ON GEOLOGICAL TIME. 23 



influence of the sun and moon is analyzed by 

 mathematical reasoning, it is found that there 

 would be for either separately, a tidal spheroid 

 fulfilling the condition just defined. Thus the 

 dynamical result of the tendency of either body 

 would be low ivater at the time of the high water 

 of the imaginary equilibrium tide, and vice versa, 

 on the average of the whole earth. By the lunar 

 tide, for instance, there would be low water when 



and there would be average high water of either component tide 

 when the body to which it is due crosses the meridian ; also, the 

 average times of greatest tide would still be those of new and full 

 moon. But if the depth of the sea and the configuration of the land 

 were such that the chief period of oscillation could be intermediate 

 between twelve solar and twelve lunar hours, the greatest axis of the 

 luni-tidal spheroid would be in line with the moon ; but that of the 

 soli-tidal spheroid would be perpendicular to the line joining the 

 earth's and moon's centres. In this case, the times of spring tides 

 would be those of quarter moons. In the first of these two unreal 

 cases, the effect of tidal friction would be to make the time of 

 average high water somewhat later in each component tide, than 

 the time when the body producing it crosses the meridian ; and this 

 deviation would be greater for the sun than for the moon. Thus, 

 the time of spring tides would be, as it is, somewhat later than the 

 times of new moon and full moon. But, in the second of the 

 imagined cases, the effect of friction would be to advance the time 

 of solar high water and to retard the time of lunar high water ; and 

 thus the time of spring tides would be somewhat before the times of 

 the quarter moons. 



