456 POPULAR LECTURES AND ADDRESSES. 



some results of mathematical calculation and 

 experimental observation on these subjects 

 I may tell you that great waves at sea will 

 travel for hours or even for days, showing 

 scarcely any loss of sensible motion or of 

 energy, if you will allow me so to call it 

 through viscosity. On the other hand, look at 

 the ripples in a little pond, or in a little pool 

 of fresh rain water lying in the street, which are 

 excited by a puff of wind ; the puff of wind is 

 no sooner gone than the ripples begin to subside, 

 and before you can count five or six the water 

 is again perfectly still. The forces concerned in 

 short waves such as ripples, and the forces 

 concerned in long waves such as great ocean 

 waves, are so related to time and to speed that, 

 whereas in the case of short waves the viscosity 

 which exists in water comes to be very potent, in 

 the case of long waves it has but little effect. 



Allow me then for a short time to treat 

 water as if it were absolutely free from vis- 

 cosity as if it were a perfect fluid ; and I shall 

 afterwards endeavour to point out where vis- 



