The exact import of the terms " vis viva" and " inertia," as understood by writers on physics, is difficult of comprehension by ordinary minds, and difficult to explain clearly when comprehended. * An engineer of some note once remarked to us, " I know exactly what I mean by l vis viva/ but I find it very difficult to tell it." We do not propose to here enter upon an elaborate discussion of the doctrines of " vis viva" and " inertia," but merely to notice some recent opinions published in the American reprint of the London Chemical News, and also a paper by Prof essor Henry Morton, on the " Resistance and Transmission of Motion/' published in the Journal of the Franklin Institute. Prof. Morton charges that the subject has been inade-- .iely treated by some even of the highest authorities, an opinion with which we perfectly coincide. He says, " we say and know that ' vis viva/ or work done by a moving body, varies with the square of its velocity, while we know, by our previous reasoning, that the force expended in giving it that velocity, only varies with the velocity itself. Thus, the force of gravity will- give a falling body a double velocity in a double time, during which it must have exerted a double force upon it. Here, then, we have a double force doing a quadruple work. Is this because, by some wonderful and recondite property inherent in ' velocity/ the double power has been indued with an again double efficiency ? Many writers leave us to think so ; but we, on the contrary, believe that the work done only seems to increase more rapidly than the power implied in the increased velocity, by reason of a loss of efficiency in the resistances, in the overcoming of which the 'work' consists, and in fact, that work in this sense, is no true measure of force." This argument is most forcibly and clearly expressed, and is further sustained by reasoning and illustrations which evince close thought upon this abstruse subject. We would be glad to notice this able article more at length, but want of space compels brevity. The Chemical News says, the statements in the works on physics in regard to " vis viva" and momentum, are in its judgment, not sustained by reason or experience. It denies that the power required to maintain a train or a ship in uniform motion, varies as the square of the velocity, and asserts that there is really no such mathematical relation, and there is no close approximation to it. It asserts, moreover, that the case of a ship is so different from that of the train, that many engineers, who strive to measure facts on a procrustean bed of simple mathematical formulae, represent that the power required to drive a ship varies as the cube of the velocity ; and and no experienced engineer will say that within ordinary limits of speed four times as much power is ever required to maintain a train at double velocity. It sums up the case by stating that as it understands the subject of " vis viva/' it relates only to change in velocity, and does not apply to the maintenance of a uniform velocity after it has been once attained. These papers are an index of the effort which thinking minds are now making to disencumber themselves of ideas originating in the old notion of occult force. The terms, " vis viva" and "inertia," were born ofthat notion ; as their parent may be said to be at the last gasp, we say let them die also. As soon as we shake ourselves free from these clinging errors, and discard the illogical language they have imposed upon us, we shall find our way totally unobstructed; we shall have " cleaned our path from briars." We shall have to come down at last to the simple fact that motion is force, and force is motion, that is, so far as the human mind is capable of com prehending force. Motion can only produce an equivalent amount of motion, and hence the only measure of an existing motion is a previously existing or co-existent motion. When we get on to this plane we have got out of the slough of metaphysics and are on solid ground.