TIDES. 57 



is rather nice for being admitted into a short popular 

 sketch of the year. 



The effect of gravitation toward the luminaries, al- 

 though a very complicated matter in its details, is far 

 more simple in principle. Whenever the sun or moon 

 is vertical, it acts as a power pulling up the atmosphere 

 and the ocean from the earth, and depressing them at 

 the circumference of the hemisphere of which the ele- 

 vated point is the centre ; and as the fluid parts of the 

 two hemispheres balance each other, there is a similar 

 elevation at the point directly opposite to the luminary. 

 One of these follows the sun in his annual apparent 

 path ; and the other, which is the greater one, follows 

 the moon in its monthly ^changes. Every one must 

 have observed that the moon has a motion eastward, by 

 which it passes from the sun round the whole heavens 

 to the sun again, in between 29 and 30 days; and that, 

 when on the same point of the compass with regard 

 to the sun, it is sometimes nearer the horizon, and some- 

 times farther from it; or, as we say in common lan- 

 guage, sometimes on " a low course," and, at other 

 times, on a " high." It is evident that whatever part 

 of the apparent circumference of the heavens is between 

 the places of the sun and moon, there will be the same 

 part of the circumference of the earth between the 

 points at which they cause the elevations or tides, in 

 the ocean and the atmosphere; and that, whatever 

 may be the position of this distance with regard to 

 north or south in the heavens, the distance of the two 

 tides will be the same on the earth. It is further evi- 

 dent that, as each of the luminaries produces two tides, 

 the one diametrically opposite to the other, a tide pro- 



