LE TERRIER AND ADAMS* PLANET. 



and more fully, by thtJse of his late relative M. Auguste Struve, 

 in 1848-9. ^ 



Prom these observations it appears, that the distance of the satel- 

 lite from the planet at its greatest elongation subtends an angle of 

 18" at the sun ; and since the diameter of the planet subtends an 

 angle of 2 "-8 at the same distance, it follows, therefore, that the 

 distance of the satellite from the centre of the planet is equal to 

 thirteen semidiameters of the latter. 



The mean daily angular motion of the satellite round the centre 

 of the planet is, according to the observations of Struve, 61 0% 2625, 

 and, consequently, the period of the satellite is 

 Sfirt 



or 5 d 21 h l8 m , a result which is subject to an error not exceeding 

 5 minutes. 



If the semidiameter of the planet be 18750 miles, the actual 

 distance of the satellite is 



18750 X 12 = 225000 miles, 



being a little less than the distance of the moon from the earth's 

 centre. 



22. If it excite surprise that the dimensions of a globe so 

 enormously distant from the earth as that of Neptune should be 

 so exactly and so easily measured, it will not create less astonish- 

 ment when we affirm that the mass of matter in that globe can be, 

 and has been weighed, and not only weighed, but weighed with 

 as much or even more precision than that which is attained by 

 the chemist in the operations conducted upon the small masses of 

 matter under his hands. 



What then, it will naturally enough be asked, can be the form 

 and structure of the balance, by which an operation so wonderful 

 can be performed ? 



Let us see whether we cannot explain this. 



If a mass of matter attached to the extremity of a string, 

 the other extremity of which is attached to a fixed point, 

 be whirled round in a circle, of which that fixed point is the 

 centre, the string will be, as every one knows, stretched with 

 a certain force, and that this force will be greater and greater 

 as the velocity with which the body is whirled is increased. 

 Now the moon whirls round the earth with just such a circular 

 motion, and if it were connected by a string with the centre of 

 the earth, that string would be stretched with a force depending 

 upon the velocity of the moon's motion ; but since no such string 

 exists, something else must exist which will exercise the same 

 force upon the moon as the string would, and that something is 

 the earth's attraction. 

 188 



