440 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



We will shortly explain the mode of action of the steam in this 

 engine. (Plate XV.) 



The boiler furnished steam at a pressure a little above that of the 

 atmosphere. At the moment it was set to work, and the piston was 

 at the upper end of the cylinder, the steam filled the latter and drove 

 the air out by an orifice called the snifting valve. Then the tap 

 of a pipe is opened, and the cold water injected into the cylinder 

 condenses the steam, and when the tap is closed, the outside pressure 

 of the atmosphere works on the piston and drives it to the bottom 

 of the cylinder. 



At this moment a slide-valve opens the communication of the 

 cylinder with the boiler, so that the steam below and the atmo- 

 spheric pressure above the piston are balanced. The piston would 

 remain then in this situation but for a counterpoise attached to the 

 beam of the engine, which forces it up to the top of the cylinder : 

 another condensation now makes it descend, and so on, and the two- 

 and-fro motion is produced. 



We see now the reason of the name, atmospheric engine, given to 

 it, for it is the pressure of the external air that is the real motive 

 force. The steam comes into use only to balance it daring the ascent 

 of the piston. During the descent the condensation of the steam 

 produces a vacuum, and it is the pressure of the air again that makes 

 the piston descend. 



The atmospheric engines were chiefly employed as pumping en- 

 gines for mines. They were also employed for distributing the water 

 in London. Notwithstanding the immense improvements introduced 

 during a centu^ arid a half, into engines which work by steam, 

 Newcomen's engines appear to have been used for a long time in 

 places where coal was cheap. 



The steam-engine, with a few unimportant improvements of detail, 

 remained in the state into which Newcomen, Savery, and Cawley 

 brought it, until the year 1769. Sixty-four years thus passed away 

 without fruit, as we may say, until the genius of Watt, seconded by 

 the rapid progress of physical science in that half century, made of 

 it the powerful motor, the incomparable engine which we have 

 described in choosing the beam-engine which still bears the name of 

 Watt for our type. 



