BOOK III. CHAP. VIT. 667 



The water of the brackifli rivers on the coaH: yields about i drachm 

 of fea fait from 2 quarts, or i ounce from 4 gallons. 



I have no meafurcmcnt of the quantity contained in a gallon of our 

 fea- water J but it would be curious to try the experiment in the hotter 

 months, and compare with that of the Britifh Channel, whofc produce 

 has been computed at 5 § ounces per gallon. 



SECT. XIII. 



'Tides, Currents, and Mcigfietic F'ariations. 



The centrifugal force of the earth, arifing from its diurnal motion, is 

 computed by Sir liaac Newton, to raife the water at the Equator to the 

 height of 85,472 feet above the water at the Poles ; and the united force 

 of the fun and moon raifes the ocean 10 1 feet; confequently the attrac- 

 tion or gravitation muft be greatefl: in this part, from the greater proxi- 

 mity of thefe two bodies. If the moon were conftituted at the iEqui- 

 nodial, there would be always high water under that circle, and low 

 water at the Poles; and, therefore, the nearer the moon approaches to 

 the i^i^quinoftial, the lefs is the agitation of ocean in that part of the 

 globe. That the tides may have their full motion, the ocean in which 

 they are produced ought to be extended, from Eaft to Weft, a quarter 

 of the great circle of the earth at leaftj becaufe the places where the 

 moon raifes moft, or moft deprefles the water, are at that diftance from 

 one another j hence it appears, that it is only in the great oceans, that 

 fuch tides can be produced ; and why in the large Pacific ocean they ex- 

 ceed thofe in the Atlantic ocean; why the tides are not fo great in the 

 Torrid Zone, where the ocean is narrowed, as in the Temperate Zones on 

 either fide; and why they are fo fmall at iflands that are far diftant from 

 continental fliores. As the tides pafs over fiioals, and run through 

 fl:raits and bays of the fea, their motion becomes various, and their 

 height depends on fo many circumftances, that it is impoffible they fliould 

 be regular. 



Thus the tide at Bermudas fets varioufly, and does not flow above i; 

 feet ; and that only when the fun is in the Southern Tropic; at other 

 times, not above 3 feet. 



At Jamaica, it rarely flows (according to my obfervation) above 1 3 

 inches or 2 feet at mofi; ; though the violence of fea breezes, and Norths, 



4 Q 2 will 



