THE TIDES 297 



found continual motions of the surface of the earth, and 

 which was not the primary object of their investigation 

 is in some respects more interesting than what they sought 

 and did not find. The delicate investigation thus opened 

 up promises a rich harvest of knowledge. These disturb- 

 ances are connected with earthquakes such as have been 

 observed in a very scientific and accurate manner by Milne, 

 Thomas Gray, and Ewing in Japan, and in Italy by many 

 accurate observers. All such observations agree in show- 

 ing continual tremor and palpitation of the earth in every 

 part. 



One other phenomenon that I may just refer to now as 

 coming out from tide-gauge observations, is a phenome- 

 non called seiches by Forel, and described by him as having 

 been observed in the lakes of Geneva and Constance. He 

 attributes them to differences of barometric pressure at the 

 ends of the lake, and it is probable that part of the phe- 

 nomenon is due to such differences. I think it is certain, 

 however, that the whole is not due to such differences. The 

 Portland tide curve and those of many other places, notably 

 the tide curve for Malta, taken about ten years ago by Sir 

 Cooper Key, and observations on the Atlantic coasts and 

 in many other parts of the world, show something of these 

 phenomena; a ripple or roughness on the curve traced by 

 the tide gauge, which, when carefully looked to, indicates a 

 variation not regular but in some such period as twenty or 

 twenty-five minutes. It has been suggested that they are 

 caused by electric action ! Whenever the cause of a thing 

 is not known it is immediately put down as electrical! 



I would like to explain to you the equilibrium theory, and 

 the kinetic theory, of the tides, but I am afraid I must 

 merely say there are such things; and that Laplace in his 

 great work, his Mecanique Celeste, first showed that the 

 equilibrium theory was utterly insufficient to account for 

 the phenomena, and gave the true principles of the dynamic 

 action on which they depend. The resultant effect of the 

 tide-generating force is to cause the water to tend to become 

 protuberant towards the moon and the sun and from them, 

 when they are in the same straight line, and to take a 

 regular spheroidal form, in which the difference between 



