418 PHILOSOPHICAL TRANSACTIONS. [aNNO 1747-8. 



to practice for the discovery of some of its most elegant deductions. This 

 points out to us the great advantage of cultivating this, as well as eveiy other 

 branch of natural knowledge, by a regular series of observations and experi- 

 ments. 



The progress of astronomy indeed has always been found to have so great a 

 dependance on accurate observations, that till such were made it advanced but 

 slowly: for the first considerable improvements that it received, in point of 

 theory, were owing to the renowned Tycho Brahe ; who far exceeding those 

 that had gone before him in the exactness of his observations, enabled the saga- 

 cious Kepler to find out some of the principal laws relating to the motion of the 

 heavenly bodies. The invention of telescopes and pendulum clocks affording 

 proper means of still further improving the praxis of astronomy ; and these being 

 also soon succeeded by the wonderful discoveries made by our great Newton, as 

 to its theory ; the science, in both respects, had acquired such extraordinary ad- 

 vancement, that future ages seemed to have little room left for making any great 

 improvements. But in fact we find the case to be very different , for as we ad- 

 vance in the means of making more nice inquiries, new points generally offer 

 themselves that demand our attention. The subject of the present letter is a 

 proof of the truth of this remark ; for as soon as I had discovered the cause, and 

 settled the laws of the aberrations of the fixed stars, arising from the motion of 

 light, &c. of which an account is given in N° 406 of the Philos. Trans. ; my 

 attention was again excited by another new phenomenon, viz. an annual change 

 of declination in some of the fixed stars ; which appeared to be sensibly greater 

 about that time, than a precession of the equinoctial points, of 30" in a year, 

 would have occasioned. The quantity of the difference, though small in itself, 

 was rendered perceptible, through the exactness of my instrument, even in the 

 first year of my observations ; but being then at a loss to guess, from what cause 

 that gi-eater change of declination proceeded, I endeavoured to allow for it in my 

 computations, by making use of the observed annual difference, as mentioned in 

 the same Transaction. 



From that time to the present I have continued to make observations at Wan- 

 sted, as opportunity offered, with a view of discovering the laws and cause of 

 this phenomenon : for, by the favour of my very kind and worthy friend Mat- 

 thew Wymondesold, Esq. my instrument has remained where it was first erected ; 

 so that I have been able, without any interruption, which the removal of it to 

 another place would have occasioned, to proceed on with my intended series of 

 observations for the space of '20 years : a term somewhat exceeding the whole 

 period of the changes that happen in this phenomenon. 



When I shall mention the small quantity of the deviation which the stars are 

 subject to, from the cause that I have been so long searching after ; I am appre- 



