THE TIDES. 



215 



the other will be in the proportion of thirty to thirty-one ; and the difference I 

 of the attraction will be in the proportion of the squares of those numbers. * 

 In the case, therefore, of the sun, the difference of the distances to the whole, 

 then, is in proportion of one to one hundred and twenty thousand ; whereas, 

 in the case of the moon it is in the proportion of one to thirty. 



Still, although the difference of the attractions of the sun on different sides 

 of the earth is infinitely less than those of the moon, it is not imperceptible ; 

 and the sun does actually produce sensible tides on opposite sides of the earth, 

 as the moon does. When the sun and moon, therefore, are either on the same 

 side of the earth, or on the opposite sides of the earth ; in other words, when 

 it is new or full moon, then their effects in producing tides are combined, and 

 the spring tide is produced ; the height of which is equal to the solar and lunar 

 tides taken together. These positions are represented in the preceding dia- 

 gram, where S is the sun, A the moon when new, and B the moon when full. 

 Hence it is that, at the epochs of new and full moons, we have tides of ex- 

 traordinary elevation, called spring tides. 



On the other hand, when the sun and moon are separated from each other 

 by a distance of one fourth of the heavens, that is, when the moon is in the 

 quarters, the effect of the solar tide has a tendency to diminish that of the lunar 

 tide. This position is represented in the annexed diagram. 



If Q and R represent positions of the moon, and 5 that of the sun at the 

 epochs of the quarters, then the lunar tides would cause the waters to be col- 

 lected at Z and N; whereas the solar tides would take place at B and F. The 

 tendency, therefore, of the sun, would be to draw the water from Z and N 

 toward B and F; and to the same extent would diminish the effect of the 

 moon's attraction. The lunar tides would be less, under these circumstances, 

 than in other positions of the moon. These have, therefore, been called the 

 neap tides. 



If physical effects followed immediately, without any appreciable interval 

 of time, the operation of their causes, then the tidal wave produced by the 

 moon would be on the meridian of the earth directly under and opposite to 

 that luminary ; and the same would be true of the solar tides. But the waters 

 of the globe have, in common with all other matter, the property of inertia, and 

 it takes a certain interval of time to impress upon them a certain change of 

 position. Hence it follows that the tidal wave produced by the moon is not 

 formed immediately under that body but follows it at a certain distance. In 

 consequence of this, the tide raised by the moon does not take place for 2 or 3 

 hours after the moon passes the meridian ; and as the action of the sun is still 



