92 DISSERTATION SECOND. [pai*i*i 



earth, the force of gravity at that distance must be re- 

 duced to the StiOOth part of what it is at the earth's 

 surface. Was the deflection of the moon then from the 

 tangent of her orbit, in a second of time, just the 3600th 

 part of the distance which a heavy body fails in a second 

 at the surface of the earth ? This was a question that 

 could be precisely answered, supposing the moon's dis- 

 tance known not merely in semidiameters of the earth 

 but in feet, and her angular velocity, or the time of her 

 revolution in her orbit, to be also known. 



" In this calculation, however, being at a distance from 

 books, he took the common estimation of the earth's 

 circumference that was in use before the measurement of 

 Norwood, or of the French Academicians, according to 

 which, a degree is held equal to 60 English miles. This 

 being in reality a very erroneous supposition, the result 

 of the calculation did not represent the force as adequate 

 to the supposed effect ; whence Newton concluded that 

 some other cause than gravity must act on the moon, 

 and on that account he laid aside, for the time, all farther 

 speculation on the subject. It was in the true spirit of 

 philosophy that he so readily gave up an hypothesis, in 

 which he could not but feel some interest, the moment 

 he found it at variance with observation. He was sensi- 

 ble that nothing but the exact coincidence of the things 

 compared could establish the conclusion he meant to de- 

 duce, or authorize him to proceed with the superstruc- 

 ture, for which it was to serve as the foundation. 



It appears, that it was not till some years after this, 

 that his attention was called to the same subject, by a 

 letter from Dr. Hooke, proposing, as a question, To de- 

 termine the line in which a body let fall from a height 

 descends to the ground, taking into consideration the 

 motion of the earth on its axis. This induced him to 



