[ 125 ] 
sether), in the manner he has propofed it, can never 
be fufficient for that purpofe. Not that I intend im- 
pudently to blame that truly great man, but to en- 
deavour to inveftigate the caufe of reflection and re- 
fraction of light, by means not made fenfible to 
mankind in his time. I may alfo abferve, that, if 
his hypothefls had not preceded his calculation, the 
world had never been obliged to him for his great 
doCtrine of attraction : for at firfl: he only fuppofed, 
that the power of gravity may extend to the moon ; 
and that the may be retained in her orbit thereby i 
but when he made his calculation (having miflaken 
flxty Englifh miles for the meafure of a degree on 
the furface of our earth), he thought, that fome other 
power mull: co-operate with that of gravity, to keep 
the moon in her orbit ; and' for that time laid his 
attempt afide; till Picart, in France, meafured a 
degree, and. found, that it 'contained about fixty-nine 
Englifh miles and an half; and then, or fome years 
after, our great author having refumed his compu- 
tation, found, that the power of gravity was alone 
fufficient to keep the moon and planets in their or- 
bits ; and thereby proved his hypothefls. 
Indeed, in works of invention, I cannot fee how 
it is poffible to proceed without fomething hypo- 
thetical j for the fuppofltion muft ever precede the 
proof. I am not for eftablifhing hypothefes as prin- 
ciples ; but if an hypothefls is rationally founded, I 
think it is worth the while to inquire into the truth 
of it ; and thus invention may proceed. 
I ffiall not fay any thing in j unification of my own, 
becaufe I have not any vanity to gratify ; for if my let- 
ter has not any thing worth the Society’s or your accept- 
ance^ 
