INERTIA, in physics, denotes the mechan ical continuance, or persistence, of energy in existence. The German language has two words for our word "inertia." An explanation of these should serve as an illustration of the way in Which inertia is used in a dual sense by English-speaking physicists. Beharrung, the better of the two German terms, because it is a positive expression, has been defined above. The other German word is a literal German translation of what we commonly mean by inertia. But there are some experimental as well as theoretical considerations which reveal that whenever one understands by in ertia the non-appearance of new energy, or motion, or activity,— and this is the meaning of Trogheit— one does so by laying all the em phasis on a merely negative characteristic of inertia. A gentleman of Vienna, J. Popper, alluded while conversing with Prof. Ernst Mach, a distinguished physicist, to an interesting parallelism subsisting between the meaning of Inertia as used in physics and the meaning of heredity as employed by biologists. Mach's version of Popper's remarks was given in an inaugural address delivered on assuming the rectorate of the University of Prague, from which we quote as follows: "Take a body in motion; the body retains the velocity acquired in (or inherited from) the interval of time just preceding, except it be changed in the next moment by an accelerating force. In the case of the body in motion the change of velocity (abanderung) was lkoked upon as a matter of course, while the dikovery of inertia (or per sistence) created great surprise; in Darwin's case, on the contrary, the heredity (or per sistence) was taken for granted, while the principle of variation (abanderung) appeared novel.) From this is revealed the notion of continuity or persistence underlying both the idea of heredity and the idea of inertia. In ertia thus places emphasis on the idea that so long as nothing interferes to bring about change everything in nature will remain as it is. The law of inertia indeed explains itself. Wher ever we meet with it, it need not be accounted for. An explanation becomes necessary only
when inertia appears to be lacking.
The term is thus used to denote the law of the material world that all bodies are absolutely passive or indifferent to a state of rest or motion, and would continue forever at rest, or persevere forever in the same uniform recti linear motion, unless disturbed by the action of some extrinsic force. Even in ancient times thinkers attributed to matter a certain in aptitude, reluctance or renitency to motion. But that a body in motion required the opera tion of an extrinsic cause (potential or actual) to bring it to rest was first discovered by Galileo. He was led to this discovery through his examination of the principle of the height of ascent. Kepler, conceiving the disposition of a body to maintain its motion as indicating an exertion of power, prefixed vis; and the compound expression vis inertia ("force of in though less accurate than inertia merely, .has been nevertheless generally retained. Huy gens, upon whose shoulders the mantle of Galileo fell, formed a sharper conception than the latter had of the law of inertia. He achieved this by generalizing the principle re specting the heights of ascent, a principle laid under contribution by Galileo. Huygens writes: "If gravity did not exist, nor atmos phere obstruct, the motions of bodies, a body would keep up forever the motion once im pressed upon it, with equable velocity in a straight line." It may be mentioned in passing, that the principle respecting the heights of ascent employed by Huygens in this connection, is identical with the principle of excluded per petual motion. Newton showed that if the property of inertia is possessed to an equal de gree by two different substances these sub stances will have equal heaviness or weight. So it appears that that part of the principle of in ertia involving continuance in rest if undis turbed was known even to the ancients, and by them attributed to a certain repugnance of mat ter to motion. It was thus reserved for Galileo to show that the remaining portion of the principle was equally true and general.