VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. '223 



air so fast, that he could not (at least in the 10 minutes) make the barometer- 

 gage indicate a degree of exhaustion of more than 152. 



To determine whether it was the moisture in the leather from which the va- 

 pour arose, Mr. N. made other experiments ; viz. with a piece of white leather 

 fresh from the leather-sellers ; then with the same piece of leather dried by the 

 fire till it would lose no more of its weight ; and 3dly, with the same piece of 

 leather held in the steam of hot-water till it bad regained the 20 grains weight it 

 had been deprived of by the drying. Hence it appeared that in the 1st and 3d 

 of these 3 states the barometer-gage indicated an exhaustion of only about 140, 

 while the pear-gage showed about 100,000; but in the 2d case they showed 

 nearly alike, the former showing an exhaustion of about 270, and the latter of 

 about 280. 



Being now perfectly satisfied, from these and other experiments, that the va- 

 riation in the testimony of the pear and barometer-gages was occasioned by the 

 moisture contained in the substances which had been put into the receiver assum- 

 ing the form of vapour ; Mr. N. determined next to try what would be the 

 effect of the vapour which might arise from small quantities of different fluids, 

 and from some other substances containing moisture of various kinds. And 

 observing by these experiments that the small quantity of moisture which ex- 

 haled from the substances under the receiver prevented the pump from exhaust- 

 ing it to any very considerable degree, he began to suspect that whenever wet 

 leather had been used to connect the receiver with the plate, there must have 

 arisen so great a quantity of vapour as to have prevented the degree of exhaus- 

 tion being near so great as in some of the foregoing instances. These suspi- 

 cions induced him to make still further experiments. 



The great difference in the testimony of the pear-gage in these last six experi- 

 ments appeared to him exceedingly astonishing, for the leathers seemed each of 

 them to be as moist at last as at first. By which he was convinced how effec- 

 tually the use of leather soaked in water, or in water and spirit of wine, prevents 

 the pump from exhausting to any considerable degree. He made a number of 

 experiments of the same kind as these ; but never was able to exhaust, under 

 such circumstances, to a greater degree than between 30 and 6o, when the 

 heat of the room was about 57" by a thermometer of Fahrenheit's scale. 



From these and many other experiments, it evidently appears, says Mr. N., 

 that the air-pump of Otto Guericke, and those contrived by Mr. Gratorix, and 

 Dr. Hooke, and the improved one by Mr. Pappin, both used by Mr. Boyle, also 

 Hauksbee's, S'Gravesande's, Muchenbroeck's, and those of all who have used 

 water in the barrels of their pumps, could never have exhausted to more than 

 between 40 and 50, if the heat of the place was about 57 ; and though Mr. 

 Smeaton, with his pump, where no water was in the barrel, but where leather 



