You will ask, how do we learn all this? As nobody could ever make such a journey, how can we feel certain that the sun is so excessively hot? I know that what I say is true for various reasons, but I will only mention one, which is derived from an experiment with the burning-glass, that most boys have often tried.
Fig. 1.—How to use the Burning-glass.
Fig. 2.—The Noonday Gun.
We may use one of those large lenses that are intended for magnifying photographs. But almost any kind of lens will do, except it be too flat, as those in spectacles generally are. On a fine sunny day in summer, you turn the burning-glass to the sun, and by holding a piece of paper at the proper distance a bright spot will be obtained ([Fig. 1]). At that spot there is intense heat, by which a match can be lighted, gunpowder exploded, or the paper itself kindled. The broad lens collects together the rays from the sun that fall upon it, and concentrates them in one spot, which consequently becomes hot and bright. If we merely used a flat piece of glass the sunbeams would go straight through; they would not be gathered together, and they would not be strong enough to burn the paper. The lens, you see, is not flat; its faces are curved, and they thus acquire the power of bending in rays of light or heat, so as to unite their effect on that one point which we call the focus. When a great number of rays are thus collected on the same spot, each of them contributes a little warmth.
Fig. 3.—A Tell-tale for the Sun.
Some ingenious person has turned this principle to an odd use, by arranging a burning-glass over a cannon in such a way that just when noon arrived the spot of light should reach the touch-hole of the cannon and fire it off. Thus the sun itself is made to announce the middle of the day ([Fig. 2]).
Another application of the burning-glass is to obtain a record of the number of hours of sunshine in each day. You will understand the apparatus from [Fig. 3]; the lens is here replaced by a glass globe, which acts as a burning-glass. As the sun moves over the sky the bright spot of light also moves, and therefore burns its track on a sheet of paper marked with lines corresponding to the hours. When the sun is hidden by clouds the burning ceases, so by preserving each day the piece of paper, we have an unerring tell-tale, which shows us during what hours the sun was shining brightly, and the hours during which he was hidden. You see, the burning-glass is not merely a toy, it can be made useful in helping us to learn something about the weather.