OF TIDES. 329 



it the level of the Mediterranean also ; since the floor of the cathe- 

 dral at Pvevenna ;s now several feet lower with respect to the sea 

 than it is supposed to have been formerly, and some steps have been 

 found in the rock of Malta, apparently intended for ascending it, 

 which are at present under water. 



[Young. 



Tor other valuable explanations of the flux and reflux of the 

 sea, the reader may consult the very excellent paper of Halley in 

 the Philosophical Transactions, year 1697 3 the prize essays of Ca- 

 valleri, Bernouilli, Maclaurin, and Euler. Lalande, Traife du Flux 

 et Reflux ; and La Place, Mechanique Celeste. As also Dr. Ro- 

 bison's very excellent paper on this subject printed in the Encyclo- 

 pedia Bjitannica ; in which he observes, that the smallest solar 

 retardation of the tides is to the greatest, as the difference of the 

 solar and lunar influence is to their sum ; that is, from Dr. Maske- 

 lyne's ^observations at St. Helena, as 37 to 8? ; whence the suu's 

 effect is to that as 2 to 4.96". 



2. Hypothesis of St. Pierre y concerning the Tides^ compared 

 uilh the common Doctrine. 



By Samuel Woods, Esq. 



THE tides are two periodical motions actuating the ocean (called 

 the flux and reflux, or ebb and flow), which succeed each other 

 alternately, at an interval of about six hours ; the period of a flux 

 and reflux being, upon an average, 12 hours 24 minutes, the double 

 of which, 24 hours 48 minutes, corresponds to that of a lunar day, 

 or the time elapsing between the moon's passing a meridian and 

 coming to it again. These alternate elevations and depressions of 

 the ocean so- exactly correspond with the course of the sun and 

 moon, as to time and quantity, that the influence of those lumina- 

 ries lias in all ages been considered as the cause of their produc- 

 tion ; but it was reserved for modern times to ascertain the prin- 

 eiple of their la\v.s, and to calculate, with precision, the effects 

 produced by the different situations of the sun and moon, and the 

 proportions of their power. This principle is no other than gravi- 

 tation. It is evident that, if the earth were entirely fluid and quies- 

 its particles, by their mutual gravity, would form the whole 



