PNEUMATICS. 



ed previously to the introduction of the 

 fresh mercury. But that column is charged 

 with a weight double of the former, since 

 a pressure of thirty inches of mercury is 

 added to an equal pressure exerted by 

 the atmospheric air, and which is not con- 

 sidered as being diminished ; for we may 

 neglect the small difference which results 

 from this, that the thirty inches which 

 terminate the atmospheric column at bot- 

 tom are actually occupied by the mercu- 

 ry. In general, if we take the ratio be- 

 tween the first pressure from the column 

 of the atmosphere, and any other pres- 

 sure whatever exerted by that same co- 

 lumn, and by the mercury superadded, 

 the corresponding spaces, occupied by 

 the compressed air, will be respectively 

 in the inverse ratio of the pressures ; 

 whence it is obvious, that the air contracts 

 itself, as we have stated, in proportion to 

 the weights compressing it. If we after- 

 wards take out the mercury at several 

 distinct times, the air will expand by rea- 

 son of its elasticity, and the spaces which 

 it will successively occupy in a contra- 

 ry order will still conform to the inverse 

 ratio of the pressures. 



Having given this brief account of the 

 general properties of air, we shall refer 

 to a few experiments, and the instru- 

 ments which are commonly used in per- 

 forming these experiments; beginning 

 with the air-pump, which has been alrea- 

 dy described in a general way. Fig. 2, 

 is an air pump, much in use. A A are two 

 brass barrels, each containing a piston, 

 with a valve opening upwards. They 

 are worked by means of the winch, B, 

 which has a pinion that fits into the teeth 

 of the racks, C C, which are made upon 

 the ends of the pistons, and by this means 

 moves them up and down alternately. On 

 the square wooden frame, D E, there is 

 placed a brass plate, G, ground perfectly 

 flat, and also a brass tube, let into the 

 wood communicating with the two cylin- 

 ders and the cock, I, and opening into 

 the centre of the brass plate at a. The 

 glass vessel, K, to be emptied or exhaust- 

 ed of air, has its rim ground quite flat, and 

 rubbed with a little pomatum, or hog's- 

 lard, to make it fit more closely upon the 

 brass plate of the pump. Sometimes thin 

 slips of moistened leather are used for this 

 purpose. These vessels are called receiv- 

 ers. Having shut the cock, I, the pistons 

 are worked by the winch, and the air be- 

 ing suffered to escape when the piston is 

 forced down, because the valve opens up- 

 wards, but prevented from returning in- 

 to the vessel, for the same reason the re- 



VOL V. 



ceiver is gradually exhausted, and will 

 then be fixed fast upon the pump-plate. 

 By opening the cock, I, the air rushes 

 again into the receiver. 



To the air pump is attached the guage, 

 z, or instrument for measuring the de- 

 gree of rarefaction, or exhaustion, pro- 

 duced in the receiver, and which is a ne- 

 cessary appendage to the air-pump. Ifa 

 barometer be included beneath the re- 

 ceiver, the mercury will stand at the 

 same height as in the open air, but when 

 the receiver begins to be exhausted, the 

 mercury will descend, and rest at a height, 

 which is, in proportion to its former 

 height, as the spring of the air remaining 

 in the receiver is to its spring before ex- 

 haustion. Thus, if the height of the mer- 

 cury, after exhaustion, is the thousandth 

 part of what it was before, we say that 

 the air in the receiver rarefied is a thou- 

 sand times. On account of the inconve- 

 nience of including a barometer in a re- 

 ceiver, a tube, of six or eight inches in 

 length, is filled with mercury, and invert- 

 ed in the same manner as the barometer. 

 This being included, answers the same 

 purpose, with no other difference, than 

 that the mercury does not begin to de- 

 scend till after about three-fourths of the 

 air is exhausted : it is called the short ba- 

 rometer guage. This is generally plac- 

 ed detached, but communicating with 

 the receiver by a tube concealed in the 

 frame, as is represented in the figure; 

 another and better guage was invented 

 by Mr. Smeaton, and called, from its form, 

 the pear-guage. It consists of a glass ves- 

 sel, in the form of a pear (fig. 3.) and suf- 

 ficient to hold about half a pound of mer- 

 cury : it is open at one end, and at the 

 other end is a tube, hermetically closed 

 at top. The tube is graduated, so as to 

 represent proportionate parts of the 

 whole capacity. This guage, during the 

 exhaustion of the receiver, is suspended 

 in it by a slip wire, over a cistern of mer- 

 cury, placed also in the receiver. When 

 the pump is worked as much as is thought 

 necessary, the guage is let clown into the 

 mercury, and the air re-admitted. The 

 mercury will immediately rise in the 

 guage ; but if any air remained in the re- 

 ceiver, a certain portion of it would be in 

 the guage ; and as it would occupy the 

 top of the tube above the mercury, it 

 would shew by its size the degree of ex- 

 haustion ; for the bubble of air would be 

 to the whole contents of the g'uug'e, as 

 the quantity of air in the exhausted re- 

 ceiver would to an equal volume of the 

 common atmospheric air. If the receiver 



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