Another experiment with the burning-glass will also teach us something. Take a candle, and from its flame you can get a bright point at the focus. It may fall upon your hand, but you can hardly feel it, and you will readily believe that the focus is not nearly so hot as the candle. Even when a burning-glass is held in front of a bright fire there is comparatively little heat in the focus. By using a lens to condense the beams from an electric lamp, Professor Tyndall has shown how to light a piece of paper, and to produce many other effects. But, nevertheless, the focus is not nearly so hot as the arc between the two glowing carbons. You might move your finger through the focus without much inconvenience, but I would not recommend you to trust your finger between the poles of the electric light itself. The temperature obtained at the focus of a burning-glass seems thus to be always less than that prevailing at the source of heat itself. This principle will be equally true when we turn a burning-glass to the sun, and hence we know that the sun must be hotter than any heat which can be obtained by the biggest burning-glass on the brightest of summer days. But burning-glasses a yard wide have been made, and astonishing heat effects have been produced. Steel has thus been melted by the sunbeams, and so have other substances which even our greatest furnaces cannot fuse. Therefore the sun must have a higher temperature than that of molten steel; higher, indeed, than any temperature we can produce on the earth.
I have tried to prove to you that the sun is very hot; but it would be well to see what arguments might be used on the other side. Indeed, it is by considering objections that we often learn. So I shall tell you of a difficulty that was once raised when I was endeavoring to explain the heat of the sun to an intelligent man. “I am sure,” said my friend, “that you must be quite wrong. You said that the nearer you got to the sun the hotter it would be; but I know this to be a mistake. When tourists go to Switzerland, they sometimes climb very high mountains. But the top of a mountain, of course, is nearer the sun than below; and so, if the sun were really hot, the climber should have found it much warmer on the top of the mountain than at its base. But every one knows that there is abundant ice and snow on lofty Alpine summits, while down below in the valleys there may be at the same time excessively warm weather. Does it not therefore seem that the nearer we go to the sun the colder it is, and the further we are from the sun the warmer it is?”
But my friend was quite wrong in his argument. The coldness of the mountain tops depends upon something which he had not taken into account. There is something else besides the sun which helps to make us so warm and comfortable. This other essential thing is more or less deficient at great heights. You know that we live by breathing air, and we find air wherever we go, over land and sea, all round the earth. Those who ascend in balloons are borne upwards by the air, and thus we can show that air extends for miles and miles over our heads, though it becomes lighter and thinner the loftier the elevation.
We not only utilize the air for breathing, but it is also of indispensable service to us in another way. It acts as a blanket to keep the earth warm; indeed, we ought rather to describe the air as a pile of blankets one over the other. These air blankets enable the earth to preserve the heat received from the sunbeams by preventing it from escaping back again into space. Thus warmth is maintained, and our globe is rendered habitable. You see then, that for our comfort we require not only the sun to give us the heat, but also the set of blankets to keep it when we have got it. If we threw off the blankets we should be uncomfortable, though the sun were as bright as before. A man who goes to the top of a mountain at mid-day does approach the sun to some extent, and, so far as this goes, he ought no doubt to feel warmer, but the gain is far too small to be thought of. Even at the top of Mont Blanc the increase in heat due to the approach to the sun would be only one ten-millionth part of the whole. This would be utterly inappreciable; even a thermometer would not be delicate enough to show it. On the other hand, by ascending to the top of the mountain, the climber has got above the lower regions of the air; he has not, it is true, reached even halfway to the upper surface—that is still very far over his head—but the higher layers of the atmosphere are so very thin that they form most indifferent blankets. The Alpine climber on the top of the mountain has thus thrown off the best portion of his blankets, and receives a chill; while the gain of heat arising from his closer approach to the sun is imperceptible. Perhaps you will now be able to understand why eternal snow rests on the summits of the great mountains. They are chilled because they have not so many air blankets as the snug valleys beneath.
The brightness of the sun is among the most wonderful things in nature, and there are three points that I ask you to remember, and then indeed you will agree with Milton, that the sun is “with surpassing glory crowned.” First think of the beauty and brilliancy of a lovely day in June. Then remember that all this flood of light comes from a single lamp at a most tremendous distance; and thirdly, recollect that the sun is not like a bull’s-eye lantern, concentrating all his light specially for our benefit, but that he diffuses it equally around, and that we do not get on this earth the two-thousand-millionth part of what he gives out so plenteously! When we think of the brightness of day, of the distance from which the light has come, though Nature has not adjusted any vast lenses to direct the light specially in our direction, we begin to comprehend the sun’s true magnificence.
FURTHER BENEFITS THAT WE RECEIVE FROM THE SUN.
I want to show you how great should be the extent of our gratitude to the sun. Of course, on a bright summer’s day, when we are revelling in the genial warmth and enjoying the gladness of sunshine, it needs no words to convince us of the utility and of the beneficence of sunbeams. So we will not take midsummer. Let us take midwinter. Take this very Christmas season when the days are short and cheerless, the nights are long and dark and cold. We might be tempted to think that the sun had well-nigh forgotten us. It is true he only seems to pay us very occasional visits, and between fogs and clouds we in England see but little of him; but, visible or invisible, the sun incessantly tends us, and provides for our welfare in ways that perhaps we do not always remember.
Let me give an illustration of what I mean. You will go back this dull and cold afternoon to the happy home where your Christmas holidays are being enjoyed. It will be quite dark ere you get there, for the sun in these wintry days sets so very early. You will gather around a cheerful fire. The curtains will be drawn, the lamps will be lighted, and the disagreeable weather outside will be forgotten in the pleasant warmth and light within. Five o’clock has arrived, the pretty wicker table has been placed near mamma’s chair; on it are the cups and saucers and the fancy teapot. Under the table is a little shelf, with some tempting cakes and a tender muffin. Two or three welcome friends have joined the little group, and a delightful half-hour is sure to follow.
But you may say, “What have tea and muffins, lamps and fireplaces to do with the sun? Are they not all mere artificial devices, as far removed as possible from the sunbeams or the natural beauties which sunbeams create?” Well, not so far, perhaps, as you may think. Let us see.
Poke up the fire, and while it is throwing forth that delicious warmth, and charming but flickering light, we will try to discover where that light and heat have come from. No doubt they have come from the coal, but then, whence came the coal? It came from the mine, where brave colliers hewed it out deep under the ground, and then it was hoisted to the surface by steam engines. Our inquiry must not stop here, for another question immediately arises, as to how this wonderful fuel came into the earth? When we examine coal carefully, by using the microscope to see its structure, we find that it is not like a stone; it is composed of trees and other plants, the leaves and stems of which can be sometimes recognized. Indeed, the fossil trunks and roots of the great trees are occasionally conspicuous in the coal-pit. It is quite plain that these are only the remains of a vegetation which was formerly growing and flourishing, and on further inquiry we learn that coal must have been produced in the following manner:—