1907.] on Seiches in the Lakes of Scotland. 663 



them. If the trough were infinitely long, thev would come to rest after 



the solitary wave had passed away. Each particle comes to rest in a 



position at, or at least near, its original one. It is the wave form, 



and not the constituent water, that really travels, as you will see by 



watching the splash of red ink thrown on top of the wave as it passes. 



Theory and observation agree in giving the formula Y = V(^ h) for 



the velocity of the highest point of the solitary wave. 



It is important to observe that a wave of this kind, travelling 



backwards and forwards in a lake of uniform breadth and depth, would 



produce a periodic disturbance at one end of the lake having a period 



2 I 

 -7-^^— . It should be noticed, however, that the curve on the lim- 



^ (^ ^0 



nogram would not in general be a sinusoid, but something more 01 

 this shape mmJ^ »rs^my\ •v— , where periods of positive 

 denivellation alternate with periods of no disturbance. 



Progressive Waves Generated hij a Wind Current. — AVe now show 

 the effect of a steady wind current blowing along the surface of water. 

 So long as the current is below a certain strength (0 * 45 miles an hour), 

 there is no disturbance of the mirror-surface ; above that limit, and 

 under a velocity of about '1 miles an hour, there is disturbance which 

 is transient, i.e. does not long survive the disturbing causes (cats- 

 paws). For higher current velocities a regular train of so-called pro- 

 gressive waves is formed, which increase in height and in length as 

 you go down the wind. In nature, the water is comparatively calm 

 at the windward end of the lake, but more — it may be very much — 

 agitated at the other. Even at their greatest, these waves, as you see, 

 are very short (say A = 0-25 feet) ; their period also is short (say T 

 = 0-22). If you watch the thin stream of red ink dropped from the 

 pipette into the water in the trough you will see that the oscillatory 

 disturbance, of which these progressive waves are the manifestation, 

 dies away as you descend from the surface, and is not appreciable at 

 any great depth. From the well known formulae 



^ = v'(f^) ^V(^^ 



it is easy to calculate the velocity of propagation and the period of 

 these waves. 



For Loch Earn, common values of the figures would be about A. 

 = 20 ft., T = 2 sec, Y = 10 ft. per sec. = 6*8 miles per hoiu'. Apart 

 from the drift of the siu'face water as a whole (which you may notice 

 by watching the cloud of red ink), the motion of the individual water 

 particles is in closed elliptic orbits. 



Generation of a Seiche hy Horizontal Stirring at the Nodes.-— 

 Following a method due to two young experimenters, Messrs. White 

 and Watson, to whose results I shall presently refer, Mr. Heath will 



