ZENITH. 



day than when it was obferved before. • Not fufpefting any 

 other caufe of this appearance, we firft concluded, that it 

 was owing to the uncertainty of the obfervations, and that 

 either this or the foregoing were not fo exaft as we had 

 before fuppofed ; for which reafon we propofed to repeat 

 the obfervation again, in order to determine from whence 

 this difference proceeded ; and upon doing it on Dec. 20th, 

 I found that the flar pafled ftill more foutheriy than in the 

 former obfervations. This fenfible alteration the more fur- 

 prifed us, in that it was the contrary tuay from what it would 

 have been had it proceeded from an annual parallax of the 

 ftar ; but being now pretty well fatisfied that it could not 

 be entirely owing to the want of exaftnefs in the obfervations, 

 and having no notion of any thing elfe that could caufe fuch 

 an apparent motion as this in the ftar, we began to think 

 that fome change in the materials, &c. of the inftrument it- 

 felf might have occafioned it. Under thefe apprehenfions 

 we remained fome time ; but being at length fully con- 

 vinced, by feveral trials, of the great exaAnefs of the in- 

 ftrument, and finding by the gradual increafe of the ftar's 

 diftance from the pole, that there muft be fome regular 

 caufe that produced it, we took care to examine nicely, at 

 the time of each obfervation, how much it was : and about 

 the beginning of March, 1726, the ftar was found to be 20" 

 more foutheriy than at the time of the firft obfervation. It 

 now, indeed, feemed to have arrived at its utmoft limit fouth- 

 ward, becaufe in feveral trials made about this time, no fen- 

 fible difference was obferved in its fituation. By the middle 

 of April, it appeared to be returning back again towards the 

 north ; and about the beginning of June, it pafTed at the 

 fame diftance from the zenith as it had done in December, 

 when it was firft obferved. 



" From the quick alteration of this ftar's declination 

 about this time, (it increafing a fecond in three days,) it 

 was concluded, that it would now proceed northward, as it 

 before had gone fouthward of its prefent fituation ; and 

 it happened as was conjeftured ; for the ftar continued to 

 move northward till September following, when it again 

 became ftationary, being then near 20" more northerly than 

 in June, and no lefs than 39" more northerly than it was in 

 March. From September the ftar returned towards the 

 fouth, till it arrived m December to the fame fituation it 

 was in at that time twelve months, allowing for the dif- 

 ference of declination on account of the precefGon of the 

 equinox. 



" This was a fufficient proof that the inftrument had not 

 been the caufe of this apparent motion of the ftar ; and to 

 find one adequate to fuch an effeft feemed a difficulty. A 

 nutation of the earth's axis was one of the firft things that 

 offered itfelf upon this occafion ; but it was foon found in- 

 fufficient ; for though it might have accounted for the 

 change of declination in y Draconis, yet it would not 

 at the fame time agree with the phenomena in other 

 ftars ; particularly in a fmall one almoft oppofite in right 

 afcenfion to 7 Draconis, at about the fame diftance from 

 the north pole of the equator ; for, though this ftar feemed 

 to move the fame way as a nutation of the earth's axis 

 would have made it, yet in changing its declination but 

 about as much as ■;. Draconis in the fame time, (as appeared 

 upon comparing the obfervations of both made upon the 

 fame days, at different feafons of the year,) this plainly 

 proved that the apparent motion of the ftars was not oc- 

 cafioned by a real nutation, fince, if that had been the caufe, 

 the alteration in both ftars would have been near equal. 



" The great regularity of the obfervations left no room 

 to doubt, but that there was fome regular caufe that pro- 



duced this unexpefted motion, which did not depend on the 

 uncertainty or variety of the feafons of the year. Upon 

 comparing the obfervations with each other, it was dit- 

 covered, that in both the fore-mentioned ftars, the apparent 

 difference of declination from the maxima was always nearly 

 proportional to the verfed fine of the fun's diftance from 

 the equinoftial points. This was an inducement to think, 

 that the caufe, whatever it was, had fome relation to the 

 fun's fituation with refpeft to thofe points. But not being 

 able to frame any hypothefis at that time, fufficient to folve 

 all the phenomena, and being very defirous to fearch a little 

 farther into this matter, I began to think of ereSing an 

 mftrument for myfelf at Wanftead, that, having it always 

 at hand, I might with the more eafe and certainty inquire 

 into the laws of this new motion. The confideration, hke- 

 wife, of being able by another inftrument to confirm the 

 truth of the obfervations hitherto made with Mr. Moly- 

 neux's was no fmall inducement to me ; but the chief of 

 all was the opportunity I ftiould thereby have of trying in 

 what manner other ftars were affected by the fame caufe, 

 whatever it was. For Mr. Molyneux's inftrument being 

 originally defigned for obferving y Draconis, (in order, as 

 I faid before, to try whether it had any fenfible parallax,) 

 was fo contrived, as to be capable of but little alteration 

 m its direftion, not above feven or eight minutes of a 

 degree ; and there being few ftars within half that diftance 

 from the zenith of Kew bright enough to be well obferved, 

 he could not, with his inftrument, thoroughly examine how 

 this caufe affeded ftars differently fituated with refpeft to 

 the equinoftial and folftitial points of the ecliptic. 



" Thefe confiderations determined me ; and by the con- 

 trivance and direftion of the fame ingenious perfon, Mr. 

 Graham, my inftrument was fixed up, Auguft 19, 1727. 

 As I had no convenient place where I could make ufe of 

 fo long a telefcope as Mr. Molyneux's, I contented myfelf 

 with one of but little more than half the length of his, (w's. 

 of about 125 feet, his being 24:5,) judging from the ex- 

 perience which I had already had, that this radius would be 

 long enough to adjuft the inftrument to a fufficient degree 

 of exaftnefs ; and I have had no reafon fince to change my 

 opinion : for from all the trials I have yet made, I am very 

 well fatisfied that when it is carefully reftified, its fituation 

 may be fecurely depended upon to half a fecond. As the 

 place where my inftrument was to be hung in fome meafure 

 determined its radius, fo did it alfo the length of the arc, or 

 limb, on which the divifions were made to adjuil it ; for the 

 arc could not conveniently be extended farther than to reach 

 to about 65^ on each fide my zenith. This indeed was fuf- 

 ficient, fince it gave me an opportunity of making choice of 

 feveral ftars, very different both in magnitude and fituation ; 

 there being more than two hundred inferted in the Britifti 

 Catalogue, that may be obferved with it. I needed not to 

 have extended the limb fo far, but that I was wilhng to 

 take in Capella, the only ftar of the firft magnitude that 

 comes fo near my zenith. 



" My inftrument being fixed, I immediately began to 

 obferve fuch ftars as I judged moft proper to give me light 

 into the caufe of the motion already mentioned. There was 

 variety enough of fmall ones, and not lefs than twelve that 

 I could obferve through all the feafons of the year ; they 

 being bright enough to be feen in the day-time, when neareft 

 the fun. I had not been long obferving, before I perceived 

 that the notion we had before entertained of the ftars being 

 fartheft north and fouth, when the fun was about the equi- 

 noxes, was only true of thoie that were near the folftitial 

 colure ; and after I had continued my obfervations a few 



months. 



