M. Pouillet has estimated, with singular ingenuity, from a series of observations made by himself, that the whole quantity of heat which the Earth receives annually from the Sun is such as would be sufficient to melt a stratum of ice covering the entire globe forty-six feet deep.
By the action of the sun’s rays upon the earth, vegetables, animals, and man, are in their turn supported; the rays become likewise, as it were, a store of heat, and “the sources of those great deposits of dynamical efficiency which are laid up for human use in our coal strata” (Herschel).
A remarkable instance of the power of the sun’s rays is recorded at Stonehouse Point, Devon, in the year 1828. To lay the foundation of a sea-wall the workmen had to descend in a diving-bell, which was fitted with convex glasses in the upper part, by which, on several occasions in clear weather, the sun’s rays were so concentrated as to burn the labourers’ clothes when opposed to the focal point, and this when the bell was twenty-five feet under the surface of the water!
CAUSE OF DARK COLOUR OF THE SKIN.
Darkness of complexion has been attributed to the sun’s power from the age of Solomon to this day,—“Look not upon me, because I am black, because the sun hath looked upon me:” and there cannot be a doubt that, to a certain degree, the opinion is well founded. The invisible rays in the solar beams, which change vegetable colour, and have been employed with such remarkable effect in the daguerreotype, act upon every substance on which they fall, producing mysterious and wonderful changes in their molecular state, man not excepted.—Mrs. Somerville.
EXTREME SOLAR HEAT.
The fluctuation in the sun’s direct heating power amounts to 1/15th, which is too considerable a fraction of the whole intensity not to aggravate in a serious degree the sufferings of those who are exposed to it in thirsty deserts without shelter. The amount of these sufferings, in the interior of Australia for instance, are of the most frightful kind, and would seem far to exceed what have ever been undergone by travellers in the northern deserts of Africa. Thus Captain Sturt, in his account of his Australian exploration, says: “The ground was almost a molten surface; and if a match accidentally fell upon it, it immediately ignited.” Sir John Herschel has observed the temperature of the surface soil in South Africa as high as 159° Fahrenheit. An ordinary lucifer-match does not ignite when simply pressed upon a smooth surface at 212°; but in the act of withdrawing it it takes fire, and the slightest friction upon such a surface of course ignites it.
HOW DR. WOLLASTON COMPARED THE LIGHT OF THE SUN AND THE FIXED STARS.
In order to compare the Light of the Sun with that of a Star, Dr. Wollaston took as an intermediate object of comparison the light of a candle reflected from a bulb about a quarter of an inch in diameter, filled with quicksilver; and seen by one eye through a lens of two inches focus, at the same time that the star on the sun’s image, placed at a proper distance, was viewed by the other eye through a telescope. The mean of various trials seemed to show that the light of Sirius is equal to that of the sun seen in a glass bulb 1/10th of an inch in diameter, at the distance of 210 feet; or that they are in the proportion of one to ten thousand millions: but as nearly one half of this light is lost by reflection, the real proportion between the light from Sirius and the sun is not greater than that of one to twenty thousand millions.