WAVE OF HIGH WATER. 309 



mation to this condition/ whilst on all other open ocean 

 coasts the observed facts are mostly the reverse of what 

 the theories require to sustain them. 



It cannot J therefore, excite much surprise that neither 

 of these hypotheses has obtained support, or even claimed 

 much notice j so that, in spite of great anomalies and im- 

 perfections, the theory of free ocean waves of translation, 

 as illustrated by maps of co-tidal lines, still holds its ground. 

 But whilst little or no advance has been made towards a 

 sound causal theory of the tides, immense progress has of 

 recent years been made in formulating observations of the 

 details of the varying phases of tidal motion. This progress, 

 however, touches only the minor features of the phenomena. 

 By no existing causal theory can the direction of the pro- 

 gressive motion of the wave of high water along any given 

 coast be predicted where unknown, or accounted for when 

 known, although, when suitable observations have been 

 made at any port, the march of the fluctuations of the 

 wave, both in time and height, which in many parts of the 

 globe obey definite laws depending upon the changing 

 position of the sun and moon, can often be predicted with 

 an approach to minute accuracy. 



In all the theories to which reference has justTjeen made, 

 it is to be observed that the disturbing action of the sun 

 and moon is supposed to centre on ocean-areas^ the land- 

 areas of the globe only coming into question so far as the 

 shores of these areas form the bounding lines of the water- 

 surfaces : we, on the contrary, starting from a new hypo- 

 thesis on the relations of terrestrial matter to cosmical 

 force, have arrived at the conclusion that the tidal motions 

 of ocean- surfaces are caused by a differential action of force 

 centring on all land- areas, and reacting indirectly on the 

 margins of all ocean-areas. 



Irrespective of ideas of cause, this apparently arbitrary 

 change of stand-point is found to lead to an empirical law 



