226 OF THE PARTS OF [SECT. vn. 



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 in 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. * 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, 4 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. 

 * From the nature of the motions of the valves, slides and cocks being incompatible with the 



