180 elation of the Laws of Mechanics to Perpetual Motion. — 
this force, _ apply it to his use, he has all that he wants. He 
ery important exception to the general law, that a 
», body is Gearalie af putting itself in motion. Matter has a con- 
“.tmual and powerful tendency to move towards the earth. A 
‘body needs only:to be left to itself, to descend with a force pro- 
= ‘portioned to its waight. But he is driven even from this refuge, 
bag by a fourth general or 
Pe y; weight, communicates motion in no other 
a “way than by eieling: and when it has once reached the 
~~ earth, its operation ceases, till it is raised up again, with a force 
af equal to that with which it descended. is does not mean that 
Pie a body can, i mo case, communicate motion except by meee 
ing itself. A heavy body moving in sdb direction may im 
another in the same direction. -A cannon ball may drive Safeas 
it the object which it strikes. This is “oi done, however, by the 
_ Weight of the ball, but by the momentum which it has received 
Sag from a foreign force. What we e are now considering is the mo- 
‘tion produced by the gravity of the body, not that which is oc- 
i casioned by the application of mechanical force from without. 
PR Neither does the principle just stated imply, that a body may not, 
z x20. — weigh cause another body to move in any direction 
wards the earth. The weight in one scale of a b 
: oy cause those in the other side to rise. But to do this 
fe itself descend. Nor in the third place, is it to be under- 
} tha a body can not by ‘its weight, ‘have an effect of any 
out descending. it may; even in a state of rest, have 
ti 1s-lost in velocity. 
ns, we’ ‘may considé 
uces by. 
ui 
ih wei 
bland 
body fF 
or. if 
~equilibrio. But the moment the’ levér.is ae 
parative’importance of the smaller may, i, 
