298 



KELVIN 



the greatest and the least semi-diameter is about 2 feet for 

 lunar action alone, and I foot for the action of the sun 

 alone that is a tide which amounts to 3 feet when the sun 

 and moon act together (Fics. 124 and 125), and to I foot 

 only when they act at cross purposes (Fics. 126 and 127), 

 so as to produce opposite effects. These diagrams, FIGS. 124 

 to 127, illustrate spring and neap tides: the dark shading 

 around the globe, E, representing a water envelope sur- 

 rounding the earth. There has been much discussion on 

 the origin of the word neap. It seems to be an Anglo-Saxon 

 word meaning scanty. Spring seems to be the same as 

 when we speak of plants springing up. I well remember at 



FIG. 124 Spring Tides 



FIG. 125 Spring Tides 



the meeting of the British Association at Edinburgh a 

 French member who, meaning spring tides, spoke of the 

 grandes marees du printemps. Now you laugh at this ; and 

 yet, though he did not mean it, he was quite right, for the 

 spring tides in the spring time are greater on the whole 

 than those at other times, and we have the greatest spring 

 tides in the spring of the year. But there the analogy ceases, 

 for we have also very high spring tides in autumn. Still 

 the meaning of the two words is the same etymologically. 

 Neap tides are scanty tides, and spring tides are tides which 

 spring up to remarkably great heights. 



The equilibrium theory of the tides is a way of putting 

 tidal phenomena. We say the tides would be so and so if 



