In the year [1666] he retired again from Cambridge to his mother in & whilst he was musing in a garden it came into his thought that the power of gravity (which made an apple fall from the tree to the ground) was not limited to a certain distance from the earth, but must extend much farther than was usually thought — Why not as high as the Moon said he to himself, & if so that must influence her motion & perhaps retain her in her orbit. Whereupon he fell a calculating what would be the effect of that supposition being absent from the books & taking the common estimate in use among Geographers & our seamen before Norwood had measured the earth, that to 60 Engish miles were contained in one degree of . His computation did not agree with his Theory and inclined him then to entertain a notion that together with the power of gravity there might be a mixture of that force which the moon would have if it was carried along a vortex, but when the Tract of Picard of the measure of the earth came out shewing that a degree was about 69 1/2 English miles, he began his calculation anew & found it perfectly in agreement to his Theory.