PRESSURE IN THE AIR, 



height ; run a nail through any part of the 

 spring hammer and fix it to a post, and cut 

 off the upper part of the spring, and the mer- 

 cury will be found to continue, by the pres- 

 sure of the inferior portion, as high as it was 

 before!!!" The distinction which I have 

 shewn to exist between expansibility be- 

 tween elasticity and between weight, will, I 

 hope, show the error and futility of these ob- 

 servations ; the most elastic substance which 

 exists, whether steel or whalebone, is not elas- 

 tic per se ; if it presses at all, it presses by its 

 quantity only, either of gravity or of levity ; 

 when it presses by an elastic force, it is in 

 consequence of a power which has been exter- 

 nally applied to it, by virtue of which the par- 

 ticles of matter become compacted and com- 

 pressed, in proportion to the degree of external 

 power which has been employed ; the resist- 

 ance of the steel may prevent the force em- 

 ployed, from extending beyond the first, or 

 second circle, or it may be so great as to over- 

 come the resistance of all the circles of the 

 spring altogether, and cause pressure to be pro- 

 duced at the other extremity of the spring, 

 which pressure will make the mercury rise: 

 it will rise in proportion to the external force 

 employed ; and it will be kept at that point, if 

 the weight of the steel is sufficiently heavy to 

 balance the mercury, or the force sufficiently 



