Review of O. Gregory’s Treatise on Mechanics. 75 
investigation. The whole subject of the reduction of for- 
ces to rectangular co-ordinates, and the formulas dependant 
on it, is only a simple corollary from Newton’s laws of mo- 
tion; and all that is said of parallel forces is but a corollary 
of oblique forces acting at a poi 
In the preliminary remarks, the author says that vis iner- 
tie is improperly called a force, “* because if it were a force 
it would be of some definite quantity in a given body, and 
an impressed force less than that would not move the body; 
whereas any impressed force, however small, will move 
we fix the idea of agent, or patient to either of them. — ‘o 
destroy motion, requires the same force as to produce it; 
y the bo.iy, which was at rest, the one may as properly be 
said to be a force as the other; both bodies are inert, and 
tion, even by the least force, as its adequate effect ; and the 
quantity of motion produced,or the magnitude of the change, 
is the true measure of that force, considered either as act- 
ing or resisting. This force, though not constant, is defi- 
ite, and is used in mechanics as the measure of force gen- 
erally, This is sai rtant an axiom in the science, that 
it is astonishing that the author should doubt of its certainty. 
