160 FERGUSON'S LECTURES. 



The cylinder is about 40 inches diameter, bored so 

 smooth, and its leather piston fitting so close, that 

 little or no water can get between the piston and sides 

 of the cylinder. 



Things being thus prepared, the cylinder is placed 

 upright, and the shank of the piston is fixed to one end 

 of the beam, which turns on a center like a common 

 balance. 



The boiler is placed under the cylinder, with a com- 

 munication between them, which can be opened and 

 shut occasionally. 



The boiler is filled about half full of water, and a 

 strong fire is made under it : then, if the communication 

 between the boiler and the cylinder be opened, the 

 cylinder will be filled with hot steam ; which would 

 drive the piston quite out at the top of it. But there 

 is a contrivance by which the piston, when it is near 

 the top of the cylinder, shuts the communication at the 

 top of the boiler within. 



This is no sooner shut, than another is opened, by 

 which a little cold water is thrown upwards in a jet into 

 the cylinder, which, mixing with the hot steam, con- 

 denses it immediately ; by which means a vacuum is 

 made in the cylinder, and the piston is pressed down by 

 the weight of the atmosphere ; and so lifts up the loaded 

 pump-rod at the other end of the beam. 



If the cylinder be 42 inches in diameter, the piston 

 will be pressed down with a force greater than 20,000 

 pounds, and will consequently lift up that weight at the 

 opposite end of the beam : and as the pump-rod with 

 its plunger is fixed to that end, if the bore where the 

 plunger works were 10 inches diameter, the water would 

 be forced up through a pipe of 180 yards perpendicular 

 height. 



But, as the parts of this engine have a good deal ol 

 friction, and must work with a considerable velocity, 

 and there is no such thing as making a perfect vacuum 



