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(70.) The consequences of the property of inertia, which have been explained in the present and preceding chapters, have been given by Newton, in his Principia, and, after him, in most English treatises on mechanics, under the form of three propositions, which are called the “laws of motion.” They are as follow:—

I.

“Every body must persevere in its state of rest, or of uniform motion in a straight line, unless it be compelled to change that state by forces impressed upon it.”

II.

“Every change of motion must be proportional to the impressed force, and must be in the direction of that straight line in which the force is impressed.”

III.

“Action must always be equal and contrary to reaction; or the actions of two bodies upon each other must be equal, and directed towards contrary sides.”

When inertia and force are defined, the first law becomes an identical proposition. The second law cannot be rendered perfectly intelligible until the student has read the chapter on the composition and resolution of forces, for, in fact, it is intended as an expression of the whole body of results in that chapter. The third law has been explained in the present chapter, as far as it can be rendered intelligible in the present stage of our progress.