80 Review of O. Gregory’s Treatise on Mechames. 
is that, which is fundamental in the demonstration of the 
principle of the lever, and other mechanical powers, some 
lustration of it may not be unacceptable to the reader. 
- Let AB be a right 
line, perpendicular to = 
which two weights or | eee Snes ' 
Ages 
=~ 
are supposed to act, ww 
and to be kept in equilibrio by the reaction of the prop P, 
the pressure on the prop would then be equal to the res 
sultant of the two forces W,w. This supposed pressure 
on the prop P, when AB has no thickness, or vertical di- 
mension, or becomes a mere mathematical line, would ev- 
idently be nothing; for the forces W, w, communicate no 
CP, but the action of these is not in the direction AB, but 
oblique to the parallel forces, and the equivalent or joint 
action of all these cohesive forces will be represented in 
direction by some oblique line, drawn to some point c, 
from AB. Now in these directions are the weights W, w, 
sustained by the cohesion of the bar at the line CP, and 
Wx AP+Pe, and w x BP+Pc, whereof W xPctwx Pé; 
are annihilated by the reaction of the prop, aud there re- 
the resolved parts of the forces, viz. AP and BP, 
which are in one continued line, be what was intended by 
their action on a straight line, even in this sense the propo- 
sition is incorrect ; for the forces in the direction of AP, 
and BP do not act in the line AB, at P, but at ¢, and pre- 
vent the bar from turning about the point P. 7 
