428 STEPHENSON'S PATENT 



steam ports, and passing round the cylinder (as in fig. 4, Plate XCII.) until it clears the 

 steam chest and comes to the outside again opposite to the other cylinder ; having 

 gradually assumed a circular form three inches and three quarters in diameter, as 

 shown by the dotted lines in Plate XC. The cross area of this passage is made of 

 such dimensions that although it alters in shape, it is in all parts equal to the area at 

 the end, that of a circle three inches and three quarters diameter or 1 1 inches area. 

 On the outside of this opening in each cylinder are fixed the two ends of the branch- 

 ing copper pipe p, of the same diameter, termed a breeches piece, and having the blast 

 pipe P bolted on to it. The piece p is fixed on to the cylinders by screws put into holes 

 tapped to receive them in the solid metal, as there is no place for nuts. The steam chest 

 is also fixed on to the cylinder at the hinder end in the same manner. The flanches are 

 made rather thicker to allow for being weakened by the holes in them, and those of the 

 copper pipes are three eighths of an inch thick, and made of brass soldered on to the 

 pipe ; the pipes are made of one eighth of an inch sheet copper lapped and soldered at 

 the edges. A layer of canvas or of gasket, like that used for packing, covered with red 

 lead and oil, is placed between the flanches of the pipes and under the flanch of the 

 steam chest, and in all other similar joints, in order to make them steam-tight. 



The face of the ports that the slide valve moves upon is made quite even and 

 true by a planing machine, and the valve Is ground upon the face to make it fit 

 steam-tight ; the face is sunk down round the ports beyond that part over which 

 the slide moves, in order to diminish the surface to be planed and ground. The 

 spaces between each of the steam ports and the waste port, called the bars, are one 

 inch wide. A section of the slide valve, quarter size, is shown in fig. 14, where 

 A and B are the back and front steam ports, 



* 



C is the waste port, and DD the bars. The 

 width of the slide inside is the same as the 

 ports, as shown in fig. 4 (Plate XCII.); the 

 length E, (fig. 14,) is one-eighth of an inch 

 less than the distance between the ports, and 

 the flanches F and Cf are one-eighth of an inch wider than the ports ; so that when 

 the slide is in a central position, as shown by the dotted lines, the flanches F and 

 G lie exactly over their respective steam ports, and overlap them one-sixteenth of 

 an inch on each side. This overlap is for the purpose of ensuring that one port is 

 completely closed before the other is opened, in order that they may never com- 

 municate with each other under the slide during its motion, nor the steam be 

 allowed to enter both ports at the same time. 



The slide valve is moved backwards and forwards a distance of 3 inches, or 1-^ 

 inch on each side of the central position, and is carried beyond the inner edge of 

 each steam port alternately, a distance of nearly half of an inch, as shown in 



