A SYSTEM OF PARTICLES. 261 



consequently, pressure is not produced in this manner, except this 

 mode of summation by integrating is not allowable, which I am 

 rather inclined to think is the case. 



27. This, however, would not deter me from still retaining the 

 above theory for the mot/on of air producing sound. 



Pressure may be produced, and I think there is good reason for 

 supposing this the case, by the actual contact of particles. It appears 

 to be the most simple hypothesis to which we can refer it. I do 

 not conceive it necessary to such a supposition that an atom should 

 be variable in its form and dimensions. It would be more simple, 

 to consider each particle as a collection of atoms arranged about a 

 central nucleus of attractive or repulsive imponderahle matter. Thus 

 the whole pressure on a surface would be the pressure produced by 

 the material particles actually in contact with it, the only effect of 

 the immaterial particles being to compress the material ones together, 

 and their sensible effect being consequently zero. Dr Dalton, however, 

 supposes each atom surrounded by an atmosphere (as he terms it) of 

 its own, which hypothesis although far more simple, does not, I 

 think, so readily solve the difficulties, as that which I have adopted. 



28. We recur then to the motion of the particles on the hypo- 

 thesis of a law of force : the transmission not being an effect of 

 actual contact, but of agitation amongst the immaterial particles. Of 

 course such an agitation will produce, or be produced by, variations 

 in the actual pressure, and thus the pressures may in one sense be 

 said to produce the motion. 



Having given reasons above for supposing the particular form of 

 the law to be that of the inverse square of the distance, we shall in 

 future adopt it. 



I 



Let us now proceed to consider the effect of the factor - on the 



velocity. Conceive an atmosphere of one hind; let r, r' be the dis- 

 tances of two strata from the centre of the Earth ; e, e' the distances 



