SECT, v.] NONCONDENSING ENGINES. 175 



and at the instant of its change of motion, the steam is fully on it. The com- 

 pression which the steam left in the cylinder receives when the cock is closed, is 

 not only a means of changing the motion without loss of force, but also occupies 

 the space at the end of the stroke, so as to require only a small quantity to refill it 

 with steam. We might arrange the motion so that the cocks would be half 

 turned, and all the passages closed just at the end of the stroke : this, however, 

 would not be so good a method, as when the cock turns with proper quickness 

 there would be DO sensible accumulation of steam to recoil the piston, and the 

 force of that in the boiler would not be fully on, till a part of the stroke was 

 made ; and the waste at the terminations of the strokes would be greater. Hence, 

 to complete the motion of the cock with the termination of the stroke, is the better 

 method. 



364. In the construction, Fig. 1. at every double stroke there is a loss of 

 the force of all the steam contained in the passages between the cock and the 

 cylinder. This defect may be avoided by the use of the slide, Fig. 2 and 3. The 

 motion of the slide should terminate with the stroke in the same manner as with a 

 cock ; and in this construction the recoil of the compressed steam is greater, because 

 it has less space of passage to retreat into. Valves may be placed to give similar 

 advantages, but slides or cocks are in my opinion better adapted to high pressure 

 engines. 



365. The modes of giving motion to the cocks, slides, or valves are various : 

 they depend chiefly on the nature of the action the engine is intended for. The 

 same methods are applicable to engines of all species, and therefore are described 

 together. (See Sect, vn.) The power is usually regulated by a throttle valve, but 

 more perfectly by means of Field's valve. (See Sect, vm.) 



366. The Proportion of Parts. The length of the stroke of the steam piston 

 should not, if possible, be less than twice its diameter, (art. 327.) The velocity in 

 feet per minute should be 103 times the square root of the length of the stroke in 

 feet, (art. 337.) And, as 4800 is to the velocity thus found, so is the area of the 

 cylinder to the area of the steam passages, (art. 154.) The strength, proportions, 

 and construction of the parts are given in Sect. vn. and the methods of equalization 

 and regulation in Sect. vm. 



367. The Power of a Noncondensing Engine may be calculated with con- 

 siderable accuracy, from knowing the excess of the force of the steam in the 

 boiler, above the atmospheric pressure, as shown by the steam gauge, the diameter 

 of the cylinder, and the velocity of the piston. The effective pressure on the piston 

 is less than the force in the boiler when that force is represented by unity, 



