“The fact, then,” said her father, “is simply this; since the air is elastic, or capable of yielding to pressure, so, of course, the lower parts must be more dense, or in a greater state of compression than those which are above them. In a pile of fleeces of wool, are not the lower fleeces pressed together by the weight of the superior ones, and do they not lie light and loose, in proportion as they approach the uppermost fleece, which receives no external pressure, and is confined merely by the force of its own gravity?”
“Clearly,” said Louisa.
“Well then, we will suppose, for example, that the whole column of the atmosphere were divided into a hundred parts, and that each of these parts weighed an ounce; would not the earth, and all things on its surface, be, in such a case, pressed upon with the whole hundred ounces?”
“No one can deny that,” said Tom.
“The lowest stratum of air,” continued Mr. Seymour, “would be pressed upon by the ninety-nine ounces above it; the next by ninety-eight; and so on, until we arrived at the ninety-ninth stratum from the bottom, which would, of course, be subjected to no more than one ounce of pressure, or to the weight of the last and highest stratum.”
The children were perfectly satisfied with this simple explanation; and Tom enquired whether, for the same reason, the water at the bottom of the sea must not be very dense, and unlike that we are accustomed to observe on the surface: his father, however, corrected this notion, by stating that water, not being, like air, elastic and compressible, would not suffer any material diminution in volume, although pressed even by the enormous weight of the superincumbent ocean.[(30)]
“I have before alluded to the relative compressibility of air and water, and the present appears a good opportunity for proving the fact by an amusing experiment. See! here are the ‘Bottle Imps,’ vicar, which you may remember I promised to introduce to your respectful notice,” said Mr. Seymour. “In this jar of water, carefully closed, as you may perceive, by parchment, are two little enamelled figures, which shall be made to rise and fall, by alternately pressing upon and removing the hand from the cover: thus.”
“Why, the spirit of Simon Magus must surely possess thee!” exclaimed the vicar.
The children, as may be readily imagined, were much astonished at so singular an effect, and expressed much anxiety to be informed by what mechanism it was produced. Their father accordingly proceeded with the following explanation.