130 FERGUSON'S LECTURES. 



LECT. about as far as e: this will condense the rarefied air in 

 ^^s the pipe between e and C to the same state it was in 

 before ; and then, as its spring within the pipe is equal 

 to the force or pressure of the outward air, the water 

 will rise no higher by the first stroke ; and the valve b, 

 which was raised a little by the dilatation of the air 

 in the pipe, will fall and stop the hole in the box H ; 

 and the surface of the water will stand at e. Then, 

 depress the piston or bucket from C to B, and as the 

 air in the part B cannot get back again through the 

 valve b, it will (as the bucket descends) raise the valve 

 a, and so make its way through the upper part of the 

 barrel d into the open air. But, upon raising the bucket 

 G a second time, the air between it and the water in 

 the lower pipe at a will be again left at liberty to fill 

 a larger space ; and so its spring being again weakened, 

 the pressure of the outward air on the water in the ves- 

 sel K will force more water up into the lower pipe from 

 e to f; and when the bucket is at its greatest height 

 C, the lower valve b will fall, and stop the hole in the 

 box H as before. At the next stroke of the bucket or 

 piston, the water will rise through the box H towafds 

 B, and then the valve b, which was raised by it, will 

 fall when the bucket G is at its greatest height. Upon 

 depressing the bucket again, the water cannot be pushed 

 back through the valve b, which keeps close upon the 

 hole whilst the piston descends. And upon raising the 

 piston again, the outward pressure of the air will force 

 the water up through H, where it will raise the valve, 

 and follow the bucket to C. Upon the next depression 

 of the bucket G, it will go down into the water in the 

 barrel B ; and, as the water cannot be driven back 

 through the now closed valve b, it will raise the valve a 

 as the bucket descends, and will be lifted up by the 

 bucket when it is next raised. And now, the whole 

 space below the bucket being full, the water above it 

 cannot sink when it is next depressed ; but, upon ils 



