TIDES. 47 



west to east, advances eastward rather more than 13 degrees 

 daily. During the same period the sun apparently advances 

 somewhat less than one degree. The moon, therefore, increases 

 her angular distance from the sun by rather more than 12 

 degrees daily, and consequently in somewhat less than 7 days 

 (7 days 9 hours 11 min. 1 sec.) after new or full moon she 

 is at right angles to a line joining the sun and earth, or at 

 her first or third quarter, as the case may be. 1 Her attractive 

 power, at such times, raises the water on the sides of the earth 

 in line with herself, and draws it away from the sides in line 

 with the sun, while the sun is playing a similar part, though 

 with less effect (^ that of the moon), on a line at right angles 

 to that through which the moon acts. In this way the high 

 tide of the moon is made to coincide with the low tide of the 

 sun, and vice versa ; the result being that the tides of the sun 

 and moon tend to neutralize one another, and the water does 

 not, therefore, rise so high or fall so low as at " springs." 



The tides at this period are known as "neap tides," the 

 range of which, as compared with spring tides, is about as 

 5 to 13. 



When the sun and moon are on the same side of the earth, 

 the upper tide of the sun coincides with the upper tide of the 

 moon, and the under tide of the sun coincides with the under 

 tide of the moon, their respective volumes being added. 



When the sun and moon are on opposite sides of the earth, 

 the upper tide of the one coincides with the under tide of the 

 other, and the effect is practically the same. 



The water-spheroids are but imperfectly formed, owing to 

 the distribution of land, irregularity in the depth of the ocean, 

 and the fact that the movements of the earth and moon do not 

 give time for their proper formation. 



With reference to the distribution of land, although the 

 tidal wave is dealt with, theoretically, as though it revolved 

 round and round the globe, the same wave cannot traverse the 

 globe twice, because, with the exception of a comparatively 

 narrow channel (about 500 miles) between Cape Horn and South 

 Shetland or the land which it is believed stretches northward 

 from the south pole, there is a practically continuous land 

 barrier from pole to pole at about 70 west longitude. The 



1 The moon passes the meridian in periods of 24-8H hours, or 48 min. 50 sec. 

 later every day. 



