“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.” [Illustration: Two devil-like figures within an upturned bottle bouncing about.] “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. “I have here,” said he, “a figure exactly similar to those in the bottle, which we will now examine. You will observe, that in its centre there is a cavity terminating in a small orifice in the lower part; this cavity may be made to contain any quantity of air, so as to give the required buoyancy to the figure: now mark!--I press my hand upon the parchment cover, and the figure, you perceive, descends; I now remove the pressure, and see, it immediately reascends. The water in the bottle, as I have told you, is incompressible; when, therefore, I press upon the surface, it rises into the interior of the figure, and, consequently, by compressing the air into a less space, renders it less buoyant; but no sooner is the hand removed, than the enclosed air resumes its former volume, and expels the intruding water; in consequence of which the figure regains its former lightness, and reascends. Do you understand me?” asked Mr. Seymour. “Perfectly,” said Tom, “and many thanks for the explanation;” and in this opinion did the whole party concur. “Well, then,” continued Mr. Seymour, “you will now understand the use of the air-bladder in fish, for it is constructed upon a precisely similar principle. When the fish desires to descend, it presses upon the bladder by means of its muscles, and thus condenses the included air into a smaller volume.”[37] “I now also perceive why the water at the bottom of the sea cannot be much more dense than that on the surface; but, if we could dig a pit to the centre of the earth, the air, in that case, would be highly dense, because, unlike water, it is compressible,” said Tom.