138 FERGUSON'S LECTURES. 



LECT. moves, c is beginning to move downward, which will by 

 v^-^'-x^ its piston continue the propelling force upon the water : 

 and when c is come down to the position of b, a will be 

 in the position of c. 



The more perpendicularly the piston rods move up 

 and down in the pumps, the freer and better will their 

 strokes be : but a little deviation from the perpendicu- 

 lar will not be material. Therefore, when the pump- 

 rods D, E. and F go down into a deep well, they may 

 be moved directly by the cranks, as is done in a very 

 good horse-engine of this sort at the late Sir James 

 Creed's, at Greenwich, which forces up water about 64 feet 

 from a well under ground, to a reservoir on the top of 

 his house. But when the cranks are only at a small 

 height above the pumps, the pistons must be moved by 

 vibrating levers, as in the above engine at Blenheim; and 

 the longer the levers are, the nearer will the strokes be 

 to a perpendicular. 



v Cal f"th ket us suppose that in such an engine as Sir James 

 quantity of Creed's, the great wheel is 12 feet diameter, the trundle 

 ^ anc ^ * ne radius or length of each crank 9 inches, 



raised by working a piston in its pump. Let there be three 

 gine. n " pumps in all, and the bore of each pump be four inches 

 diameter. Then, if the great wheel has three times as 

 many cogs as the trundle has staves, the trundle and 

 cranks will go three times round for each revolution of 

 the horses and wheel, and the three cranks will make 

 nine strokes of the pumps in that time, each stroke be- 

 ing 18 inches (or double the length of the crank) in a 

 four-inch bore. Let the diameter of the horse-walk be 

 18 feet, and the perpendicular height to which the water 

 is raised above the surface of the well be 64 feet. 



If the horses go at the rate of two miles an hour 

 (which is very moderate walking) they will turn the 

 great wheel 187 times round in an hour. 



In each turn of the wheel the pistons make 9 strokes 

 in the pumps, which amount to 1683 in an hour. 



