[ 3S7 ] 



ous vapours, but even of permanently elaftic air from the rea, 

 contrary to what has been generally imagined, viz. that fuch 

 aqueous vapours would encreafe the rarefadion as fhewn by 

 the pear-gage, in the fame degree as they would diminifh it, ac- 

 cording to the teftimony of the baromi^-gage. 



The reafon why mere moifture in the recr cannot be pumped 

 out, though it be in the form of elaftic vapour, when the air 

 is moift (for in dry weather it adually can, by long working, 

 as I have often found) I take to be this, that when it is got 

 above the pifton, as foon as the valve opens, the preffure of the 

 external air inftantly reduces the vapour to water, which fubfides 

 in a dew on the top of the pifton, and the upper parts of the 

 pump (the little air which gets in through the valve, and by 

 fuppofition moift, being infufficient to dry it up) and refolves 

 into vapour again, when it can pafs by the circulating-pipe, 

 opened below, into the vacuum under the pifton : Thus I fup- 

 pofe it is kept circulating within the barrel and cannot be dif- 

 charged. But why any permanently elaftic fluid in the rec"" 

 could not, in damp weather, be exhaufted, as it was when the 

 air was dry, is a queftion of great difficulty. I had long before 

 iufpeded that air was produced within the pump, notwith- 

 ftanding its excellent performance juft mentioned; of the reality 

 of which, from the agreement of the gages^ and chiefly from 

 the eleftrical phajnomena (to be hereafter recited) I could not 



fee 



