Construction of Stecijn- Engines, i 35 



its volume, without any limitation but what is imposed by 

 the frangible nature of every material of which boilers and 

 the other parts of steam-engines have been or can be made j 

 and prudence dictates that the expansive force should never 

 be carried to the utmost the materials can bear, but rather 

 be kept considerably within that limit. 



Having thus briefly explained the nature of Mr". Woolf *3 

 discovery, we shall proceed to give a description of his im-» 

 provements grounded thereon 5 and for which he has ob- 

 tained his majesty's royal letters patent. Mr. Woolf in 

 his specification states, that in describing his invention he 

 has found it necessary to mention the entire steam-engine 

 and its parts, to which, as an invention well known, he nei- 

 ther can nor does assert any exclusive claim : he observes, 

 however, that from the nature of his aforesaid discovery, 

 and its application, there can be no difficulty in distinguish- 

 ing his said improvements from the improved engine (of 

 Mr. Watt) as to its other common and well known parts, 

 and then gives the following account of an engine embracing 

 his new improvements. 



" If the engine be constructed originally with the inten- 

 tion of adopting my said improvement, it ought to have two 

 steam vessels of different dimensions, according to the tem- 

 perature or the expansive force determined to be com- 

 municated to the steam made use of in working the engine; 

 for the smaller steam vessel or cylinder must be a measure 

 for the larger. For example, if steam of forty pounds the 

 square inch is fixed on, then the smaller steam vessel 

 should be at least one fortieth part the contents of the 

 larger one; each steam vessel should be furnished with a 

 piston, and the smaller cylinder should have a commu- 

 nication both at its top and bottom (top and bottom being 

 here employed merely as relative terms, for the cylinders 

 may be worked in a horizontal or any other required posi- 

 tion, as well as vertical) : the small cylinder, I say, should 

 have a communication both at its top and bottom with the 

 boiler which supplies the steam, which communications, 

 by means of cocks or valves of any construction adapted to 

 the use, are to be alternately opened and shut during the 

 working of the engine. The top of the small cylinder 

 should have a communication with the bottom of the larger 

 cylinder, and the bottom of the smaller one with the top of 

 the larger, with proper means to open and shut these al- 

 ternately by cocks, valves, or any other well known con- 

 trivance. And both the top and bottom of the larger cy- 

 linder or steam vessel should, while the engine is at work, 



L 2 com- 



