tOMMENTATIO ASTRONOMICA. 1193 



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% were owing lo tlie renowned Tycho Brah6; who far exceeding those Ihat had gone 

 -« before him, in the exncliiess of his observations, enabled Ihe sagacious Kepler lo 

 « find out some of llie' principal laws, relaling to the motion of the heavenly bodies. 

 « The invention of lelescopes and pendulum-clocks affording proper means of still farther 

 « improving the praxis of aslronomy; and Ihese being also soon succeeded by the won« 

 « derfiil discoveries made by our great Newton, as to its theory ; the science , in 

 « both respccls, had acquired such exlraordinary advancemeiit , that future ages seeraed 

 « lo hare liltle room Icft, for making any great improvements. But, in fact, we find 

 «the case to be yery difi'erent ; for, as we advance in the means of roaking more nice 

 « inquiries, new points generally offer thomselves, Ihat demand our attention. The 

 « subject of my present letter to your Lordschip, is a proof of the trulh 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, etc., whereof I gave an account 

 « in n°. 406 of the Philosophical Transactions ; my altenlion was again excited by 

 « another new phaenomenon , 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 equinoclial points of 50" in a year would have occasioned. The quanlily of 

 « the difFerence tho' small in itself, was rendered perceptible, thro' the exactness of my 

 « Instrument, even in the firstyear of my observations; but being ihen at a loss to guess, 

 « from what cause that greater change of decHnation proceeded, I endeavoured to allow 

 '« for it in my computations , by making use of the observed aunual difference , as mea- 

 « tioned in p. 652 of the same Transacdon. " 



« From that time to fhc present, I have conlinued to make observations al Wansted, 

 w as opportunity offered , with a view t)f discovering the laws and cause of this phaeno- 

 « menon. For, by the favour of my very kiud and worthy friend Matthew Wymon- 

 « deso'Id Esq , my instrument has reraaincd, where it Jwas first erected ; so that I have 

 « been able , without any interruption , which the removal of it to another place would 

 w have occasioned, to proceed on with my intended series of observations, for the space 

 « of twenly years: a term soraewhat exceeding the whole period of the changes, that 

 x( happen in this phaenomenon. " 



« When I shall mention the small quanlity of the devialion, which tlie slars are sub- 

 « ject to , from the cause that I have been so long searcbing after ; I am apprehensive , 

 « that I may incur the censure of some persons , for having spent so much time in the 

 <f pursuit of such a seeming trifle : but the candid fovers of science will, I hope , make 

 « due allowance for that natural ardour, with which the mind is urged on towards the 

 « discovery of trnths , in themselves perhaps of small moment, were it not that tfaey 

 ^i tend lo illustrate others of greater use. " 



Bb Dein- 



