214 



THE TIDES. 



when it is the greatest distance, or at the point called apogee, the tides will be least ; 

 not because the entire attraction of the moon in the former case is greater than 

 in the latter, but because the diameter of the globe bearing a greater proportion 

 to the lesser distance than the greater, there will be a greater inequality of at- 

 traction. 



It will doubtless occur to those who bestow on these observations a little 

 reflection, that all which we have stated in reference to the effect produced by 

 the attraction of the moon upon the earth, will also be applicable to the attrac- 

 tion of the sun. This is undoubtedly true ; but in the case of the sun the 

 effects are modified, in some very important respects, as will readily be seen. 

 The sun is at four hundred times a greater distance than the moon, and the 

 actual amount of its attraction on the earth would, on that account, be one hun- 

 dred and sixty thousand times less than that of the moon ; but the mass of the 

 sun exceeds that of the moon in a much greater ratio than that of one hundred 

 and sixty thousand to one. It therefore possesses a much greater attracting 

 power in virtue of its mass, compared with the moon, than it loses by its in- 

 creased distance. The effect is, that it exercises upon the earth an attraction 

 enormously greater than the moon exercises. Now, if the simple amount of its 

 attraction were, as is commonly supposed, the cause of the tides, the sun ought 

 to produce a vastly greater tide than the moon. The reverse is, however, the 

 case, and the cause is easily explained. Let it be remembered that the tides 

 are due solely to the inequality of the attraction on different sides of the earth, 

 and the greater that inequality is, the greater will be the tides, and the less that 

 inequality is, the less will be the tides. 



Now in the case of the sun, its total distance from the earth is one hundred 

 millions of miles, and the difference between its distance from one side of the 

 earth, and from the other, is only eight thousand miles, or about one hundred 

 and twenty thousandth part of the whole distance. The inequality of the at- 

 traction of the sun, therefore, on different sides of the earth will be in the pro- 

 portion of the square of the numbers one hundred and twenty thousand and one 

 hundred and twenty thousand and one to each other, a proportion which it will 

 be evident, is extremely small. But in the case of the moon, the distance 

 of that object being about two hundred and forty thousand miles, or thirty 

 diameters of the earth, the difference between its distance from one side to 



