424 RISE AND PROGRESS OF 



in reviewing briefly the principal steps which led to its complete 

 discovery. 



In the year 1666, the visitation of the plague compelled the 

 young philosopher, then about the age of twenty-four, to leave 

 Cambridge and retire into the country : and we are told that as 

 he sat one day in a garden, meditating on the wonders of nature, 

 his attention was arrested by the falling of the apples from the 

 trees ; and it at once occured to him that, as the force of gravity 

 appeared to extend from the earth's surface to the tops of trees 

 and of houses, and even to the summits of the highest hills, 

 without any apparent diminution, might it not reach even to the 

 moon itself ? And may not our satellite be retained in its orbit 

 by this very force, combined with an original impulse just as a 

 projectile at the earth's surface is forced to describe a curve under 

 the same influences ? 



But another question here arose. Although the force of 

 gravity does not appear to vary sensibly in the limited distance 

 from the earth's surface at which we can make observations, yet, 

 at the distance of the moon from the earth, this force (supposing 

 it to extend so far) may be greatly diminished ; and if so, it 

 would be necessary to ascertain the law of the diminution, before 

 we could submit the question to calculation. Here the mind of 

 Newton made another vast stretch. If the moon be retained in 

 her orbit round the earth, by the attraction of the latter body, 

 may not the earth, also, and the other planets, be held in their 

 paths round the sun by the action of a similar force tending to 

 that luminary? Now, it was not difficult to prove that the 

 centripetal forces of bodies, revolving in circles round a common 

 centre, must be inversely as the squares of their distances from that 

 point, provided that (as is the case with the earth and planets) 

 the squares of their periodic times varied as the cubes of their 

 distances from the centre. If this law, therefore, be extended by 

 analogy to terrestrial gravity, at the distance of the moon (which 

 is about sixty semi-diameters of the earth), that force must be less 

 than at the earth's surface, in the duplicate ratio of sixty to one. 

 Is then the space through which the moon is bent in a second, 

 from the rectilinear direction towards the earth, the 3600th part 

 of that through which a body will fall, in the same time, at the 

 earth's surface ? 



Here was a question that could at once be submitted to 



