PNEUMATICS. 265 



escape into another vessel, where a jet of cold wa- 

 ter is introduced, the condensation is much more 

 complete, and the heat of the cylinder being pre- 

 served, the steam possesses its full elasticity. 



This improvement was made by Mr WATT, and com- 

 pletely changed the character of the steam-engine. 

 In the old engines, the power was reduced nearly to 

 half its real value, so that the moving force, instead 

 of amounting to 14 Ib. on every square inch of the 

 area of the piston, was reduced to little more than 

 seven. In Mr WATT'S engines, the moving force is 

 not less than 12 Ib. on the square inch. 



358. A farther improvement has been made on 

 this engine, by injecting the steam into the cylin- 

 der, alternately above and below the piston, so 

 that the whole motion is produced by the elastici- 

 ty of steam, and has no dependence on the weight 

 of the atmosphere. 



This improvement is also due to Mr WATT, and could 

 not have been made without the previous contri- 

 vance of condensing the steam in a separate vessel. 

 It is particularly accommodated to the production 

 of a rotatory motion by means of a steam en- 

 gine. 



In the double-stroke engine, the piston-rods require 

 to be forced down as well as to be drawn up, in the 

 same vertical line. The method by which Mr 

 WATT has accomplished this depends on a geometri- 

 cal theorem. 



359. If 



