrong position for the thermometer, since it is now known that the temperature of the water tank may be different by several degrees to that of the egg drawer; for with a fall of external temperature that of the latter necessarily tends to fall more rapidly than the former. But, none the less, in skilful hands this incubator gave See also:good results. T. See also:Christy's incubator, which we shall describe next, has passed through several forms. We shall consider the most recent one (11894). The incubator (fig. 1) is double walled, and the space between the two walls is packed with a non-conducting material. In the upper T C p H 0 See also:part of the incubator there is a water tank (T) divided by a See also:horizontal See also:partition into two chambers, communicating with each other at the See also:left-See also:hand side. Below the tank is the incubation drawer (E), which contains the eggs and also a temperature regulator or thermostat (R). The tank is traversed by a ventilating See also:shaft (V), and inserted into this is a smaller sliding See also:tube passing up to it from a hole in the bottom of the incubator drawer. The floor of the incubator drawer is perforated, and beneath it is an enclosed air space which opens into the sliding air shaft just described. Fresh air is let into the incubator drawer from a few apertures (I) at its top. The ventilating shaft (V) is closed externally by a cap (C), which can be raised from or lowered down upon its orifice by the horizontal See also:arm (H) working upon See also:pivot See also:joints at (P). This arm is operated by the thermostat (R), through the agency of a vertical See also:rod. The water in the tank is heated by an external boiler (B) through two pipes, one of which (T) serves as an inlet, and the other (L) as an outlet channel from the tank. These two pipes do not open directly into the tank, but into an outer See also:vessel (0) communicating with it. Communication between this vessel and the tank may be made or broken by means of a sliding valve (S), which is pierced by an aperture that corresponds in position with the upper of the two in the wall of the tank when the valve is up. When this valve is in its upper position, the tank (T) communicates with the outer vessel (0) by two apertures (A and A'), the top one being the inlet and the See also:lower one the outlet. These coincide in position with the tubes from the boiler. This latter (B) is a conical vessel containing two spaces. The heated water is cons tained in the outer of these spaces, while the central space is an air shaft heated by a lamp flame. This particular form of the boiler results in the water at its top part being more heated than that in its lower. As a consequence of this, a continual circulation of water through the tank ensues. The more heated water, being specifically lighter, passes into the outer vessel, where it remains among the higher strata, and therefore enters the tank through the upper aperture. In passing along the upper See also:division of the tank it becomes slightly cooled and sinks therefore into the lower compartment, passes along it, and out through the aperture A'. Hence it passes into the lower portion of the boiler, where it becomes warmed and specifically lighter; in consequence it becomes pushed upwards in the boiler by the cooler and heavier water coming in behind and below it. Should the temperature in the incubator drawer rise, the bimetallic thermostat (R) opens out its coil and pulls down the vertical rod. This simultaneously effects two things: it raises the cap (C) over the ventilating shaft and allows of a more rapid flow of fresh air through the incubator drawer, and it also lowers the slide-valve (S) so that the tank becomes cut off from communication with the outer vessel (0) and therefore with the boiler. The temperature thereupon begins to fall and the thermostat, coiling closer, raises the vertical rod, closes the ventilating shaft, and once more places the tank in communication with the boiler. The structure of the thermostat is liven below. The See also:Chantry Incubator (See also:Sheffield) is also an incubator with a hot-water tank, the circulation of which is maintained by an outside boiler. Its temperature is regulated by a metal regulator. In Schroder's and Christy's incubators the hot-water pipes from the boiler simply entered the warm-water tank but did not See also:traverse it. In the two incubators to be next described the hot-water pipes are made to pass through the water in the tank, and are so arranged as to minimize the possibility that the outside of the tank may become colder than the centre. Both of them are also fitted with an ingenious though slightly complex valve for maintaining an approximately constant temperature. Halsted's incubator was the earliest of this type. Since his See also:original form was constructed he has designed an improved one, and it is this latter which will be described. The egg drawer (E, fig. 2) lies beneath the warm-water tank (T), and above this is a nursery (N). The egg drawer is ventilated by two tubular shafts (V), of which only one is represented in the See also:illustration; the tubesare about 21 in. in See also:diameter, and each one is fitted at its upper end, where it opens into the nursery, with a See also:swing-valve (V') which turns upon a horizontal See also:axis (A), in its turn connected, by means of cranks (C) and shafts (S), with the heat regulating apparatus (R). A space of about 2 in. between the top of the incubating drawer and the warm-water tank is necessary for the insertion of this :-apparatus. The water in the tank (T) is heated by means of the boiler (B) ; the tank and boiler are connected by the two pipes (I) and (0), of which one is the inlet and the other the outlet channel. The boiler consists of an FIG 2.-Halsted's Incubator. inner (I') and an outer (0) division in communication with each other below. The latter is cylindrical in form, while the outer wall of the former is cylindrical and its inner wall conical. The conical wall of the inner boiler is the surface which is heated by the lamp (L). The arrangement of the inlet and outlet tubes is important. In the illustration, for the See also:sake of clearness, they are represented as one above the other. In reality they lie in the same See also:plane, and the See also:fork (F) of the inlet pipe similarly lies in the horizontal plane and not vertically as represented. The inlet pipe not only differs from the outlet pipe in the 1111111111111 V' See also:possession of a forked end, but it is carried to the farther end (not closely, according as the flame is to be smaller or larger respectively. shown in the See also:diagram) of the water tank, while the outlet pipe opens from about the middle of the tank. The inlet pipe is connected with the inner portion of the boiler and the outlet one with the outer portion. The result of this adjustment of the parts is that the warmer water of the inner boiler, being specifically lighter than the cooler water of the outer boiler, rises up and passes through the inlet pipe (I) and is discharged into the tank through the two divergent orifices of the fork (F). Here the water strikes the side wall of the farther end of the tank and is reflected back along the back and front walls towards the nearer side. Hence it is again reflected, but in the opposite direction, and now forms a central current, which is directed towards.the centrally situated orifice of the outlet tube (0). Through this it passes to the outer boiler, and sinking towards the bottom, reaches the See also:base of the inner boiler. Here it becomes heated and lighter and consequently rises to the top, and once more passes through the inlet pipe to the water tank. The warm water thus travels round the outer walls of the tank and the cooled water is conducted away along the middle portion. A more equable See also:distribution of temperature over the roof of the incubating chamber is thus ensured than would be the case if the heated water were discharged either into the centre or at any other single point only of the tank. To a very large extent, the efficiency of this apparatus depends l serted over the orifice of the upon the approximately perfect performance of the lamp. A good, I ventilating shaft (A) which steadily burning one should be employed, and only the best oil used; for, should the wick become fouled the flame cannot freely burn. For this reason it is better to use gas, whenever obtainable. The maintenance of an approximately uniform temperature is obtained by allowing the heated air of the egg-drawer to escape through the two ventilating shafts (V). The swing-valves of these are opened or closed by means of the regulator (R). This latter consists of a glass bowl prolonged into a tube, about 8 in. long and three-eighths of an See also:inch in diameter. The glass tube swings upon an axis (A) which is situated as near as possible to the bowl of the regulator. The axis is connected with a See also:crank (C') which is disposed so as to act as a See also:lever upon the vertical shaft (S), which in its turn is connected with the upper crank (C) ; this See also:works the axis (A') of the swing-valves, and so can open or See also:close the apertures of the ventilating pipes. The bowl of the regulator is filled with See also:mercury to such an extent that at the temperature of zoo° F., and when the tube is slightly inclined upwards from the horizontal it just flows slightly into the tube from the bowl. On the lever-crank (C') a See also:weight is slung by a sliding adjustment, and is so placed that when the temperature of the egg-drawer is Io3° it just balances the tube of the regulator when it is slightly inclined upwards. Should the temperature of the drawer now rise higher the mercury flows towards the distant end of the tube and, causing it to fall down, brings about a rotation of the regulator axis and as a consequence the opening of the ventilat- See also:ing valves. A transverse stay prevents the See also:limb of the regulator that of See also: The incubator itself (fig. 3) consists of an incubating or egg-drawer (E) heated from above by a warm-water tank (T). Below the egg-drawer is a tank containing cold water, the vapour of which passes through the perforated floor of the former and keeps the air of the egg-chamber slightly humid. Above the warm-water tank is an air chamber (AC) to serve as a non-conducting See also:medium and to prevent therefore undue loss of heat. Above this is a nursery or drying chamber (N), closed in, with a movable lid. The warm-water tank is heated by means of a simple boiler (B) from which an inlet tube (I) carries heated water to the tank; the tube traverses the length of the tank and discharges at its farther end (not shown in the diagram). From the nearer end of the tank an outlet tube (0) passes out and opens into the boiler at a slightly higher level than the inlet one. The boiler is heated by an evenly burning lamp below, of special construction. The rectangular tube through which the wick passes is bevelled at its outer end, and upon this bevelled edge a metal flap (F) is allowed to See also:rest more or less The wick is, of course, bevelled to correspond to the form of its tube. The metal flap is raised or depressed by means of levers connected with the heat-regulator. When it is depressed upon the wick the flame is lessened; and it becomes proportionately bigger as the flap is raised more and more. The heat-regulator consists of a glass tube (T) which runs the whole width of the incubation chamber and lies in contact with the floor of the warm-water tank ; it is filled with See also:alcohol. Externally to the incubator this tube is connected with a U-shaped one containing mercury. The See also:free limb of the U-tube contains a See also:piston (P) which rests upon the surface of the mercury in that limb. From the piston a piston rod (PR) passes vertically upwards and is connected with a lever (1.) which operates, through the agency of a second lever (L') the movements of the ventilating valve (V) in F opens from the roof of the incubator drawer. The lever (L) is further connected with a See also:spiral See also:spring (S) which works the metal flap of the lamp already described. The height of the piston in the U tube can be so adjusted, by varying the quantity of mercury in the tube, that when the temperature of the incubation drawer is 103° F., the ventilating valves are closed and the wick is burning to its full extent. Should the temperature rise, the alcohol in the glass tube (T) expands and causes the mercury in the free limb of the U tube to rise. This carries with it the piston, and this See also:movement brings about the opening of the ventilating valves, and at the same time, through the agency of the lever (L) and the spiral spring (S) the metal flap is brought down upon the wick, cutting off more or less of the flame. Should the temperature then fall to Io3° or lower, the contraction of the alcohol reverses these movements, the valve closes, and the wick once more See also:burns to its full extent. In practice, the boiler and the temperature regulator are duplicated, there being a set on both sides of the incubator. Any slight irregularity on the one side may be thus compensated for by the other side, Graves's incubator has the reputation of being a good machine. Among the most recent type of incubators made in England is in the simplicity and ingenuity of the heat regulator, and in that the tubes which traverse the water tank are hot-air flues, carrying the air heated by the flame and not warm water. Consequently a further simplification is introduced inasmuch as no boiler is required. The essential features of this incubator are shown in fig. 4. The See also:internal parts of the incubator are insulated by a double wall, the interspace being packed by a non-conducting material, which is not shown in the figure. The incubation or egg-drawer (E) is heated by the warm-water tank (T). Beneath the egg-drawer is a zinc tray (Z), so constructed that in the central part the floor is raised up into a See also:short See also:cylinder. Around the raised cylinder is a wide trough containing water and into this dips a See also:canvas See also:cloth which is stretched out over a perforated zinc support (F). By this means an extended moistened surface is produced which allows of a rapid evaporation. F E S r...F_ z v ~l The floor of the incubator, which is raised by short feet from the table on which it stands, is perforated in the central portion by a number of holes, and which are so situated that they lie beneath the raised cylinder of the cold-water tray (Z). The incubation-drawer is thus supplied continuously by a slow current of moistened air because the air in the upper part of the drawer, i.e. in contact with the floor of the warm-water tank, is the warmest and lightest. It therefore tends to diffuse or pass through the narrow slits between the drawer and the walls of the incubator, and also through the aperture in the front wall of the egg-drawer, through which a thermometer is placed. To replace the air thus lost, fresh air passes in through the holes in the bottom of the incubator, and on its way must pass through the pores of the damp canvas which dips into the water in the zinc tray (Z). The warm-water tank is heated by an inlet (I) and outlet (0) flue which are, however, continuous. The inlet flue opens out from a vertical chimney (C), the air in which is heated either by a gas flame or that of an oil lamp. The outlet or return flue passes back through the width of the tank and opens independently to the exterior. The vertical chimney (C) is capped by a lid (L) capable of being raised or lowered upon its orifice by the lever (L'). When the cap is resting upon the chimney all the heated air from within the latter passes through the flues and heats the water in the tank. If the cap is widely raised, practically all the heated air passes directly upwards through the chimney and none goes through the flues. If the cap be but slightly raised, part of the heated air goes through the flues and part directly escapes through the aperture of the chimney. The movement of the lever (L') which raises the cap (L) is deter-See also:mined by the thermostatic See also:capsule (S), situated within the egg-drawer. The principle upon which this capsule is designed is that the boiling point of a liquid depends not only upon temperature but also upon pressure. A given liquid at ordinary atmospheric pressure will See also:boil at a certain degree of temperature, which varies for different substances. But if the pressure be increased the boiling point of the liquid is raised to a higher degree of temperature. A liquid when it boils passes into a gaseous See also:condition and in this See also:state will occupy a very much larger See also:volume some two or three See also:hundred times—than in the liquid condition. If, therefore, a hermetically sealed capsule: with flexible sides be filled with some liquid which boils at a given temperature, the sides of the capsule will distend when the temperature of the air round the capsule has been raised to the boiling point of the liquid within it. The distension of this capsule can be used to raise the lever (L'). The thermostatic capsule is placed on a fixed See also:cradle (F) and is filled with a mixture of See also:ether and alcohol, the proportions being such that the boiling point of the mixed liquid is too° F. Between the capsule and the lever (L') is a vertical rod (V), articulating with the lever as close as possible to its fulcrum (M). The See also:articulation with the lever is by means of a See also:screw, so that the necessary See also:nice adjustment between the height of the rod (V), the thickness of the capsule and the position of rest of the damper (L) upon the chimney, can be accurately made. The temperature at which it is desired that the liquid in the capsule shall boil can be determined by sliding the weight (W) nearer or farther to the fulcrum of the lever (L'). The farther it is moved outwards, the greater is the pressure upon the thermostatic capsule and consequently the higher will be the boiling point of its contained liquid. By means of the milled-See also:head screw (A), the height of the lever at its outer end can be so adjusted that when the liquid of the capsule is not boiling the damper (L) closes the chimney, but that when it does boil the damper will be raised sufficiently high from it. If the weight is pushed as far as it will go towards the fulcrum end of the lever, the temperature of the egg-drawer will never rise more than toe F. because at this temperature and under the pressure to which it is then subjected, the liquid in the capsule boils, and consequently brings about the raising of the damper. It matters not, therefore, how high the flame of the gas or lamp be turned, the temperature of the egg-drawer will not increase, because the extra heat of the enlarged flame is passing directly outwards through the chimney, and is not going through the flues in the tank. In See also:order to raise the temperature within the incubation chamber to 102° or 103°, or any other desired degree, the weight (W) must be moved outwards along the lever (L'), about t in. for every degree of temperature increase desired. This thermostatic capsule works admirably, and the incubator will work for months at a time and requires no adjustment, however much, within the limits of our See also:climate, the external temperature may vary. The capsule, like all other thermostats in which the expansible substance is a liquid, is, however, dependent upon external pressure for the point at which its contained liquid boils and therefore, for the degree of temperature prevailing within the incubator drawer. It is therefore responsive to See also:variations in atmospheric pressure, and as the See also:barometer may fall t or 2 in., this may possibly make a difference of two or three degrees in the fluctuation of temperature within the egg-drawer. It is not, of course, often that such large oscillations of the barometer occur, and as a See also:matter of practical experience, under ordinary conditions, this incubator will work for months together without See also:attention with only See also:half a degree variation round the point at which it was set. ' See also:Greenwood's incubator (fig. 5), named the See also:Bedford, resembles Hearson's in that hot-air flues (F and F') and not hot-water pipes, 363 traverse the water tank (T). And the method of regulation of the temperature is much the same, i.e. a thermostat (V) operating upon a lever which raises a cap (C) from off the aperture of the See also:main flue (F) and thus allows all the heat of the flame to pass directly outwards, without passing through the series of flues (F) which horizontally traverse the water- tank. Fresh air enters through a wide circu- W See also:lar aperture (A) which surrounds the main flue, and it thus becomes partially warmed before entering the egg-chamber. The eggs are placed upon a perforated floor (E) lying over water See also:baths (B). The water tank (T) lies in the centre of the incubation chamber and is traversed through its central axis by the main hot-air flue (F). From this, four horizontal flues pass outwards through the water and open into small vertical flues, which in their turn communicate with the exterior. The thermostat (V) consists of a glass tube of See also:peculiar form. This is closed at the end of its short limb and open at its other extremity on the long limb. The See also:bent portion of the tube is filled with mercury and between the mercury See also:column and the closed end is a small quantity of ether. The thermostat is lodged in a box (G), which forms part of the lever (L). At one end this lever is pivoted to a fixed arm, and at the other to the vertical rod .which operates the ventilating cap (C). If the temperature should rise, the ether in the thermostat expands and pushes the mercury column up along the inclined long limb. This disturbs the See also:equilibrium of the lever (L), and it descends downwards, pulling with it the vertical rod, and thus raising the cap over the main flue. If the temperature falls the See also:reverse series of changes occur. The temperature at which the cap will be raised can be adjusted within limits by the position of the weight (W) and by the adjustment of the degree of inclination of the thermostat. The See also:Proctor incubator, made at See also:Otley, is apparently, in its main features, similar to the Greenwood. Somewhat similar, in certain features, to the Greenwood is the Winchcombe. Its improved form, in which metal replaces the wood casing, is named the See also:Gladstone. In it there is a See also:combination of the hot-air and the water-tank systems of warming the incubation chamber. The wall of the incubator is double, and the space between the outer and inner wall is packed with a non-conducting material. The incubation chamber is heated above by a water-tank (fig. 6 T) A which is traversed by a main vertical flue (F) and four subsidiary horizontal ones which See also:discharge externally. The main flue, how-ever, in passing up to enter the water tank traverses the egg-chamber, and therefore serves to warm it, as in the hot-air type of incubator, by the heat of the flue itself. Around the lower half of the flue is a water vessel consisting of two concentric containers (C), holding water. In the space between these concentric containers, fresh air passes in through the aperture (A), and before it reaches the egg-chamber it passes through coarse canvas which dips into the water in the containers, and is therefore kept permanently moist. The containers are filled from a water tank (S) outside the incubator. Air passes out from the egg-chamber through the aperture (0). The temperature is regulated by a bimetallic thermostat (see below), which operates two levers, that by their arrangement can raise or depress the cap (D) over the main flue (F). The temperature at which this occurs will be determined, within limits, by the position of the adjustable weight (W). Tomlinson's incubator, designed in 188o, is novel in principle. Additional information and CommentsThere are no comments yet for this article.
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