AND EXTERNAL RESISTANCE. 303 



column of air from the bladder back into the 

 receiver, which, consequently^ rose the mer- 

 cury as high as before. The observation was a 

 very just one, but it did not apply to the ob* 

 jection which he wanted to make ; for although 

 it was very true, that the same given quantity 

 of air entered into the receiver, as had been 

 pumped out of it, a separation existed, total 

 and complete, between the air in the receiver, 

 and the column of air external to it, if it were 

 therefore, the weight of the air, which forced 

 up the mercury in the tube, it could only be 

 the actual weight of the air alone which the 

 receiver contained, a weight of 20 grains, ele- 

 vating to the height of 29 or 30 inches, a column 

 of mercury of the weight of 30 Ibs. ! ! !" A third 

 sagaciously remarked, " I do not pretend to 

 say, that it is the absolute weight of the air, 

 which causes the elevation of the mercury, but 

 it is owing to the regular and progressive in- 

 crease of weight, which the whole column ac- 

 quires, that the elastic, or expansible, foree of 

 air is increased at the bottom!!!'' And this 

 knock me down argument, was supposed to be 

 proved, by an illustration which was made by 

 a fourth ; " Take (he said) a spiral spring of 

 *ny height, place it on the mercury in th ba- 

 ion apply a given force upon it, to overcome 

 the weight of the mercury, and the mercury 

 jnay by that means be raised to any given 



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