MOON OF NEPTUNE. 



It is proved by theory, and verified by experiment, that the 

 force with which the string connecting such a body with the 

 centre would be stretched, would increase in the same proportion 

 as the square of the velocity of the revolving body increases, other 

 things being the same. If, therefore, the moon's velocity in 

 whirling round the earth were twice what it is, the force with 

 which it would react against the earth's attraction, would be four 

 times greater than it is, and as the earth's attraction would still 

 be the same, the moon, in that case, would escape from her orbit, 

 and would depart to a greater distance from the earth. If, on the 

 other hand, the moon moved with half its present velocity, the 

 force with which it would stretch the string would be four times 

 less than it is, and being then less than the earth's attraction upon 

 it, the moon would fall towards the earth to a much less distance. 

 But since the moon neither departs to greater distances nor 

 approaches to less distances, it follows that the attraction of the 

 earth upon it is neither more nor less than that with which a string 

 would be stretched which would connect the moon with the centre 

 of the earth. 



Now we have seen by what has been explained, that Neptune, 

 as well as the earth, has a moon, and moreover, that this moon 

 whirls round Neptune at a distance a little less than that at which 

 our moon moves round the earth. To simplify the question, let 

 us suppose for a moment that these distances are equal. If then 

 Neptune's moon had the same velocity as ours, a string connecting 

 it with Neptune would be stretched with the same force as that 

 with which a string would be stretched connecting the moon with 

 the earth ; and since the attraction of the two planets on their 

 respective moons is represented by the tension of such string, it 

 would follow, in that case, that the two planets would exert equal 

 attractions on moons revolving at equal distances from them. But 

 since these attractions depend only on the quantities of matter in 

 the two planets, or what is the same on their weights, it would, 

 in that case, follow, that the weight of Neptune would be equal 

 to that of the earth. 



But Neptune's moon, instead of revolving in the same time as 

 ours, revolves as it appears, by what has been explained, in 5-8763 

 days. Now we have just explained that the force with which the 

 ^whirling body would stretch a string increases, other things being 

 the same, in the proportion of the square of its velocity, and 

 since our moon takes 27-322 days to make a complete revolution, 

 while that of Neptune makes a revolution in 5-876 days, the 

 velocity of the latter will be greater than that of the former in 

 the proportion of 5876 to 27322 ; and consequently the forces 

 with which they will react upon the planetary attractions which 



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