JLIRPUMP FOR A PERFECT TACUUM. 10/ 



top. When themfrcury rises 1 inch in the common baro- 

 meter« it will rise only fan inchin this, on account of the de- 

 pression in the other end, which will also be equal to |^ an 

 inch: it is however evident, that this diminution of range 

 cannot at all affect the sensibility of the instrument ; as it 

 will be increased in an equal degree, in the opposite tube. 

 But it is by no means necessary, that thelegsof the siphon 

 should be of equal diameter; although I have supposed 

 them to be so, in the present instance, in order to make the 

 action of the sliding scale more apparent. The scale may 

 be affixed to any common barometer, having either a bulb, 

 or an open reservoir; as will be evident, by inspecting the 

 figure. 



The note which is added to my paper on the airpurap Airpump. 

 is perfectly just. I had however purposely avoided mak- 

 ing use of the expression a ** perfect ractmm" ; and sub- 

 stituted that oi exhavstion, in the room of it: meaning, that 

 provided the construction of the pump were perfect, there 

 would be no limit to the exhaustion it was capable of pro- 

 ducing, i. e. that as long as any air remained in the receiver, 

 a portion of it might still be expelled, by continuing the 

 action of the pump; and although, strictly speaking, this is 

 not producing ii perfect exhaustion, yet it may be carried 

 on in infinitum*. 1 was so well convinced of the impossibi- 

 lity of obtaining a perfect vacuum, by this, or any other 

 pump, that it was my intention to have added a few words 

 on this subject, and on the only means, by which, I con- 

 ceive, it may be procured. The vacuum of the baro- Toricellian va- 

 meter cannot be considered as perfect, even when air and ^""™* 

 moisture are entirely excluded; on account of the atmos- 

 phere, formed by the evaporation of the mercury itself, as 

 was ascertained by Lavoisier, (Kerr's transl. p. 59, Ed. 3,) 

 and also by Dr, Priestley. (See his Experiments Vol. 5, 



• Let us suppose the capacity of the barrel to be greater than that of 

 ^ the receiver. By the first stroke of the piston, a quantity of air greater 

 than the half will be taken away : by th« second stroke, more than half 

 ef the remainder will be removed, and so on : in this case, (hen, it is evi- 

 <lent, that if the action of the pump be continued, there will at last re* 

 inain a quantity less than any that caii be assigned. This is approaching 

 f€»y nev to perfection, 



p. 22^.) 



