244 POPULAR LECTURES AND ADDRESSES. 



condition, so keenly desired by Newton and 

 Faraday, of being explained by continuous 

 action through an intervening medium. The 

 law of the mutual force in our model, however, 

 is not the simple Newtonian law, but the much 

 more complex law of the mutual action between 

 two electromagnets- with the difference, that in 

 the hydrokinetic model in every case the force 

 is .opposite in direction to the corresponding 

 force in the electromagnetic analogue. Imagine 

 a solid bored through with a hole and placed 

 in our ideal perfect liquid. For a moment let 

 the hole be stopped by a diaphragm, and let an 

 impulsive pressure be applied for an instant 

 uniformly over the whole membrane, and then 

 instantly let the membrane be dissolved into 

 liquid. This action originates a motion of the 

 liquid relatively to the solid, of a kind to which I 

 have given the name of " irrotational circulation," 

 which remains absolutely constant however the 

 solid be moved through the liquid. Thus, at 

 any time the actual motion of the liquid at any 

 point in the neighbourhood of the solid will be 



