346 Astronomy. [Lecture 22. 



highest tides are when the moon is at once near 

 the equator, in perige, and in the syzigies. 



8dly, In northern climates the spring tides 

 are higher in the evening during winter ; and in 

 the summer they are higher in the morning *. 



It is evident from the detail of these phe- 

 nomena, that the tides have a marked connexion 

 with the motions of the moon; and that they 

 are also in some degree governed by those of the 

 sun. Whence we may fairly conclude that these 

 luminaries, and particularly the former, are the 

 principal natural causes of the phenomena of the 

 tides. 



Kepler had long ago conjectured that the gra- 

 vitation of the earth towards the sun and moon 

 was the cause of the tides. " If the earth ceased," 

 said he, " to attract the waters of the ocean, they 

 would be elevated towards die moon; for the 

 moon's sphere of attraction extends to our earth, 

 and evidently acts upon the waters." What was 

 mere conjecture in this great astronomer was 

 reduced to certainty by the superior genius of 

 Newton: upon his principles, therefore, I shall 

 endeavour to exhibit a popular view of the theory 

 of the tides. 



* The days on which the highest tides may be expected 

 are always given in White s Ephemeris before mentioned. 

 That very useful almanac also exhibits the time of morn- 

 ing and afternoon high water daily, as computed accurately 

 for London Bridge; with subsidiary rules, by which the 

 respective times of high water at several other ports may 

 readily be found. 



