AND EXTERNAL RESISTANCE. 299 



bottom of the tube, at the same level with that 

 in the basin, immediately ascended in the ex- 

 hausted and unresisting medium which the 

 Torricellian tube contained, to the elevation 

 of 29|- inches, being of the same level as one 

 exposed to the open air. It must, there- 

 fore, follow, that if the elevation of the mercury 

 under the receiver was the consequence of 

 weight, it must have been accomplished by 

 the weight of the air which had passed from 

 the bladder into the receiver, and which before 

 I had ascertained to have amounted to 30 

 grains only. With a view of seeing whether the 

 same weight of ponderable matter, lead for in- 

 stance, would have the power of elevating themer- 

 cury from the basin into the tube, I placed 100 

 grains of lead on the mercury in the basin; on 

 exhausting the air out of the receiver, so far 

 from that weight of lead being adequate to pre- 

 serve the mercury in the tube to the same ele- 

 vation, as the pressure produced by one tenth 

 part of the same weight of air ; I found, on 

 exhausting' the air out of the receiver, that the 

 mercury in the tube sunk to the same level as 

 the mercury in the basin. I endeavoured to 

 ascertain what quantity of weight was adequate 

 to raise the mercury in the tube exposed to the 

 open air above the standard it was then of 29J 

 inches. I, therefore, placed a piece of paste- 

 board over the mercurv in the basin, as well 



