ATTRACTION OF THE MOON. 25 



satellite. Those parts of the earth which come 

 nearest to the moon, are more strongly attracted, 

 and are therefore made to move somewhat quicker 

 than those at the mid-distance. The parts at the 

 greatest distance from the moon are less strongly 

 attracted than those in the middle, and must 

 therefore, if they are liquid, remain somewhat 

 behind these others. From this cause the waters 

 at the two opposite sides of the earth in the 

 direction of the straight line between the centres 

 of the earth and moon, are at the same time 

 raised above the level of the proper surface of 

 the earth; and, in the same manner, when they 

 come to the half-way point of mean distance 

 from the moon, both at the same time, sink down 

 again. 



The points of strongest and of weakest attraction 

 move with the moon from east to west, and thus 

 in the period of the apparent revolution of the 

 moon round the earth, that is, in one of nearly 

 twenty-five hours, they likewise accomplish their 

 journey round the earth. During this time, 

 therefore, there are brought about, at every part 

 of the liquid surface of the earth, two uprisings 

 of the water — two flood-tides — the one at the 

 time the moon passes the meridian above, the 

 other when it crosses that line below, and two 

 ebbs in the intervals. It is clear, too, that this 

 action must be exerted most strongly on the parts 



