PRODUCED BY UNEQUAL ATTRACTION. 



the earth, there would assuredly be no tides. "We are to look for 

 the cause, then, not in the attraction of the moon, but in the 

 inequality of its attraction on different parts of the earth. The 

 greater this inequality is, the greater will be the tides. Hence, 

 as the moon is subject to a slight variation of distance from the 

 earth, it will follow, that when it is at its least distance, or at the 

 point called perigee, the tides will be greatest ; and when it is at 

 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 attraction. 



It will doubtless occur to those who bestow on these observa- 

 tions 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 attraction 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 hundred 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 increased distance. The consequence 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, 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 ine- 

 quality is, the less will be the tides. 



o. The rate at which the attraction decreases with the increase 

 of distance being clearly understood, nothing can be more easy of 

 solution than the question of the difference between the influences 

 of the sun and the moon in raising a tide. 



The distance, M o, of the moon from the earth's centre is in 

 round numbers sixty semi-diameters of the earth. Its distance, 

 M H, from the nearest part of the earth's surface is therefore fifty- 

 nine semi- diameters of the earth. Now since its attraction upon 

 the entire solid mass of the earth is the same as if it were 

 collected at the centre, o, and since we may regard its attraction 



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