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 11 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 JB 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 F, (fig. 14,) is one-eighth of an inch 
less than the distance between the ports, and 
the flanches A 7 and G 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 
Fig. 14.