TIDES, WAVES 107 



the range from high-water to low-v/ater being 

 greatest about the time of full moon and new 

 moon, when the tides are called " spring- 

 tides," and least about the time of the moon's 

 first and third quarters, when the tides are 

 called " neap-tides." The tide-generating 

 effect of the moon is more than double that 

 of the sun, because of the very much greater 

 distance of the sun, in spite of its greater 

 mass. When the sun and moon are both 

 on the same side of the earth, as at new moon, 

 and when they are diametrically opposed to 

 each other, as at full moon, their tide-genera- 

 ting effects are additive, but when they are at 

 right angles to each other, as at half moon, 

 waxing and waning, the effects are subtractive, 

 so that the spring-tides have a range three 

 times greater than the neap-tides. 



It is usual to regard the sun and moon as 

 producing a tidal wave which develops freely 

 in the Southern Ocean, where a zone of water 

 encircles the earth. This wave has a very 

 great length with high- water at the crest and 

 low-water in the trough. On passing the 

 opening between Africa and South America 

 it gives rise to a lateral wave, moving from 

 south to north through the whole Atlantic, 

 producing tidal effects in Northern Europe 

 and America. Besides this wave there is 

 formed another tide-wave in the Atlantic 



