218 FERGUSON'S LECTURES. 



LECT. i on the pump- plate, and cover both it and the hole with 

 ^^, the receiver M ; and turn the wire so by the top P, 

 that its hook may take hold of the little receiver by a 

 ring at its top, allowing that receiver to stand with its 

 own weight on the plate. Then, upon working the 

 pump, the air will come out of both receivers ; but the 

 large one M will be forcibly held down to the pump by 

 the pressure of the external air ; whilst the small one O, 

 having no air to press upon it, will continue loose, and 

 may be drawn up and let down at pleasure, by the wire 

 P P. But, upon letting it quite down to the plate, and 

 admitting the air into the receiver M, by the cock k, 

 the air will press so strongly upon the small receiver O, 

 as to fix it down to the plate ; and, at the same time, 

 by counterbalancing the outward pressure on the large 

 receiver M, it will become loose. This experiment 

 evidently shews, that the receivers are held down by 

 pressure, and not by suction, for the internal re- 

 ceiver continued loose whilst the operator was pump- 

 ing, and the external one was held down ; but the 

 former became fast immediately by letting in the air 

 upon it. 



12. Screw the end A of the brass pipe ABF 

 into the hole of the pump-plate, and turn the 

 cock e until the pipe be open ; then put a wet 

 leather upon the plate cd, which is fixed on the 

 pipe, and cover it with the tall receiver G H, 

 which is close at top : then exhaust the air out 

 of the receiver, and turn the cock e to keep it 

 out ; which done, unscrew the pipe from the 

 pump, and set its end A into a bason of water, 

 and turn the cock e to open the pipe ; on which, 

 as there is no air in the receiver, the pressure of the 

 atmosphere on the water in the bason will drive the 

 water forcibly through the pipe, and make it play up in 

 a jet to the top of the receiver. 



