OF TIDES. 331 



to descend, after it lias passed at 90 distance from the point below 

 the moon, to a like distance of about half a quadrant ; and there- 

 fore in open seas, where the water flows freely, the time of high 

 water does not exactly coincide with the time ot the moon's coming 

 to the meridian, but is some time after. Besides, the tides do not 

 always answer to the same distance of the moon from the meridian, 

 since they are variously affected by the sun's action, which brings 

 them on sooner when the moon is in her first and third quarters, 

 and keeps them back later when she is in her second and fourth : 

 because, in the former case, the tide raised by the sun alone tvould 

 be earlier than the tide raised by the moon : in the latter case, 

 later. 



We have hitherto considered the moon as the principal agent in 

 producing tides, but it is obvious that the inequality of the sun's 

 action must produce a similar effect ; so that, in reality, there are 

 two tides every natural day occasioned by the sun, as well as two 

 tides every lunar day occasioned by the moo, and subject to the 

 same laws : on account, however, of the sun's immense distance, 

 his action is considerably inferior to that of the moon. By com- 

 paring the spring and neap tides at the mouth of the Avon, below 

 Bristol, Sir Isaac Newton calculates the proportion of the moon's 

 force to the sun's as nine to two nearly. Dr. Horsley, in his edition 

 of the Principia, estimates it at 5,0409 to 1 ; and, considering the 

 elevation of the waters by this force as an effect similar to the ele- 

 vation of the equatorial above the polar parts ot the earth, it will 

 be found that the moon is capable of producing an elevation of 

 about ten feet, the sun of about two feet ; which corresponds 

 pretty nearly to experience. 



In order to understand the cause of spring and neap tides, we 

 must consider, that the moon, revolving round the earth in an ellip- 

 tic orbit, approaches nearer and recedes further from it, than her 

 mean distance, in every revolution or lunar month. When nearest, 

 her attraction is strongest, and vice versa : when both luminaries 

 are in the equator, and the moon in perigeo, the tides rise highest, 

 particularly at opposition and conjunction : at the change, when 

 the attractive forces of the sun and moon are combined, the tide is 

 raised to a greater height : at the full, when the moon raises the 

 tide under and opposite to her, the sun, acting in the same line, 

 raises the tides under and opposite to him, whence their conjunct 



