110 Sullivan on the Revolving Engine. 
_ chines, and that oleaginous substances interposed lessen the 
friction essentially. All these things being equal, the law 
above stated applies, nor is there any particular portion in. 
which it is peculiarly great. The most disadvantageous 
moment is, when the piston has reached the end of the 
stroke, and starts in the other direction: but it starts gently, 
and when in the other cylinder, the power to help it, is 
ereatest—the substances in contact are a polished surface of 
iron and oiled hempen packing; there cannot be much at- 
trition between these; every other part of the engine is 
lubricated, and moves always the same way. 
The Rotary Valves seem the most subject to this 
objection at first view. Here are two surfaces moving up- 
on each other, one of iron the other of brass, both perfect- 
ly polished, and occasionally oiled; they are kept to- 
gether by springs, elastic enough to preserve the contact ; 
for the tendency is to separate; there is little or no weight or 
pressure to cause friction between them and it cannot possi- 
bly be great. 
You have witnessed the operation of a large engine of 
this kind ; and must recollect with how little force of steam 
it moved. ‘ 
I believe nothing in mechanics is more difficult to esti- 
mate than friction, what is ever incident to machinery ; but 
it should not be confounded with the obstacles to be over- 
come in the imperfections of work as well as of plan. 
Its unavoidable existence however, shews the expediency 
of reducing the steam engine to as light a construction as 
possible, as well as to get rid of those massive members 
which waste the original power on their own movements. 
Experience is our surest guide in mechanics, and perhaps 
the late Mr. Evans’ heirs may have nothing to fear from 
what Mr. Doolittle calls the rivality of Morey’s invention. 
But I can assure him of the fact, that the same boilers which 
once carried a twenty horse engine of Evans’ in my first 
steam tow-boat, that could not be made to tow more than 
one boat, now applied to a small single revolving engine, can 
tow four boats faster than that one was carried and con- 
sume not half so much fuel. 
Dr. Young justly observes that the beauty of a contrivance, 
and the skill of the contriver depend, principally, on the sim- 
plicity of the means, and the safety, and durability of the ma- 
