Whence, then, does the augmentation pointed out by Laplace arise? I would ask your best attention while I endeavor to make this knotty point clear to you. If air be compressed it becomes smaller in volume; if the pressure be diminished, the volume expands. The force which resists compression, and which produces expansion, is the elastic force of the air. Thus an external pressure squeezes the air-particles together; their own elastic force holds them asunder, and the particles are in equilibrium when these two forces are in equilibrium. Hence it is that the external pressure is a measure of the elastic force. Let the middle row of dots, Fig. 13, represent a series of air-particles in a state of quiescence between the points _a_ and _x_. Then, because of the elastic force exerted between the particles, if any one of them be moved from its position of rest, the motion will be transmitted through the entire series. Supposing the particle _a_ to be driven by the prong of a tuning-fork, or some other vibrating body, toward _x_, so as to be caused finally to occupy the position _a′_ in the lowest row of particles: at the instant the excursion of _a_ commences, its motion begins to be transmitted to _b_. In the next following moments _b_ transmits the motion to _c_, _c_ to _d_, _d_ to _e_, and so on. So that by the time _a_ has reached the position _a′_, the motion will have been propagated to some point _o′_ of the line of particles more or less distant from _a′_. The entire series of particles between _a′_ and _o′_ is then in a state of condensation. The distance _a′ o′_, over which the motion has travelled during the excursion of _a_ to _a′_, will depend upon the elastic force exerted between the particles. Fix your attention on any two of the particles, say _a_ and _b_. The elastic force between them may be figured as a spiral spring, and it is plain that the more flaccid this spring the more sluggish would be the communication of the motion from _a_ to _b_; while the stiffer the spring the more prompt would be the communication of the motion. What is true of _a_ and _b_ is true for every other pair of particles between _a_ and _o_. Now the spring between every pair of these particles _is suddenly stiffened_ by the heat developed along the line of condensation, and hence the velocity of propagation is augmented by this heat. Reverting to our old experiment with the row of boys, it is as if, by the very act of pushing his neighbor, the muscular rigidity of each boy’s arm was increased, thus enabling him to deliver his push more promptly than he would have done without this increase of rigidity. The _condensed_ portion of a sonorous wave is propagated in the manner here described, and it is plain that the velocity of propagation is augmented by the heat developed in the condensation.