EBB AND FLOW. 205 



motion in the quarter of a day, so that the waters being- 

 cooped in on both sides, the ebb and flow of the sea would 

 become visible twice a day, since there is a double advance 

 and also a double recoil. Now if these two islands were 

 extended through the waters like cylinders or columns, of 

 equal dimensions, and with rectilinear shores, that mo 

 tion might be easily perceptible, and might be pointed out to 

 any one, which now seems to be perplexed and obscured by 

 so great a variety of position of land and sea. For it is not 

 difficult to form some conjecture what degree of velocity it 

 is proper to ascribe to that motion of the waters, and what 

 distances it may describe in one day. For if there be 

 selected, in order to form a judgment of this matter, some 

 of those coasts which are less mountainous or low lying, 

 and which are contiguous to the open sea, and then the 

 measure of the space of the globe interjacent between the 

 extreme points of the flux and reflux, and that space be 

 quadrupled on account of the four movements of the tide 

 each day, and that number again doubled on account of the 

 tides at the opposite shores of the same ocean; and to this 

 number there be something added over and above on ac 

 count of the height of the shores which always rise to a 

 certain elevation above the channel of the sea; that calcu 

 lation will give the space which this sphere of water, were it 

 free from obstruction, and moving in progression round tjie 

 enveloped globe of earth, would describe in one day, which 

 certainly would not be great. 



Now, with respect to that difference which coincides 

 with the measure of the moon s motion, and forms the 

 period of a lunar month ; we think that the explanation is 

 this, that the period of six hours is not the exact measure 

 of this reaction, just as the diurnal motion of any of the 

 planets is not accomplished in twenty-four hours precisely, 

 and least of all that of the moon. Wherefore the measure 

 of the ebb and flow of the tide is not a quarter of the 

 motion of the fixed stars, which is twenty-four hours, but 

 a quarter of the diurnal motion of the moon. 



DIRECTIONS. 



Let it be inquired, whether the hour of the tide on the 

 coast of Africa be before the hour of tide about the Straits 

 of Gibraltar. Let it be inquired whether the hour of the 

 tide about Norway is before the hour of the tide about 

 Sweden, and that, in like manner, before the hour of the 

 tide at Graveling? 



