zr, POPULAR LECTURES AND ADDRESSES. 



of the whole ocean due to resistance against 

 the tidal motions. 



IT. The accompanying woodcut illustrates 

 the position of the sun and moon, and of the 

 longest axis of the resultant tidal spheroid at 

 the average time of spring tides for the whole 

 earth. It represents a section in the plane of 

 the equator, in which for simplicity the sun 

 and moon are both supposed to be. The spec- 

 tator being supposed to look at the diagram 

 from the north side, sees the earth rotating, 

 and the moon revolving round the earth's centre, 

 each in the direction opposite to the motion of 

 the hands of a watch. If there were no tidal 

 friction, QMS would be in one straight line, 

 and I III', the longest axis of the tidal spheroid 

 would be perpendicular to it. What observa- 

 tion on the time of spring tides proves, is simply 

 that OM i* inclined to OS forwards by the 

 angle through which the moon advances before 

 the Min, in the time by which the spring tides 

 late. If this time were twelve hours, the 

 angle M()S would be 6. The dynamical theory 



