AND EXTERNAL RESISTANCE. 307 



great to raise it. The elastic force whicfr 

 the steel has acquired, is entirely from with- 

 out ; and it ceases the instant that force is 

 taken away -the external force is the agent, 

 the steel is the instrument only; and it is by 

 the combination of both together that the effect 

 is produced ; the one without the other is uur 

 equal to the task. 



With respect to expansible bodies such as 

 air, it is far otherwise ; it contains within itself 

 both the agent and the instrument instead of 

 acting, like elastic bodies, mediately it act$ 

 immediately although an external force may 

 jcompress and condense it, into a smaller vo- 

 lume ; it was expansible before that external 

 force was applied. It acts, by its own inherent 

 power, independently of external influence, 

 wherever the least resistance prevails; and, 

 consequently, overcomes the weight of the mer- 

 cury, when it is placed at the bottom of a tube, 

 having in it an unresisting, because an exhaust- 

 ed medium. It is becausethere is less resistance 

 in the medium above, than there is below that 

 the natural tendency in the air of expanding 

 equally in every direction, is rather directed 

 and diverted from the bottom towards the top; 

 that, the air, rather ascends than descends ; 

 and, consequently, that the observation made 

 by the former gentleman, is an erroneous one 

 " that the whole weight of the incumbent colunm 



