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XXX. On Friction in Machinery ; and on Wheel-Carriages. 
By Mr. Hexry Meike. 
To Mr. Tilloch. 
London, Feb. 22, 1819. 
Sir, -- Turre are some circumstances affecting friction and 
several mechanical movements, that seem as yet to have been 
rather overlooked : and as the diminution of friction and of every 
unnecessary waste of moving power is undoubtedly of great im- 
portance, I shall venture to give my opinion in some cases, 
hoping that it may excite the notice of others who consider them- 
selves better qualified for the task. 
It is now pretty generally understood, that friction remains. 
constant although the velocity varies; and indeed, many experi- 
ments are favourable to this notion—but there are certainly some 
exceptions. Most people, it is presumed, will be ready to ad- 
mit, that where the greater wear is, there will also, ceteris part- 
bus, be the greater friction. But. I have often observed, that 
where one surface passed over another with great velocity and 
under strong pressure, there was always a tendency to generate 
heat, which was also followed by a wearing of the parts consi- 
derably greater than if the velocity had been small. Indeed I 
cannot help regarding heat as having a powerful influence on 
friction. It certainly softens the Wearing parts consisting of 
metal—it tends rapidly to dry up the unctuous substances applied 
to lessen friction—it probably promotes the tremulous or vibra- 
tory motion of the parts of bodies 3 and besides, it alters even 
their size and shape. 
Such being some of the more obvious effects of heat, we need 
not be surprised that theories of friction which do not condescend 
to take notice of such circumstance, should often be at variance 
with observation ; and probably a theory including every ano- 
maly would, from its intricacy, be scarcely intelligible to the 
framer of it himself. 
Another circumstance affecting friction, and often attended 
with an immediate loss of power, is the application of the moving 
power in a direction oblique to that in which the body is moved. 
This, as is well known, takes place on many occasions; often 
needlessly too; and is particularly observable in the case of 
wheels acting on one another with teeth that are much tapered 
towards their extremities. ‘The consequence is, that the mutual 
action of the wheels not being directed in a line perpendicular 
to the line joining their ceitres, they repel each other powerfully, 
which produces an enormous strain and friction at their axes. 
There are various sorts of tapering teeth; but the epicycloidal 
kind invented by De la Hire, as it has been long celebrated, and 
N3 is 
