4 6 



HARBOUR CONSTRUCTION. 



The water on the opposite side of the earth, owing to its 

 greater distance from the attracting bodies, is acted upon in 

 a less degree than the great mass of the earth itself, and con- 

 sequently "sags" or falls away from the earth, to an extent 

 almost the same as that to which the water immediately under 

 the sun and moon is raised. 



The effect is therefore practically the same as if a force were 

 drawing away the water from the earth on that side, and the 

 general water-surface takes approximately the form of a prolate- 

 spheroid, or that of a solid formed by revolving an ellipse about 

 its longer axis. Inasmuch as the moon, as a tide-producer, is 

 much more powerful than the sun, the tides are governed by 

 her, and they follow her apparent passage round the earth, thus 

 giving us two tides in the course of each lunar day. 



SPRING TID'ES 



V* ^i / 



V..t.x 



( SUN j 



FIG. 9. Diagram of the tides. 



Spring Tides. When the sun, moon, and earth are in one 

 straight line called the line of syzygies which occurs twice 

 in each lunar month, the sun and moon act in unison to raise 

 the water on the two opposite sides of the earth, to an extent 

 proportionate to the sum of their effective attractive powers, 

 which, according to the laws of gravity, as already pointed out, 

 are about as 1 to 2J. Thus the highest, or what are known 

 as " spring tides " are produced. 

 Neap Tides. The moon, in her passage round the earth from 



to be 93,000,000 miles, it may be shown that the difference between the sun's 

 attractive force on the near and far sides of the earth is only ss 1 ?? of its whole 

 attraction. 



Thus the proportion which the moon's effective attraction for tide-producing 

 purposes bears to that of the sun is as ^5 to ^$, or as 2-3 to 1. 



