CH. V] MECHANICAL RECREATIONS 101 



difference than might be thought by a landsman. In the case 

 of boats sailing on smooth ice the assumptions made are sub- 

 stantially correct, and the practical results are said to agree 

 closely with the theory. 



Boat moved by a Hope. There is a form of boat-racing, 

 occasionally used at regattas, which affords a somewhat curious 

 illustration of certain mechanical principles. The only thing 

 supplied to the crew is a coil of rope, and they have, without 

 leaving the boat, to propel it from one point to another as 

 rapidly as possible. The motion is given by tying one end of 

 the rope to the after thwart, and giving the other end a series 

 of violent jerks in a direction parallel to the keel. I am told 

 that in still water a pace of two or three miles an hour can be 

 thus attained. 



The chief cause for this result seems to be that the friction 

 between the boat and the water retards all relative motion, 

 but it is not great enough to affect materially motion caused by 

 a sufficiently big impulse. Hence the usual movements of the 

 crew in the boat do not sensibly move the centre of gravity of 

 themselves and the boat, but this does not apply to an impul- 

 sive movement, and if the crew in making a jerk move their 

 centre of gravity towards the bow n times more rapidly than 

 it returns after the jerk, then the boat is impelled forwards 

 at least n times more than backwards: hence on the whole 

 the motion is forwards. 



Motion of Fluids and Motion in Fluids. The theories 

 of motion of fluids and motion in fluids involve considerable 

 difficulties. Here I will mention only one or two instances — 

 mainly illustrations of Hauksbee's Law. 



Hauksbee's Law. When a fluid is in motion the pressure 

 is less than when it is at rest*. Thus, if a current of air is 



* See Besant, Hydromechanics, Cambridge, 1867, art. 149, where however 

 it is assumed that the pressure is proportional to the density. Hauksbee was 

 the earliest writer who called attention to the problem, but I do not know who 

 first explained the phenomenon; some references to it are given by Willis, 

 Cambridge Philosophical Transactions, 1830, vol. m, pp. 129—140. 



