14

MECHANICS.

[Chap. 1.

the body would continue its course in a right line. Thepreceding remarks may serve to illustrate the second, aswell as the first law.

The third law is confirmed by all our observations onthe motions of the heavenly bodies, and by all our expe-riments. If a glass bottle be struck by a hammer, or ahammer by a glass bottle, the bottle will in either casebe broken by the same degree of moving power: werethe hammer equally fragile with the bottle, both wouldbe broken. If a stone be thrown against a pane of glass,the glass will'be broken, and the stone be retarded, inexact proportion to the resistance offered by the glass.

To assert the contrary of this law, would be to main-tain an absurdity; for if action and reaction be not equal,one must be greater than the other, which would be tosay that the effect was greater than, or not equal to, thepause.

article 4 ,

OX ABSOLUTE AND RELATIVE MOTION.

The idea intended to be conveyed by the term mo-tion is too familiar to require a definition.

Motion is either absolute, or relative.

Absolute motion is the removal of a body from onepart of space to another, as the motion of the earth inits orbit.

Relative motion is the change of place which one bodyundergoes in relationship to another: such, for example,as the difference of motion in the flight of two birds, orthe sailing of two ships.

Were all the articles upon the surface of the earth toretain their respective situations, they would still be inabsolute motion with the earth in space, but they wouldexperience no relative motion, and would appear to usto be at rest.

In the theory of mechanics, much information is de-rived from our knowledge of the laws observed by theheavenly bodies in their absolute motions; but, in prac-