CHAMBERS'S INFORMATION FOR THE PEOPLE. 



volume is again doubled by drawing the piston to 

 .?, its elastic force falls to one-fourth, or 3f pounds. 

 Thus the law of Mariotte holds for rarefied air as 

 well as for condensed air. 



HEIGHT OF THE ATMOSPHERE. 



Air at the level of the sea being, in round num- 

 bers, 11,000 times lighter than mercury, it would 

 require a column of air of that density to be 1 1,000 

 times 30 inches, or nearly five miles, high, to 

 balance the mercury in the barometer ; and this 

 would actually be the height of our atmosphere if 

 air were like water, of uniform density through- 

 out its whole depth. But we could conclude even 

 without trial, that the air must become rarer 

 and rarer as we ascend, there being at every 

 stage a diminished mass pressing it down ; and 

 experience verifies the conclusion. At the height 

 of about three miles, the barometer shews that one 

 half the mass of the air is below, so that the 

 density at that point is reduced to one half. 

 While, therefore, an ascent of 500 feet above sea- 

 level makes the barometer fall half an inch, an 

 ascent of 500 feet above three miles produces only 

 half that fall. That the air does not extend in- 

 definitely into the planetary spaces, we infer from 

 the consideration that its elastic force diminishes 

 as it becomes rarer, and is further weakened by 

 the increasing cold of the upper regions. There 

 must thus be a limit at which the tendency of 

 a molecule to recede from the rest is balanced 

 by the action of gravity drawing it towards the 

 earth. From various appearances, it is inferred 

 that the atmosphere extends to the height of at 

 least ico miles, if not much farther. 



The regular decrease in density which would 

 follow from Boyle's law, is interfered with by 

 the decrease of temperature, which is subject to 

 much irregularity and uncertainty. Were it not 

 for this, the barometer would furnish a ready 

 means of measuring heights. As it is, the rule is 

 too complex for explanation here. 



THE AIR-PUMP. 



The air-pump, by which air is removed from 



Fig. 29. 

 vessels, acts by means of the expansibility of the 



238 J 



fluid. There are various forms of the instrument ; 

 fig. 29 represents one of the simplest. R is the 

 glass-receiver, or vessel to be exhausted, standing 

 on a smooth plate P, and fitting so exactly, that 

 no air can penetrate between them. From an 

 opening in the middle of the plate, a tube passes 

 down to the sole of the frame, and then along to 

 the end, where it opens into the bottoms of the 

 two pumps or syringes, as is seen in fig. 30, which 

 represents the syringes A, A', in section. The 

 piston-rods, R, R', are alternately raised and 

 depressed by turning the pinion o backwards and 

 forwards by the handle H (which, for facility in 

 working, is often made double, as represented in 

 fig. 29). As the piston, P, descends, the valve, V, 

 is closed, and that in the piston opens and allows 

 the inclosed air to escape. When the piston has 

 reached the bottom, and begins to ascend, the 

 valve in P closes, and a vacuum would be formed, 

 were it not that the air in the receiver, R, and the 

 connecting tube being re- 

 lieved of the pressure of the 

 outward atmosphere, ex- 

 pands, and opening the 

 valve at V, fills the barrel 

 below the piston. Thus, 

 when the piston reaches the 

 top of the barrel, the air 

 that was at the outset con- 

 fined in the receiver and 

 tube, now fills a larger space, 

 and is therefore so much 

 rarer. The action of the 

 other syringe is exactly the 

 same ; and the two do not 

 interfere with each other, as 



Fig. 30. 



the valve at V is always shut when that at V is 

 open. Some air-pumps have only one syringe ; 

 and the chief use of two is for rapidity of ex- 

 haustion. Although every stroke of each piston 

 removes a barrelful of the inclosed air, it must be 

 remembered that that air is continually becoming 

 rarer, so that the absolute quantity removed 

 becomes less and less for every stroke. The 

 whole of the air could never thus be removed, even 

 in theory ; and in practice, a limit is put to the 

 degree of rarefaction by the elasticity of the 

 remaining air becoming too feeble to raise the 

 valves. The syphon-gauge at G shews to the 

 eye how far the exhaustion has proceeded. It 

 is constructed on the principle of the bent- 

 tube barometer, fig. 27. When the process ot 

 rarefaction has reached a certain point, the 

 mercury begins to descend in the closed leg of 

 the tube ; and if the exhaustion could be made 

 complete, the two surfaces would be on the same 

 level. 



The condensing pump, or syringe, is the counter- 

 part of the exhausting air-pump. By means of it, 

 a receiver can be charged with an amount of air 

 that would fill it many times at the ordinary 

 density, and experiments can thus be tried on 

 bodies inclosed in the receiver, under the pressure 

 of many atmospheres. 



A condensing syringe is used for charging the 

 chamber of air-guns, which consists of a strong 

 hollow copper ball. The valve of this chamber 

 being then suddenly opened by the trigger, a 

 portion of the highly compressed air is allowed to 

 escape into the barrel behind the ball, which it 

 propels with great velocity. 





