Polytechnic Association Proceedinos. 591 



therefore, remain at that fixed amount under all variations of load 

 or i^ressure of steam; for any increase or diminution of speed will 

 cause either the balls or weight to preponderate, and the point of 

 cut-oflf to be changed until the speed is again brought to the 

 standard where the two forces are in equilibrium. 



Any desired speed can be obtained by altering the weight, W, 

 and the action of the governor will be as perfect in one case as in 

 any other. A spiral on the rod, o, serves to advance or retire the 

 crank, m, relatively to the main crank, so as to cause the cut-ofl' to 

 occur eai'lier or later in the stroke, as the balls diverge or con- 

 verge; and the amount of this adjustment is such that the cut-off 

 may be varied from nothing to seven-eighths strokes. 



The following indicator diagrams were taken from engines of this 

 construction, and elucidate its action and economy. 



The importance of the indicator as a means of studying the 

 action of any given engine, and of comparing the relative values 

 of different constructions and proportions, though known from the 

 time of Watt, has but recently been fully appreciated by engineers; 

 and, in fact, not until within a very few years has there been an 

 instrument manufactured capable of being used with any satisfac- 

 tory degree of accuracy upon the quick moving engmes now 

 employed for most stationary purposes. To its employment the 

 world is indebted for its most satisfactoiy practical knowledge of 

 the action of steam, and the best means of obtaining the hio:hest 

 economical results. 



But in order to compare one engine with another, they should 

 be in precisely similar circumstances. As, however, this rarely 

 occurs, it is necessary to have some standard by which all engines 

 may be compared, and their relative performances determined. 

 The best means for doing this, at present within our reach, is to 

 compare each engine with a theoretically perfect engine of the 

 same size, under the same circumstances; and the engine which 

 more nearly approaches the theoretical is evidently the best 



The expansion of steam follows certain laws, and the quantitj^ 

 of steam being known, as well as the space which it occupies, it is 

 possible to tell the correct pressure for each variation in the space 

 occupied. A curve can thus be calculated which will give a 

 diagram of the theoretical action of a given amount of steam in a 

 given size of cylinder, and the diagrams taken from any engine 

 may thus be compared with a theoretical diagram for the same quan- 

 tity of steam used in the same sized cylinder, the ratio existing 



