[ 3^2 ] 



appearances ; for if this circumftance took place at all, it would 

 do fo equally in every ftroke and in all the cafes mentioned ; 

 whereas it happened only in the firfl: cafe ; in the fecond it did 

 not ; and in the third the contrary efFeft occurred. Moreover, the 

 fame phasnomena were exhibited, when the pump was covered 

 by the valve, and none hut greatly rarefied air was over the 

 pifton. It muft be remembered, that all the air which once got 

 under the pifton, muft always have remained there until it 

 paffed into the reC ; and would, if it had been produced as by 

 this fuppofition, accumulate in every ftroke. Neither was this 

 furreptitious air owing to the foulnefs of the barrel, it being 

 both foul and equally fo in the circumftances in which this air 

 raifed the mercy in the gage ; in thofe in which it did not, 

 and in thofe in which the mere/ funk in it ; and after the barrel 

 was cleaned, the effedls were the fame as before. 



It fcems therefore, on the view of thefc fads, to be a necef- 

 fary conclufioo, that the caufe of the phaenomena, is to be 

 looked for in the barrel itfelf, and in this, from the effed: pro- 

 duced by the motion of the piflon ; which efFed was declared by 

 the gage to be fometimes a produdion of air de novo^ and 

 fometimes an extindion or abforption of it again. 



To underftand thefe efFeds, let it be confidered, that though 

 the pump barrel be perfedly clean, and the pifton leathers be 



even 



