I. A Letter to the Right honourable George 
Rarl of Macclesfield concernmg an appa- 
rent Motion obferved in fome of the fixed 
Stars ; by James Bradley D. D. Aflronomer 
Royal , a?id F. R. S, 
Read at a Meeting 
of the Royal Society, 
Fcbr. 14. 1747. 
T ' 
My Lord, 
HE great Exaflnefs, with 
which Inftruments are now 
conftru&ed, hath enabled 
the Aftronomers of the 
prefent Age to difeover feveral Changes in the Po- 
rtions of the heavenly Bodies 5 which, by reafon of 
their Smallnefs, had efcaped the Notice of their Pre- 
deceffors. And altho’ the Caufes of fuch Motions have 
always fubfitted, yet Philofophers had not fo fully 
confider’d, what the Effefls of thofe known Caufes 
would be, as to demonftrate a priori the Fhano- 
mena they might produce; fo that Theory itfeif is 
here, as well as in many other Cafes, indebted to 
Practice, for the Difcovery of fome of its mod ele- 
gant Dedu&ions. This points out to us the great 
Advantage of cultivating this , as well as every other 
Branch of Natural Knowledge, by a regular Series 
of Obfervations and Experiments. 
The Progrefs of Aftronomy indeed has always 
been found, to have fo great a Dependence upon 
A accurate 
