AND EXTERNAL RESISTANCE. 293 



done in me. I shall, therefore, proceed to show, 

 that the various phenomena which take place 

 by the mechanical power of the air, are not 

 referable to its weight, but to its expansible 

 power alone. 



L It must evidently appear, that if it be the 

 weight of the air which, in one instance, press- 

 ed and kept the Magdeburgh hemispheres, 

 when exhausted of air, by a bond so close and 

 firm, &s to be equal to a weight of 140 pounds, 

 and in the other to a power of 20 horses, be- 

 fore they could be separated ; it must require 

 a weight equal to that force to compress them. 

 So far, however, from that being the case, I 

 placed, as many have done before rne, the 

 common hemispheres under a glass-receiver, 

 containing four cubic feet of air, the weight of 

 which in vacuo did not amount to one drachm, 

 and from which the atmospherical air, external 

 to the receiver, was entirely excluded; the 

 firmness of union by which they were held 

 together in the open air, did not appear to 

 be in the least abated, although exposed within 

 the receiver to a weight so extremely small. 

 On exhausting, however, that small quantity of 

 air out of the receiver, and rarifying thereby 

 the air without to the same state as it was 

 within, the one hemisphere immediately sepa- 

 rated from the other. If the same hemispheres 



