444 Mr. S. T. Preston on the Mode of 



(or the motion of the molecules towards all directions) which 

 produces this equilibrium of pressure is therefore also neces- 

 sarily self-adjusting. This fact will be at once evident by sup- 

 posing an extreme case. Thus, supposing the component mo- 

 lecules of an imaginary cubic foot of gas to have their motion 

 interfered with in such a way that the molecules only move 

 longitudinally (i. e. in directions parallel to each other), then 

 this cubic foot of gas will cease to exert any transverse or 

 lateral pressure ; for a pressure cannot be exerted at right 

 angles to the line of motion. The pressure of the surrounding 

 gas will therefore cause this cubic foot of gas to collapse late- 

 rally (owing to the absence of a lateral opposing pressure); 

 and in this act of collapse, by the lateral inrush of the sur- 

 rounding gas, the molecules of this cubic foot will receive a 

 forcible lateral acceleration which they previously wanted, 

 the irregularity of motion being thus soon corrected and the 

 equilibrium of pressure restored. It is clear that in the actual 

 fact no such state of things as this could occur ; for the rapid 

 interchange of motion going on among the molecules of a gas 

 necessarily corrects any incipient disturbance of the equili- 

 brium of pressure immediately on its occurrence — a continual 

 self-acting adjustment thus going on which entirely prevents 

 any abnormal movement of the molecules from developing 

 itself, the movement of the molecules equally towards all 

 directions being thus automatically maintained. To summa- 

 rize, therefore, we observe : — 



That a special form of motion is required to produce equi- 

 librium or uniformity of pressure in all directions within a gas, 

 viz. uniformity of motion in the molecules of the gas towards all 

 directions (so that an equal number of molecules are moving in 

 any two opposite directions); and, further ', that this uniformity 

 of motion is self-adjusting, or the gas itself automatically adjusts 

 the motion of its molecides, so that they move uniformly towards 

 all directions. 



4. Mode of the Propagation of Waves. — These considera- 

 tions enable the mode of propagation of waves in a gas to be 

 illustrated in a very simple manner. Thus, since the mole- 

 cules of a gas move in such a way that an equal number of 

 molecules are moving in any two opposite directions, we may 

 therefore represent the molecules of a gas by a row of spheres 

 or ivory balls (fig. 1), colliding among each other in such a 

 way that at any given instant half are Fig. 1. 



moving in one direction and half in the 

 opposite. The odd balls 1, 3, &c. may be 

 supposed to move simultaneously in one 

 direction, during the time that the even balls 2, 4, &c. move 



