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Zetetic Astronomy, by 'Parallax' (pseud. Samuel Birley Rowbotham), [1881], at sacred-texts.com
If it is argued that "there are times when the surface of the sea is perfectly calm, and that at such times at least, if the earth is a plane, the telescope ought to restore the hull of a ship, irrespective of distance, providing its power is great enough to magnify it," the reply is that practical experiments have proved that during what is called a "dead calm," the undulations or waves in the water amount to more than 20 inches, as will be seen from the following extracts:--
"This interesting subject was very fully entered into at a recent meeting of the Academy of Sciences, by Admiral Coupvent de Bois:
"It is not easy to ascertain the height of the waves of the ocean; nevertheless, the method adopted for the purpose is capable of affording sufficiently exact results. The point in the shrouds corresponding with a tangent to the tops of the highest waves is ascertained by gradually ascending them, and making observations until it is reached. That point being determined, the known dimensions of the ship give the height of the waves above the line of flotation, which corresponds with the horizon of the sea, in the trough of the wave. In this way the following results were obtained:--
|
With |
a smooth sea |
the waves were |
1.97 |
feet. |
|
" |
fair weather |
" |
3.28 |
" |
|
" |
a slight swell |
" |
4.921 |
" |
|
" |
a full swell |
" |
7.546 |
" |
|
" |
a great swell |
" |
10.827 |
" |
|
" |
a very great swell |
" |
15.42 |
" |
|
" |
a heavy sea |
" |
20.67 |
" |
|
" |
a very heavy sea |
" |
28.543 |
" |
"The lengths of the waves have also been measured, and it has been found that, for example, waves of 27 feet in height, are about 1640 feet in length." 1
It is well known that even on lakes of small dimensions and also on canals, when high winds prevail for some time in the same direction, the ordinary ripple is converted into comparatively large waves. On the "Bedford Canal," during the windy season, the water is raised into undulations so high, that through a powerful telescope at an elevation of 8 inches, a boat two or three miles away will be invisible; but at other times, through the same telescope the same kind of boat may be seen at a distance of six or eight miles.
During very fine weather when the water has been calm for some days and become as it were settled down, persons are often able to see with the naked eye from Dover the coast of France, and a steamer has been traced all the way across the channel. At other times when the winds are very high, and a heavy swell prevails, the coast is invisible, and the steamers cannot be traced the whole distance from the same altitude, even with a good telescope.
Instances could be greatly multiplied, but already more evidence has been given than the subject really requires, to prove that when a telescope does not restore the hull of a distant vessel it is owing to a purely special and local cause,