344 Astronomy. [Lecture 22. 



Where the motion of the waters is not re- 

 tarded by capes, islands, or straits, or other 

 similar obstacles, three periods are remarkable in 

 the tides The daily period, the monthly, and 

 the annual. 



The mean daily period is 24 hours 49 minutes, 

 during which there are two flood and two ebb 

 tides. This interval of 24 hours 49 minutes 

 is the time in which the moon performs her 

 mean apparent daily revolution round the earth. 

 During this diurnal period we observe, 



1st, That the high tide reaches the Eastern 

 harbours and roads, sooner than those to the 

 West. 



2dly, That between the tropics the tide always 

 seems to proceed from East to West. 



3dly, That in the torrid zone, unless there is 

 some particular obstacle, the flood tide comes 

 regularly at the same time to all places under the 

 same meridian. On the contrary, in the tempe- 

 rate zones it comes sooner to a lower than to a 

 higher latitude ; but beyond 65 of latitude the 

 tide is not sensible. 



The monthly period is distinguished, 1st, by 

 this circumstance, that at the new and full moons 

 the tides rise much higher than at other periods ; 

 and these are called spring tides ; and when the 

 moon is in the quarters, the tides are lowest, and 

 are called neap tides. The new and full moons 

 are called the syzigies, the quarters, the quadra- 



