226 
OF THE PARTS OF 
[sect. vit. 
motion by the crank shaft of a double engine, causing the shaft Z to revolve by 
means of the wheels 7, 8; then if on Z two eccentric wheels 4, 4, be fixed under 
two rods which slide vertically in guides, (see z, z, Fig. 2.) and provided with 
friction rollers 3, 3, the revolution of the shaft Z will alternately raise and depress 
the rods, which by the arms 9, 10, 11, 12, raise and depress the valves by their 
stems. The lever or handle 13 is used to open or close the valves by hand in 
setting to work, &c. It will be remarked that this construction does not admit of 
cutting off the steam without also shutting the condenser. 
481. As far as regards opening and closing the passages more rapidly, a 
good improvement has been made on the eccentric motion, by altering the form 
of the portion fixed on the shaft so as to act more nearly as a tooth or cam, and 
by placing adjustible spanners on the eccentric rod; but why not at once form it 
as a tooth, or a series of teeth, in the best manner to produce the movements 
required ? Suppose the object be to cut off the steam at some part of the stroke 
by a slide or cock, then there must be two motions, the one double the length of 
the other. Let A B, Fig. 1 and 2. Plate ix. be the first, and B G the second, 
and from the centre D describe circles through these points; set off A E for the 
time to be expended iu closing the passage to the condenser, and A F for the 
time of opening the passage for the steam; then, that the action may be easy, the 
curve H G should be drawn, so that each of its parts may be a parabola, the one 
with its vertex at H, that of the other at G. 1 To produce the second motion, 
another wheel should be placed on the same axis, behind the first one, with the 
curve I K. If these curves have corresponding ones, and act on connected rollers, 
the motion will be certain, and the range confined, and the motions of the engine 
may be made to reverse in the case of boat or carriage engines; for the posi 
tion of the slide being changed by hand, the pressure of the steam will impel the 
crank shaft in the contrary direction, and the toothed wheel will move the slide or 
cock in the proper directions. 
In order that the steam may be cut off at any period of the stroke, according to 
the resistance or the work on the engine, the wheel with the curve I K may be 
made to slide round on its axis, and the curve I K may be placed so that the 
period of cutting off the steam may be varied from N to O. 
482. If valves are to be opened, the weight of the valves and rods is generally 
sufficient to close them ; hence the rods do not require to be connected so as both 
to push and draw, but on the other hand a separate rod for each valve is required 
for a valve engine to work expansively, 2 and the toothed wheels or cams to move 
1 The best curve for generating motion from rest is the common parabola. See Emerson’s 
Mechanics, 4to ed. prop. 91. case 3. 
2 From the nature of the motions of the valves, slides and cocks being incompatible with the