4 HISTORY OF 



led by any other, the sun must quickly have attracted all the 

 bodies of our celestial system to itself; but it is equally coun- 

 teracted by another power of equal efficacy ; namely, a pro- 

 gressive force, which each planet received when it was 



the earth, it orcuired to him, that as this power 13 not sensibly diminished at any 

 distance to which we can recede frona the earth's centre, there seemed reason 

 to conclude that it extended much farther than was commonly supposed, and 

 even might extend as far as the moon ; and if this were true, he concluded 

 that her motion would be influenced by it, and that probably it was this 

 very force which retained her in her orbit. However, although the force of 

 gravity be not sensibly less at the tops of the highest mountains, than at the 

 ordinary level of the earth's surface, he conceived it to be very possible, that 

 at so great a distance as that of the moon, it might be considerably different. 

 To make an estimate of what might be the degree of the diminution, he con- 

 sidered that if the moon be retained in her orbit by the force of gravity, no 

 doubt the primary planets are carried round the sun by a like power ; and by 

 comparing the periods of the several planets with their distances from the 

 6un, he found that if any power hke gravity kept them in their orbits, its 

 strength must decrease in proportion, as the squares of the distances increase : 

 but in making this conclusion, he supposed that the orbits of the planets 

 were circles, having the sun in their centre, from which figure the greater 

 part of them do not much differ. Supposing, therefore, the force of gravity 

 to extend as far as the moon, and to decrease according to this ratio, he com. 

 puted whether that force would be sufficient to keep the moon in her orbit ; 

 but having no books at hand, by which he might ascertain the true magni- 

 tude of the earth, he was obliged to employ in his calculation the erroneous 

 estimate at that time commonly received among geographers and seamen, 

 namely, that a degree of latitude on the earth's surface was 60 English miles. 

 Now, as the degree contains in reality about 69J miles, his computation of 

 course did not agree with the phenomena ; and on this account, he laid aside 

 at that time all further consideration of the subject. Some years after, in con. 

 sequence of a letter he received from Dr Hooke, he investigated the nature 

 of the path which a body would describe, if it were let fall from any high 

 place, taking into account the rotation of the earth ; and on this occasion, he 

 resumed his former train of reflections concerning the motion of the moon. 

 He had now, however, the advantage of knowing pretty nearly the exact 

 magnitude of the earth, in consequence of tlie measurement of an arc of the 

 meridian made in France by Picard ; and he had the inexpressible satisfaction 

 of finding that his calculation agreed exactly with what it ought to be, if the 

 opinion he had formed was correct. He therefore concluded that his conjee- 

 ture was correct, and that the moon was really kept in her order by the force 

 of gravity, which decreased according as the square of the distance increased, 

 agreeably to what he had supposed. It is said, that as the calculation drew 

 to a close, the mind of Newton was so much agitated by the importance of 

 the discovery he was on the point of making, that he was obliged to desire 

 a friend to finish them. This is not to be wondered at, when we considiv 

 the great revolution which he foresaw he was about to produce in the opt- 

 ions of mankind, and the immense fabric of science that might be built ' n 

 his discovery. 



