VOL. LXVII.] PHILOSOPHICAL TRANS AUCTIONS. 221 



inch of quicksilver before the water turns into vapour. It is true, that water 

 exposed to the open air will evaporate at any heat, and with any pressure of the 

 atmosphere; but that evaporation is entirely owing to the action of the air upon 

 it: whereas the evaporation here spoken of is performed without any assistance 

 from the air. Hence it follows, that when the receiver is exhausted to the 

 abovementioned degree, the moisture adhering to the different parts of the 

 machine will turn into vapour and supply the place of the air, which is con- 

 tinually drawn away by the working of the pump, so that the fluid in the pear- 

 gage, as well as that in the receiver, will consist in a good measure of vapour. 

 Now letting the air into the receiver, all the vapour within the pear- 

 gage will be reduced to water, and only the real air will remain uncondensed; 

 consequently the pear-gage shows only how much real air is left in the receiver, 

 and not how mucli the pressure or spring of the included fluid is diminished, 

 whereas the common gages show how much the pressure of the included fluid is 

 diminished, and that equally whether it consist of air or of vapour." 



Mr. Cavendish having explained so satisfactorily the cause of the disagreement 

 between the two gages, Mr. N. considered, that if he were to avoid moisture as 

 much as possible, the two gages should nearly agree: this induced him to make 

 the following experiment. 



The plate of the pump being made as clean and as dry as possible, there was 

 then put on it the beforementioned short barometer-gage, also the pear-gage, 

 with a cistern entirely of glass which held the quicksilver; they were then 

 covered with a receiver, round the outside of which was laid a cement which 

 perfectly excluded the outer air; every part, before it was put under the receiver, 

 as well as the receiver itself, being made as clean and as free from moisture as 

 possible.* The pump was then worked for 10 minutes, and the barometer-gages 

 indicated a degree of exhaustion nearly 600; the air was then let into the 

 receiver, the pear-gage indicated a degree of exhaustion, but very little more 

 than 600 also. The near agreement of the pear-gage with the barometer-gages 

 in this last experiment, in which Mr. N. had been so careful to exclude the 

 moisture as much as possible, seemed to prove beyond a doubt, that their dis- 

 agreeing in the several former experiments must have been owing, as 

 Mr. Cavendish supposed, to the moisture which in them had not been so care- 

 fully excluded. But he began now to suspect also, that there might arise a 

 vapour from some moisture that might be contained in the leather soaked in oil 

 and tallow, or in the wooden foot which was cemented to the glass cup, both 



* It may be proper here to notice, that the pump in every experiment hereafter mentioned was 

 worked 1 minutes, and the same receiver continued cemented to the pump plate, except where it is 

 otherwise mentioned. The top of this was made to open, in order to put in different things. — Orig. 



