AND EXTERNAL RESISTANCE. 291 



an air-pump, as was adopted by OTTO GUE- 

 RICKE. The question itself is one of the more 

 or the less, and is a fit object for the inquiry of 

 the mechanic, in order that he may bring the 

 machine to its utmost state oL perfection ; its 

 excellence depending upon the degree of ex- 

 haustion which it is able to produce. 



When the degree of exhaustion has been 

 accomplished as far as it is capable ; the pres- 

 sure from without which the vessel sustains, 

 ma\ r be considered a measure of the expansible 

 force of the atmosphere, with relation to the 

 unresisting medium within. Under these cir- 

 cumstances, the Magdeburgh hemispheres of 

 three inches and a half diameter, require a 

 weight suspended from them of 140 pounds, 

 before the one can be separated from the 

 other; and Otto Guericke, in prosecuting this 

 experiment on a large scale, it is said, made 

 them to such an extent, and exhausted the 

 air within them so completely, that it required 

 the power of twenty horses, before the one 

 hemisphere could be separated from the other. 



A thin glass bottle, exhausted of air, will 

 burst, in consequence of the pressure of the 

 atmosphere, as readily as it will do, when 

 full of air, arid placed under an exhausted re- 

 ceiver, the only difference will be, that in 

 one case it will burst from without, in the 



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