n: Fig. 305.]

The main valve, whose movement alternately opens the ports for the
admission of steam to, and the escape of steam from, the main cylinder,
is divided into two parts, one of which, C, slides upon a seat on the
main cylinder, and at the same time affords a seat for the other part,
D, which slides upon the upper face of C. As shown in the engravings, D
is at the left-hand end of its stroke, and C at the opposite or
right-hand end of its stroke. Steam from the steam-chest, J, is
therefore entering the right-hand end of the main cylinder through the
ports E and H, and the exhaust is escaping through the ports H 1, E 1, K
and M, which causes the main piston A to move from right to left. When
this _piston_ has nearly reached the left-hand end of its cylinder, the
tappet arm, T, attached to the piston-rod, comes in contact with, and
moves the valve rod collar O 1 and valve rod P, and thus causes C,
together with the supplemental valves R and S S 1, which form, with C,
_one casting_, to be moved from right to left. This movement causes
steam to be admitted to the left-hand end of the supplemental cylinder,
whereby its piston B will be forced towards the right, carrying D to the
opposite or right-hand end of its stroke; for the movement of S closes N
(the steam-port leading to the right-hand end), and the movement of S 1
opens N 1 (the steam-port leading to the opposite or left-hand end), at
the same time the movement of V opens the right-hand end of this
cylinder to the exhaust, through the exhaust ports X and Z. The parts C
and D now have positions opposite to those shown in the engravings, and
steam is therefore entering the main cylinder through the ports E 1 and
H 1, and escaping through the ports H, E, K and M, which causes the main
piston A to move in the opposite direction, or from left to right, and
operations similar to those already described will follow, when the
piston approaches the right-hand end of its cylinder. By this simple
arrangement the pump is rendered positive in its action; that is, it
will instantly start and continue working the moment steam is admitted
to the steam-chest, while at the same time the piston is enabled to move
as slowly as the nature of the duty may require. It will be noted that
in Figure 305, the ports of C are shown through D, whose location is
marked by dark shading. This obviously is not correct, because D being
above C should be shaded lighter than C, and again the por