The above gives some idea of the immensity of the solar system. And it is but one of the myriads of systems, and our earth a speck amidst it. If on a clear night we cast our eyes upwards, we behold a belt of closely dotted stars extending across the sky—the Milky Way. This galaxy, a luminous, cloudlike band, stretches across the heavens in a great circle, and contains myriads of stars, densely crowded together. Herschel remarks that 288,000 stars once passed across the field of his great reflector in forty-one minutes, and says: “Thus we are to think of our own sun as a star of the second or third magnitude, and of our little solar system as plunged far into the midst of the vortex of worlds, a mere atom along that
“ ‘Broad and ample road
Whose dust is gold and pavement stars.’ ”
CHAPTER II.
THE EARTH.
This earth we live on is a planet, and belongs to the solar system of planets. It shines brightly, and appears to other worlds as other planets do to us. It is nearly 25,000 miles in circumference, and has a diameter of a little over 8,000 miles. It is five and a half times denser than water, and weighs about 6,096,000,000,000,000,000,000 tons.
The atmosphere that surrounds this earth is like a shell that is two hundred to three hundred or more miles in thickness. We live at the bottom of an immense ocean of gaseous matter, which envelops everything, and presses upon everything with a force which appears, at first, perfectly incredible, but whose actual amount admits of easy proof. Gravity being, so far as is known, common to all matter, it is natural to expect that gases, being material substances, should be acted upon by the earth’s attraction, as well as solids and liquids. This is really the case, and the result is the weight or pressure of the atmosphere, which is nothing more than the effect of the attraction of the earth on the particles of air. The amount of pressure exerted upon every square inch of the surface of the earth, and the objects thereon, is from fourteen to fifteen pounds. This enormous force is borne without inconvenience by the animal frames, by reason of its perfect uniformity in every direction; and it may be doubled, or even tripled, without inconvenience. An important law which connects the volume occupied by a gas with the pressure made upon it, is expressed by Mariotte in the following manner. This law is usually called Mariotte’s law: “The volume of gas is inversely as the pressure; the density and elastic force are directly as the pressure, and indirectly as the volume.”
This law has been found to be true no matter how rarefied the air.