AND EXTERNAL RESISTANCE. 297 



Universally exerted, whenever the equilibrium 

 is destroyed, and where the least resistance 

 prevails. 



I have deemed it proper to dwell at some 

 length on this part of the subject,, in order 

 that I might put it on its proper footing, and 

 explain away the improper notions to which it 

 has given rise; notions which have led to the 

 most erroneous conclusions to the erroneous 

 suppositions that it is the weight, and not the 

 expansibility of the air or atmosphere, which 

 causes the elevation of the mercury in the Tor- 

 ricellian vacuum as it is called, as well as that 

 it is not the expansibility, but the weight of the 

 air, that causes the depression and rupture of 

 the bladder over an exhausted receiver; when 

 in fact the weight of the air is no more concern- 

 ed in elevating the one, than in cracking the 

 other. 



In order to prove the fact, I placed and im- 

 mersed the bottom of a Torricellian tube in a 

 basin of quicksilver, and placed a receiver 

 over it ; in the top of the receiver, a screw 

 was inserted, with a stop-cock attached to 

 a bladder, which had been previously filled 

 with common atmospherical air, as nearly 

 as possible of the same dimensions with those 



