SECT, vn.] STEAM ENGINES. 255 



the joints and then screw them together. A considerable degree of action and 

 reaction takes place among the ingredients, and between them and the iron sur- 

 faces, which causes the whole to unite as one mass ; the surfaces of the flanches 

 become joined by a species of pyrites, all the parts of which cohere strongly 

 together. Mr. Watt found that the cement is improved by adding some fine 

 sand from the grindstone trough. 



531. For some purposes it is more convenient to join the parts with white lead 

 paint, mixed with a portion of red lead to a proper consistence, and applied on 

 each side of a piece of thick canvas, flannel, or plaited hemp, previously shaped to 

 fit the parts, and then interposed between them before they are screwed together : 

 it makes a close and durable joint, and is generally used for those joints which 

 have occasionally to be opened, and for those which must be separated repeatedly 

 before a proper adjustment is obtained ; and when this is the case, the white lead 

 ought to be predominant in the mixture, as it dries much slower than the red. 



532. There is another cement often used by coppersmiths, to lay over the 

 rivets and edges of the sheets of copper in large boilers, to serve as an additional 

 security to the joinings, and to secure cocks, &c. from leaking. It is formed by 

 mixing pounded quick lime with serum of blood or white of egg into a paste, and 

 must be applied as soon as it is made, for it speedily gets so hard as to be unfit for 

 use. The properties of this cement have been long known to chemists, and it may 

 be found useful for many purposes to which it has never yet been applied : it is 

 cheap and very durable. 



533. Steam-tight joints may also be formed by fitting the parts very accu- 

 rately to a conical aperture, and screwing them close together with bolts of a less 

 expansible metal ; and the same method may be followed where the pressure of the 

 steam tends to close the joint. 



When two flat surfaces are to be joined, they may be made to fit together very 

 accurately, and a single ring of fine copper wire inserted in between them before 

 screwing them close. The pressure of the screws partially flattens the wire, and 

 makes it fit so accurately, as to prevent the escape of even very high pressure 

 steam. 



