Note 757 — Chapter V · (continued 2)

As an illustration of this doctrine, I cannot do better than quote the following passage from one of the most suggestive and clearly reasoned books which has been written in this century by an English physicist: ‘Wave your hand; the motion which has apparently ceased, is taken up by the air, from the air by the walls of the room, &c., and so by direct and reacting waves, continually comminuted, but never destroyed. It is true that, at a certain point, we lose all means of detecting the motion, from its minute subdivision, which defies our most delicate means of appreciation, but we can indefinitely extend our power of detecting it accordingly as we confine its direction, or increase the delicacy of our examination. Thus, if the hand be moved in unconfined air, the motion of the air would not be sensible to a person at a few feet distant; but if a piston of the same extent of surface as the hand be moved with the same rapidity in a tube, the blast of air may be distinctly felt at several yards' distance. There is no greater absolute amount of motion in the air in the second than in the first case, but its direction is restrained, so as to make its means of detection more facile. By carrying on this restraint, as in the air-gun, we get a power of detecting the motion, and of moving other bodies at far greater distances. The puff of air which would in the air-gun project a bullet a quarter of a mile, if allowed to escape without its direction being restrained, as by the bursting of a bladder, would not be perceptible at a yard's distance, though the same absolute amount of motion be impressed on the surrounding air.’ Grove's Correlation of Physical Forces, London, 1855, pp. 24, 25. In a work now issuing from the press, and still unfinished, it is suggested, with considerable plausibility, that Persistence of Force would be a more accurate expression than Conservation of Force. See Mr. Herbert Spencer's First Principles, London, 1861, p. 251. The title of this book gives an inadequate notion of the importance of the subjects with which it deals, and of the reach and subtlety of thought which characterize it. Though some of the generalizations appear to me rather premature, no well-instructed and disciplined intellect can consider them without admiration of the remarkable powers displayed by their author.