OF TIDES. 317 



other method, with which we are acquainted, thau the galvanic 

 energy. Water flowing through minerals of such different natures 

 must dissolve very different proportions of salt. In beds of the first 

 kind, it mav, in certain circumstances, become saturated with salt^ 

 and will always dissolve so much as to be entitled to the name of a 

 salt spring. In beds of the second kind, it will dissolve only a very 

 minute quantity of salt; and, in those of the third kind, if any such 

 exist of any considerable extent, it will dissolve none at all. Hence 

 waters, when they began to flow into the sea aivd into lakes, would 

 contain very different proportions of salt, according to the nature of 

 the country through which they flowed; and hence different lak.es, 

 and different parts of the sea, would possess different degrees of 

 saltness, and would increase in saltness at very different rates. Fi- 

 nally, this increase of saltness of the sea, if it takes place at all, must 

 do so with inconceivable slowness ; for the specific gravity of sea- 

 water has never been observed to increase since the first time that it 

 ever was taken, which is more than a century ago. 



[Thomson. Phil. Trans. 1715.] 



SECTION VT. 



On the Tides. 



1. Etytanation of the received or Newtonian System. 



THE height of the surface of the sea at any given place is observ- 

 ed to be liable to periodical variations, which are found to depend 

 on the relative position of the moon, combined in some measure 

 with that of the sun. These variations are called tides ; they were 

 too obvious to escape the observation even of the ancients, who in- 

 habited countries where they are least conspicuous : for Aristotle 

 mentions the tides of the northern seas, and remarks that they vary 

 with the moon, and are less conspicuous in small seas than in the 

 ocean : Cresar, Strabo, Pliny, Seneca, and Macrobius give al$9 

 tolerably accurate accounts of them. 



There are in the tides three orders of phenomena which are se- 

 parately distinguishable : the first kind occurs twice a day, the 

 second twice a month, and the third twice a year. Every day, 

 about the time of the moon's passing over the meridian, ora certain 

 number of hours later, the sea becomes .elevated above its mean 



