[ *9 ] 
IV. Of the Moon’s Di fiance and Parallax': 
A Letter to Andrew Reid, Efq\ from P. 
Murdoch, D. D. a?id F. R. S. 12 Nov. 
1763. 
S I R, 
Have at your defire wrote out what I 
was mentioning to you in our la ft con- 
verfation ; of an eafy rule for determining the Moon’s 
diftance, from the received theory of central forces : 
which I wifh may merit your approbation : it will at 
leaft ferve as a teftimony of the efteem and regard 
with which I am, &c. 
Read Jan. 
1764. 
20 , 
I. 
Sir Ifaac Newton, inveftigated the law of gravitation, 
in the duplicate ratio of the diftance of the central 
body inverfeiy, from the following data. 
1. The length of a fimple pendulum which vi- 
brates in one fecond of time, gave him, by Huygens’ 
theorem, a determinate meafure of the force of gravi- 
ty, at the place of obfervation. And, by his own 
theory, he could thence infer the like meafure for 
any other place, of a given latitude 
2. The Earth’s femidiameter was computed from 
the Abbe Picard’s meafure of a degree of the terreftrial 
meridian. 
* See one of the Eflays prefixed to Bufching’s Geography. 
5. The 
