ox PNEUMATIC MACHINES, 341 



ance, 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 preponder- 

 ance of the largx-r 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 ebul- 

 lition of etlier, or of water moderately warm. (Plate XXIV. Fig. 32.6, 

 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 instead .of 

 the short gage, the difference of the height in its two branches indicating, 

 the pressure ; and this instrument has the advantage of requiring no cor- 

 rection on account of capillary attraction, since the depressions of the two co- 

 lumns 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. 



