ON PNEUMATIC MACHINES. 261 



single barrel has nearly the same advantage, the rod of the piston working 

 'in a collar of leathers with oil, and the air heing excluded from the upper 

 part of the barrel by a valve, through which the air passes when the piston 

 is raised near to the top ; so that in the descent of the piston there is 

 a vacuum above it, and the air below opens the valve much earlier, and 

 passes more completely through it, than in the common air pump ; and 

 the piston is only exposed to the whole pressure of the atmosphere 

 during the discharge of the air through the upper valve. (Plate XXIV. 

 Fig. 325.) 



That the air is really removed by the operation of the air pump, may be 

 demonstrated by various experiments, which show the absence of its 

 resistance, of its buoyant effect, and of its pressure ; such are the descent of 

 a guinea and a feather at the same time, the equal duration of the motion 

 of two fly wheels, with their plates placed in different directions, the prepon- 

 derance of the larger of two bodies which balance each other in the open 

 air, the descent of mercury or of water in a barometrical tube, the playing 

 of a fountain urged by the expansion of a portion of confined air, and the 

 ebullition of ether, or of water moderately warm. (Plate XXIV. Fig. 

 326, 327.) 



The degree of perfection of the vacuum formed by the air pump, or the 

 rarity of the air remaining in the receiver, is measured by gages of different 

 kinds. The simplest gage is a short tube filled with mercury, and inverted 

 in a bason of the same fluid ; in this the mercury begins to descend when 

 the elasticity of the air becomes diminished in the proportion of the height 

 of the gage to that of the barometer ; but on account of the capillary at- 

 traction of the particles of mercury for each other, there is a depression 

 within the tube, differing in quantity according to its magnitude, which 

 renders it difficult to observe the exact situation of the surface when the 

 height of the column is very small, although, if that height were correctly 

 ascertained, the allowance to be made for the depression might easily be 

 calculated. It is, however, more usual to employ the long barometer gage, 

 in which the pressure is removed from the upper surface of the column of 

 mercury in proportion as the exhaustion proceeds, and the height to which 

 it is raised by the pressure of the external atmosphere, is compared with 

 that of a common barometer, the difference always indicating the density 

 of the air left in the receiver. Sometimes also a bent tube is employed in- 

 stead of the short gage, the difference of the height in its two branches indi- 

 cating the pressure ; and this instrument has the advantage of requiring no 

 correction on account of capillary attraction, since the depressions of the 

 two columns exactly counterbalance each other. But in all these cases the 

 mercury must be well boiled in the tubes ; and in the bent tube, or siphon 

 gage, the operation is somewhat difficult. 



The pressure indicated by a gage of any kind depends on the elasticity 

 of the whole of the fluid remaining in the receiver ; but this fluid is not 

 always atmospheric air alone. In all common temperatures, water, and 

 iriany other liquids, have the property of emitting a vapour which possesses 

 a very sensible degree of elasticity ; so that if either water, or any moist 

 substance, be present under the receiver, it will be impossible to procure a 



