[ H° ] 
A knowledge in rational mechanics, which com- 
prehends the whole theory of motion, upon which 
natural philofophy fo greatly depends, is chiefly con- 
fined to the learned; and the proper conftrudtion of 
engines and machines, which is the principal objedt 
of pradlical mechanics, altho’ fo very neceffary to carry 
on the feveral branches of hufbandry, manufadture, 
and commerce, upon which, the riches and power 
of a nation depend in a great meafure, is feldom at- 
tended to, but by the meet handicraftfman ; who is 
little acquainted with the principles he works on, and 
from whom no great improvements can well be ex- 
pedited ; yet it has happened fometimes, that excellent 
contrivances have been invented, for railing heavy 
weights and overcoming their refinances, by perfons 
who never took the trouble of examining into the 
caufe of gravity. . 
As this branch is certainly molt ufeful to mankind; 
and a knowledge in it, generally deemed one of the 
marks by which a civilized nation is diftinguifhcd from 
barbarians, one would imagine, it fhould have in- 
duced a greater attention to improvements in it, than 
has been generally found : But it often happens that 
mechanical powers, feemingly demonftrable in theory, 
are found very deficient in operation, from unexpedfed 
obftrudions ; which, with the expence and trouble 
that generally attend the reducing fpeculations of this 
nature into pradiice, have probably been the greateft 
obftacles to improvements in it. 
One of the greateft obftrudtions to the mechanical 
powers of engines proceeds from the fiidlion, or 
refiftance of the parts rubbing on each other; 
which in general, is greater, or lefs, as the rubbing 
D ° ' parts 
