594 



A (^ENTUKY S PROGRESS oK THE STEAM ENGINE. 



instances, or in tlie effort to accomplish u special tour de force, speeds 

 of considerably greater magnitude have been for a time maintained. 

 The figures here given, however, represent settled practice in the 

 })usiness of certain builders, or in certain classes of constructions. 

 Thus torpedo-boat builders adopt the radical practice, while con- 

 structors of small and short-route craft keep speeds down to what 

 they regard as economical and permanently safe rates. 



Speeds of engine may be measured either by speeds of piston, as 

 above, or by speeds of rotation, and it is <)])vious that the latter and 

 the U'ligtli of stroke of engine piston together determine the speed of 

 the piston. With some engines, iis those Avith detachable valve gear, 

 the speed of i-otation is limited to that at which disengagement of the 

 valve may l)e positively assured; and this with, for example, the 

 Corliss engine is jit i)resent not far from 100 revolutions per 

 minute, altliough instances of nmch higluM- speeds are known, and, in 



R. p. M 

 140 



JIG. 2. — SPEEDS OF REVOLUTION (MARINE) 180O-I9OO 



one case iit least, a speed of lt><> j-evolutions ])er miniiie was main- 

 tained for years together. 



The method of progress of rotative speeds is shown in lig. i!, and, 

 naturally, follows closely the direction observed in the preceding 

 case. Here the lowest curve in the diagi'am is that for heavy engines 

 and a very conservative practice; the intermediate line gives the 

 speeds for common good practice at the respective dates; and the 

 higher curve shows the limit of what is considered safe, and a radical 

 practice, where, as in practically all marine engines, no limit. to speed 

 of rotation is set, as in so many stationary engines, by the character 

 of the system of steam distril)ution. Each curve has been given its 

 appropriate scale, and the latter is suitably designated on the margin 

 of the diagram. 



Torpedo-boat practice illustrates the highest case, and the work of 

 the average good marine-engine builder the middle case. The lowest 



