112 DISSERTATION SECOND. [met h. 



middle of the hemisphere lengthen, in consequence of th« 

 pressure of the columns which are at a distance from the 

 middle point, of which the weight is less diminished, and 

 towards the horizon must even be increased. In the op- 

 posite hemisphere, again, the mass of the earth is more 

 drawn to the moon than the waters of that hemisphere, 

 and their relative tendencies are changed in the same 

 direction, and nearly by the same quantity. If the action 

 of the moon on all the parts of the earth, both sea and 

 land, were the same, no tide whatever would be pro- 

 duced. 



Thus, the same analysis of the force of gravity which 

 explained the inequalities of the moon, was shown by 

 Kewton to explain those inequalities in the elevation of 

 the waters of the ocean to which we give the name of 

 tides. On the principle also explained in this analysis, it 

 is, that the attraction of the sun and moon conspire to 

 elevate the waters of the ocean whether these luminaries 

 be in opposition or conjunction. In both cases the solar 

 and lunar tides are added together, and the tide actually 

 observed is their sum. At the quadratures, or the first 

 and third quarters, these two sides are opposed to one 

 another, the high water of the lunar tide coinciding with 

 the low water of the solar, and conversely, so that the 

 tide actually observed is the difference of the two. 



The other phenomena of the tides were explained in a 

 manner no less satisfactory, and it only remained to in- 

 quire, Whether the quantity of the solar and lunar forces 

 were adequate to the effect thus ascribed to them ? The 

 lunar force there were yet no data for measuring, but a 

 measure of the solar force, as it acts on the moon, had 

 been obtained, and it had been shown that in its mean 

 quantity it amounted to yfg- of the force which retains 

 the moon in her orbit. This last is ^Va of the force of 



