570 



LECTURE XLVir. 



ON THE TIDES. 



The form and structure of the solid parts of the globe have afforded but 

 few remarkable features capable of arresting our attention, except the general 

 distribution of land and water, and the permanent differences of elevation of 

 different parts of the earth. But the sea exhibits a series of phenomena far 

 more interesting to the mathematical philosopher, because they admit of a 

 methodical investigation, and of a deduction from general causes, the action 

 of which may be traced in detail. For the height of the surface of the sea 

 at any given place is observed to be liable to periodical variations, which are 

 found to depend on the relative position of the moon, combined in some 

 measure with that of the sun. These variations are called tides ; they were 

 too obvious to escape the observation even of the ancients, who inhabited 

 countries where they are least conspicuous : for Aristotle mentions the tides 

 of , the northern seas, and remarks that they vary with the moon, and are less 

 conspicuous in small seas than in the ocean : Caesar, Strabo, Pliny, Seneca, 

 and Macrobius give also tolerably accurate accounts of them, 



Tliere are in the tides three orders of phenomena which are separately 

 distinguishable: the first kind occurs twice a day, the second twice a month, 

 and the third twice a year. Every day, about the time of the moon's 

 passing over the meridian, or a certain number of hours later, the sea become* 

 elevated above its mean height, and at this time it is said to be high water. 

 The elevation subsides by degrees, and in about six hours it is low water, 

 the sea having attained its greatest depression ; after this it rises again when 

 the moon passes the meridian below the horizon, so that the ebb and flood 

 occur twice a day, but} become daily later and later by about 50^ minutes, 

 which is the excess of a lunar day above a solar one, since 284- lunar days 

 are nearly equal to S9i solar ones. 

 4 



