76 Mr Sang on some prevailing misconceptions 



In the two arrangements, however, which I have contrasted, 

 the motion will be divided among the parts in very different 

 proportions. The manner of that distribution vitally affects 

 the economy of the machine. The entire quantity of motion 

 is not and cannot be concentrated in the fly-wheel ; the piston 

 and all the other moving parts must have their share of it. In 

 the engine with the double rack, the piston and its appendages 

 possess, from the beginning of a half stroke until the end of it, 

 their full velocity. During the half-stroke, therefore, none 

 of ^the force of the steam is expended in generating the mo- 

 tion of the piston; but just when the piston has reached its 

 extreme position, its whole velocity must be extinguished and 

 generated in the opposite direction, and the consequence is, 

 that the extreme tooth of the rack will receive a blow as if 

 from a hammer as heavy as the piston and all its appendages, 

 and moving with twice its velocity ; and although no loss of 

 force would arise from this action, its continued repetition 

 would tear the machine to pieces. In the case of the balanced 

 crank-engine, on the other hand, during the first part of a half- 

 stroke, the velocity of the piston and beam is gradually in- 

 creasing; at the middle their velocity is the greatest; and 

 during the last part it gradually decreases. In the first quar- 

 ter revolution of the crank, the force of the steam is thus partly 

 expended in producing the motions of the piston, beam, and 

 connecting-rod ; but in the second quadrant, when these mo- 

 tions are being retarded, the force necessary to accomplish this 

 retardation communicates an equal accession of motion to the 

 fly-wheel. Still, then, is the whole force of the steam expended 

 in overcoming the friction, and in producing the particular ef- 

 fect which may be wanted ; still is there no force lost on ac- 

 count of the obliquity of the actions, or of the reciprocation of 

 the movements ; and still is the diminution of the friction the 

 only source whence increased effectiveness can be obtained. 



I do not expect that what I have just said will be sufficient 

 to eradicate misconceptions so prolific of crude and abortive 

 schemes. The failure itself of the contrivance is often inade- 

 quate to convince its inventor of the fallacy of his ideas. For- 

 tified in his ignorance by the fancy that theory is a different 

 thing from practice, there is small chance of his yielding to 



