NOTES ON STATIONARY ENGINES. 



39 



important total, and in which, for ono reason or another, 

 there is an incrodiblo amount ol coal frittered away to 

 small purpose. A certain amount of expansicm is good in 

 practically every case which has any pretence to good 

 practice; but whether or not this siiould be variable, and 

 if variable, whether or not it sliould bo actuated by tlio 

 "■ovcrnor, arc questions to be decided in connection with 

 each case separately. If the work is very uniform, the ex- 

 pansion may be permantly adjusted. If the work varies at 

 certain considerable and well-delined intervals, tlic expansion 

 may bo adjusted by hand, and usually it is to be preferred 

 that this should be done outside the steam cliest, while the 

 engine is running in its usual manner; in eacli of these 

 cases a throttle valve of some sort, actuated by the governor, 

 is a necessity. If however the work is liable to vary at any 

 moment (and more especially if the variation is one of 

 increase upon what may be spoken of as the ruling lioi'se- 

 power), then a cut-off motion, actuated by the governor 

 directly, and without the intervention of the driver, becomes 

 an essential condition in good practice. If sadden reduc- 

 tions in the required power occur for only short periods, 

 then the governor and throttle valve are quite sufficient 

 from any point of view. In speaking generally on the ques- 

 tion of economy in coal, wliile a,s befoi'o stated, it is of 

 immense importance, yet a practical audience need not be 

 reminded that, every case must stand upon its own merits, 

 and due allowance must be made for many elements which 

 cannot just now be discussed. 



In all engines there is one element which furnishes a 

 good index as to the mechanical design as a whole ; this is 

 the speed at which the engine can run with perfect safety. 

 It does not follow that the economical properties of the 

 enti-ine would keep pace with the mechanical ones; but there 



