CAPACITY OF THE CHEST. 245 



a weight of books we can heave up if laid upon 

 its board. 



Understanding, then, that the power of the 

 hydraulic press, in raising the lid, depends on the 

 size of the reservoir, and its relation to the tube ; 

 and again, that in pressing the fluid up through 

 the tube, the pressure upon the sides of the reser- 

 voir must be the greater the larger the cavity, we 

 can conceive how a glass-blower propels the air 

 into his blow-pipe with great ease, if he blows with 

 the contraction of the cheeks, the smaller cavity; 



but with an exhausting effort, if he blows by the 

 compression of the larger cavity, the chest. Dr. 

 Young made a calculation, the result of which 

 was, that, in propelling the air through a tube of 

 the same calibre, a weight of four pounds operat- 

 22* 



