1^6 Examination of St. Pierre's Hypothe/ts 



fequence of the earth's motion on her axis, the mod elevated 

 part of* the water is carried beyond the moon in the direction 

 of the rotation, and continues to rii'e after it has pafied di- 

 rectly under the moon, not attaining its greater! elevation till 

 it has got about half a quadrant further. It continues alfo to 

 defcend, after it has palled at 90° diftance from the point below 

 the moon,toa like diftance of about halfaquadrant; and there- 

 fore in open fcas, where the water flo'vs freely, the time of 

 high water does not exactly coincide with the time of the 

 moon's coming to the meridian, but is fome time after. 

 Befides, the tides do not always anfwer to the fame diftance of 

 the moon from the meridian, mice they are varioufly affected 

 by the fun's action, which brings them on fooner when the 

 moon is in her firft and third quarters, and keeps them back 

 later when fhe is in her fecond and fourth : becaufe, in the 

 former cafe, the tide railed by the fun alone would be earlier 

 than the tide railed by the moon: in the latter cafe, later. 



We have hitherto confide red the moon as the principal 

 agent in producing tides, but it is obvious that the inequality 

 of the fun's action muft produce a fimilar effect ; fo that, in 

 reality, there are two tides every natural day occafioned by 

 the fun, as well as two tides every lunar day occafioned by 

 the moon, and fubject to the fame laws: on account, how- 

 ever, of the fun's immenfe diftance, his action is confider- 

 ablv. inferior to that of the moon. By comparing the fpring 

 and neap tides at the mouth of the Avon, below Briftol, Sir 

 Ifaac Newton calculates the proportion of the moon's force 

 to the fun's as 9 to 2 nearly. Dr. Hoffley, in his edition of 

 the Principia, eflinrates it as 5,0469 to 1 ; and, confidering 

 the elevation of the waters bv this force as an effect fimilar 

 to the elevation of the equatorial above the polar parts of the 

 earth, it will be found that the moon is capable of producing 

 an elevation of about ten feet, the fun of about two feet j 

 which correfponds pretty nearly to experience. 



In order to understand the caufe of fpring and neap tides, 

 we muft confider, that the moon, revolving round the earth 

 in an elliptiq orbit, approaches nearer and recedes further 

 from if, than her mean diftance, in every revolution or lunar 

 month. When neareft, her attraction is ftronge'ft, and vice 



verfd ; 



