86 DETERMINATION OF THE MASS [CH. vm. 



surface, a clear space being left above it which 

 gradually increases in thickness as the cloud falls. 

 The rate of movement of the top of the cloud will 

 give the rate of falling of the individual globules of 

 which it is composed. And this brings us to the 

 next section. 



Prof. Stokes and Falling Spheres. 



Many years ago, in 1849, Sir George G. Stokes* 

 discussed the motion of solids through fluids, and 

 among others of a sphere moving through a viscous 

 fluid urged by its own weight. It is a familiar fact 

 that large bodies fall through air or water or any 

 resisting medium more quickly than small ones of 

 the same shape. Thus coarse sand settles down 

 through water quicker than fine sand, and the finest 

 powder takes a very long time to settle ; in fact 

 this difference of the rate of falling is used as a 

 practical process of separating granular materials into 

 sizes, and is called levigation. 



So it is in air : large raindrops fall violently, small 

 raindrops fall gently, and mist globules hardly fall 

 at all fall so slowly that their motion is difficult 

 to observe, but the same law governs all, so long 

 as the motion is not too violent, or so long as the 

 falling body has no edges such as will cause eddies 

 during the fall. A sphere falling slowly, controlled 

 by viscosity alone without waves or eddies, is the 

 simplest case. It soon reaches what is called a 

 terminal velocity the speed at which the viscous 

 resistance exactly balances its weight. At this 

 speed it is subject to zero resultant force, so it 

 simply obeys the first law of motion and moves at 

 a constant speed, t This constant speed or terminal 



* Camb. Trans. Phil. Soc., ix. 48. t Of. Nature, vol. 31, p. 266. 



