VOL. XLV.] PHILOSOPHICAL TRANSACTIONS. 4'2l 



great care, the true angle answering to them: and. after various trials, I 

 thoroughly satisfied myself, both of the equality of the threads of the screw, and 

 of the precise number of sounds corresponding to them. 



But though these points could be settled with great certainty, I was neverthe- 

 less obliged to make one supposition ; which perhaps to some persons may seem 

 of too great moment in the present inquiry, to be admitted without an evident 

 proof from facts and experiments. For I suppose, that the line of collimatiou 

 of my telescope has invariably preserved the same direction, with respect to the 

 divisions on the arc, during the whole course of my observations. And in- 

 deed it was on account of the objections, which might have been raised against 

 such a postulate, that I thought it necessary to continue my series of observa- 

 tions for so many years, before I published the conclusions, which I shall at pre- 

 sent endeavour to draw from them. 



Whoever compares the result of the several trials that have been made by the 

 gentlemen of the Academy of Sciences, for determining the zenith point of their 

 sector, since their return from the north ; will, I presume, allow that mine is 

 not an vmreasonable or precarious supposition : since it is evident, from their 

 observations, that the line of collimation of that instrument underwent no sen- 

 sible change in its direction, during the space of more than a whole year : though 

 it was several times taken down and set up again, in different and remote places; 

 whereas mine has always remained suspended in the same place. 



But besides such a strong argument for the probability of the truth of my sup- 

 position, I have the satisfaction of finding it actually verified by the observations 

 themselves ; which plainly prove, that at the end of the full period of the devi- 

 ations which I am going to mention, the stars are found to have the same 

 positions by the instruments, as they ought to have, supposing the line of 

 collimation to have continued unaltered from the time when I first began to 

 observe. 



I have already taken notice, in what manner this phenomenon discovered it- 

 self to me at the end of my first year's observations, viz. by a greater apparent 

 change of declination in the stars near the equinoctial colure, than could arise 

 from a precession of 50" in a year : the mean quantity now usually allowed by 

 astronomers. But there appearing at the same time an effect of a quite contrary 

 nature, in some stars near the solstitial colure, which seemed to alter their de- 

 clination less than a precession of 50" required; I was thereby convinced that all 

 the phenomena, in the different stars, could not be accounted for, merely by 

 supposing that I had assumed a wrong quantity for the precession of the equi- 

 Doctial points. 



At first I had a suspicion that some of these small apparent alterations in the 

 places of the stars might possibly be occasioned by a change in the materials, or in 



