VOL. XLVII.] PHILOSOPHICAL TRANSACTIONS. J4g 



upper plate, the piston is worked with more ease* than the common pumps 

 with two barrels : and not only so, but when a considerable degree of rarefaction 

 is desired, it will do it quicker ; for the terms of the series expressing the quan- 

 tity of air taken away at each stroke do not diminish so fast, as the series an- 

 swering to the common one. 



Having found the gages that have been hitherto made use of, for measuring 

 the expansion of the air, very unfit to determine in an experiment of so much 

 nicety ; Mr. S. therefore contrived one of a different sort, which measures the 

 expansion with certainty, to much less than the 1000th part of the whole. It 

 consists of bulb of glass something in the shape of a pear, and sufficient to hold 

 about a half a pound of quicksilver. It is open at one end, and at the other is a 

 tube hermetically closed at top. By the help of a nice pair of scales, he found 

 what proportion of weight a column of mercury, of a certain length, contained 

 in the tube, bore to that which filled the whole vessel. By these means he was 

 enabled to mark divisions on the tube, answering to a 1000th part of the whole 

 capacity, which being of about one tenth of an inch each, may by estimation be 

 easily subdivided into smaller parts. This gage, during the exhausting of the 

 receiver is suspended in it by a slip-wire. When the pump is worked as much 

 as shall be thought necessary, the gage is pushed down, till the open end is im- 

 merged in a cistern of quicksilver placed underneath : the air being then let in, 

 the quicksilver will be driven into the gage, -f- till the air remaining in it be- 

 comes of the same density with the external ; and as the air always takes the 

 highest place, the tube being uppermost, the expansion will be determined by 

 the number of divisions occupied by the air at the top. 



The degree, to which he has been able to rarefy the air in experiment, has 

 generally been about 1000 times, when the pump is put clean together: but 

 the moisture that adheres to the inside of the barrel, as well as other internal 

 parts, on letting in the air, as in the succeeding trials worked together with the 

 oil, which soon renders it so clammy, as to obstruct the action of the pump on a 

 fluid so subtile as the air is, when so much expanded ; but in this case it seldom 

 fails to act on the air in the receiver, till it is expanded 500 times : and this he 

 found it to do, after being frequently used for several months without cleaning. 

 He also generally found it to perform best the first trial at each time of using ; 



• Because, though the pressure of a column of air, equal to the diameter of the piston-rod, still 

 presses on it, yet as there is only the friction of one piston, and that not loaded with the weight of 

 the atmosphere; the friction of the leather against the side of the barrel, and that of the rack and 

 wheel, is much less: so that, notwithstanding the addition of friction in the collar of leathers, that 

 of the whole will be less. — Orig. 



+ The bulb of the gage may be emptied of its quicksilver, without taking that out of the tube ; 

 and the tube being held horizontal, the column of mercury in it will have no power to contract or 

 expand the air at the top — Orig. 



VOL. X. K K 



