Sulliwan on the Revolving Engine. 107 
some part of the force, was undoubtedly lost at every stroke, 
in giving motion to the balance wheel necessary to equalize 
the movement. Whether the loss of force by a crank is 
actual or theoretical, may be a question. Itis not one how-- 
ever which applies.to this engine so much as to others, be- 
cause it is moved by very elastic steam always operating in 
one or both of the two cylinders which compose this en- 
gine. 
Your correspondent deems this unimportant to the ques- 
tion he raises,—which I may answer more satisfactorily to 
your readers, by a quotation in point, from Dr. Young’s Lec- 
tures. He is speaking of the use of the crank before men- 
tioned, as an improvement in the Steam Engine. 
“If the rotary motion of the Crank be equable, the pro- 
gressive motion of the rod will be gradually accelerated and 
retarded, and for a considerable space of the revolution the 
force exerted will be nearly unitorm ; but if we attempt to 
communicate at once to the rod its whole velocity in each 
direction, as has sometimes been done, the motion would 
become extremely irregular, and the machinery would be 
destroyed by the strain. 
“¢ On the other hand it must be observed, that force ap- 
plied to the machinery, may in general be divided into two 
forces; the one employed in opposing the force, so as to 
produce an equilibrium only, and the other in generating. 
momentum. a 
‘With respect to the first portion, a single crank has the 
inconvenience of changing continually the mechanical ad- 
vantage of the machine; with respect to the second, its 
motion in the second quarter of its revolution is accelerated, 
instead of being retarded by the inertia, which this portion 
of the force is intended to overcome ; and from the combi- 
nation of these causes, the motion must necessarily be ren- 
dered very irregular. aa 
“¢ (>-This may however be completely removed by em- 
ploying always cranks in pairs, one of them being fixed so 
as to make aright angle with the other.” 
Here Dr. Young does not seem to think this supposable 
decision of force “ lost to all useful purposes,” but incident to 
the nature of machinery—or remediable on the same _prin- 
ciple by which steam, as a power, is applied by the double 
revolving engine. Whatever deduction is to be made then 
