THE EARTH AND MOON IN MOTION. 47 



fathoms deep, this force will only raise the surface of 

 the water about -j-^ths of an inch ; in other words, 

 it will cause no appreciable tide. But the tangential 

 force, although at its maximum (45 at either side of 

 the attracting centre) it be only equal to three-fourths 

 of the maximum normal, yet, because it does not op- 

 pose gravity, but pulls horizontally, or at right angles 

 to that force, produces the whole of the effect of the 

 lunar tide. None of its little strength is wasted in 

 contending with a power ten or twelve million times 

 its superior, but every atom is employed in pulling 

 the waters along the surface towards a point of con- 

 vergence. In this way the waters are, as it were, 

 heaped up at that point to the calculated amount of 

 58 inches under the moon. 



In thus stating the attractive force of the moon 

 on the terrestrial ocean, we have, for simplicity sake, 

 regarded both earth and moon as being immovably 

 fixed in space. But we must remember that these 

 bodies revolve on their axes and move in orbits, and, 

 consequently, are for ever changing their relative posi- 

 tion to each other, and the relative position of points 

 on their surface. The earth, by its daily revolution 

 on its axis, brings, hour by hour, one meridian after 

 another vertically under the moon, while, by its an- 

 nual progress in its orbit, and by the moon's monthly 

 progress in hers, the forces respectively exercised by 

 sun and moon are alternately diminished by opposi- 

 tion, and increased by conjunction. The result is, 

 that the lifting power of the moon and sun has 

 changed its place before the waters vertically under 



