ZENITH. 



months, I difcovered what I then apprehended to be a 

 general law obfcrved by all the ftars, viz. that each of them 

 became ftationary, or was farthcft north or fouth, when 

 they pafTed over my zenith at fix of the clock, either in the 

 morning or evening. I perceived likevvife, that whatever 

 fituation the ftars were in with refpea to tlie cardinal pomts 

 of the ecliptic, the apparent motion of every one tended the 

 fame way, when they palTed my inftrument about the fame 

 hour of' the day or night ; for they all moved fouthvvard 

 while they paffed in the day, and northward in the night ; 

 fo that each was fartheft north, when it came about fix of 

 the clock in the evening, and fartheft fouth, when it came 

 about fix in the morning. 



" Though I have fiiice difcovered, that the maxima in 

 moft of tliefe ftars do not happen exaftly when they come 

 to my inftrument at thofe hours ; yet not being able at that 

 time to prove the contrary, and fuppofing that they did, I 

 endeavoured to find out what proportion the greateft altera- 

 tions of dechnation in different ftars bore to each other, it 

 being very evident that they did not all change their de- 

 chnation equally. I have before taken notice, that it ap- 

 peared from Mr. Molyneux's obfcrvations, that y Draconis 

 altered its declination about twice as much as the fore-men- 

 tioned fmall ftar almoft oppofite to it ; but examining the 

 matter more particularly, I found that the greateft alteration 

 of declination in thefe ftars was as the fine of the latitude of 

 each refpeftively. This made me fufpedt that there might 

 be the like proportion between the maxima of other ftars ; 

 but finding that the obfcrvations of fome of them would not 

 perfectly correfpond with fuch an hypolhefis, and not 

 knowing whether the fmall diff'erence I met with might not 

 be owing to the uncertainty and error of the obfervations, I 

 deferred the farther examination into the truth of this hypo- 

 thefis, till I fhould be furniftied with a ferics of obfervations 

 made in all parts of the year, wliich might enable me not 

 only to determine what errors the obfervations are liable to, 

 or how far they may fafely be depended upon, but alfo to 

 judge wliether there had been any fenlible change in the parts 

 of the inftrument itfelf. 



" Upon thefe confiderations, I laid afide all thoughts at 

 that time about the cauf; of the fore-mentioned phenomena, 

 hoping that I ftiould the more eafily difcover it, when I was 

 better provided with proper means to determine more pre- 

 cifely what they were. 



*' When one year was completed, I began to examine 

 and compare my obfervations ; and having pretty well fatis- 

 fied myfclf as to the general laws of the phenomena, I then 

 endeavoured to find out the caufe of them. I was already 

 convinced that the apparent motion of the ftars was not 

 owing to a nutation of the earth's axis. The next thing 

 that offered itfelf was an alteration in the diredion of the 

 plumb-line, with which the inftrument was conftantly rec- 

 tified ; but this, upon trial, proved infufficient. Then 

 I confidered what refra&ion might do, but here alfo 

 nothing fatisfaftory occurred. At laft I conjeftured that 

 all the phenomena hitherto mentioned proceeded from the 

 progrejjlvs motion of light, and the earth's annual motion in its 

 orbit. For I perceived that if light was propagated in time, 

 the apparent place of a fixed objedl would not be the fame 

 when the eye is at reft, as when it is moving in any other 

 direftion than that of the line paffing through the eye and 

 objeft ; and that when the eye is moving in different direc- 

 tions, the apparent place of the objeft would be different." 



The author then proceeds to deduce from his obferva- 

 tions the relative velocities of light, and of the earth in its 

 annual orbit ; fhews what variation in the right afcenfioii 



and declination of ftars differently placed may arife out of 

 the aberration of light ; and concludes that light, agreeably 

 to fuch dcduftions, muft travel from the fun to the earth in 

 about 8' 7".5 of time. (See Aherration, Light, and 

 Stars. ) The difcovery of the earth's nutation was not, 

 however, publifhed until the year 1737. See Nutation. 



After our readers have feen what important difcoveries 

 and deductions have been derived from Graham's zenitli- 

 feftor in the hands of a flcilful aftronomer, they will be na- 

 turally difpofed to become acquainted with its conftruftion, 

 which we will now proceed to defcribe. 



Zenith-Scaor by Graham. — The zenith-feftor that we have 

 faid Graham made for Mr. Bradley, afterwards Dr. Bradley, 

 was removed to Greenwich, v^lien the proprietor became 

 aftronomer royal, and is the fame inftrument which Dr. 

 Mafl<elyne ufed with great fuccefs in adjufting, by com- 

 parifon, the zero of the large quadrants. It Itill remains 

 at Greenwich, and is yet capable of meafuring zenith-dif- 

 tances to the accuracy of half a fecond, according to Brad- 

 ley's original report, or even lefs. Fig. i. of Plate XXXIH. 

 of AJlronomical Injlrumetits, is a reprefentation of the effential 

 parts of tliis inilrument: A B reprefents the iron tube of 

 the vertical telefcope, which is fufpended by two fmall me- 

 talhc cylinders projefting at right angles from the fuperior 

 end, one of which is feen at a, and the other is hidden by 

 the tube. Thefe cylinders, which conftitute the axis of 

 motion, refi: in a pair of Ys, attached to the folid wall 

 facing the north, and alfo occafionally to another pair fixed 

 to a wall facing the fouth ; which additional pair allows the 

 ends of the cyhnders, or axis of motion, to be reverfed in 

 pofition. The brafs bar C D is fixed to the fame wall to 

 which the pair of Ys are attached, at oppofite fides of the 

 room, and bears a cock to which the micrometer-fcrew E 

 is fixed, wliich meafures the fraftional portion of a minute 

 on its head b ; and the fecond fcrew c is made to relieve it. 

 The ends of thefe fcrews prefs againft ftuds inferted into the 

 tube of the telefcope, while the weight F pulls a ftring 

 round the fixed pulleys G and H, by means of a pliable 

 cord, attached to the tube at the point J, and keeps the 

 telefcope liome. The graduated arc I K contains 12-5°, 

 each fubdivided into twelve parts, or five minute fpaces, 

 and is fixed exaftly at right angles to the tube, over the 

 point where the wires interleft the field of view. This arc 

 was originally of brafs, but Siffon put on an arc of fteel, 

 containing gold pins to receive the dots of divifion. A 

 plumb-hne fufpended from the fuperior end of the tube, over 

 the centre of one of the cylinders a, and having an adjufting 

 fcrew e, to bring the point of iufpenfion to the upper dot, falls 

 near the face of the arc, and indicates the diftance from zero 

 at the middle of the arc ; if the plumb-line covers one of 

 the dividing dots of the icale, when a ftar near the zenith 

 is cut by the horizontal wire, then the quantity is read by 

 the plumb-line and arc alone ; but otherwife the fradional 

 portion is afcertained by means of the micrometer-fcrew, 

 which is made to prefs againft the tube until the fufpended 

 line coincides with the next neareft dividing dot of the fcale. 

 The value of the micrometer-head, which is divided into 

 thirty-four equal parts, was afcertained by trying how many 

 revolutions of the fcrew would meafure a degree, or other 

 portion of the arc, exaftly ; and on an average of feveral 

 trials made in different parts of the arc, it was afcertained, 

 that one revolution was not precifely 34", but 33".6328, 

 and one of its divifions on the Iiead, therefore, only ©".9892, 

 inftead of i", as was intended by the maker. The inftru- 

 ment had originally a fingle lens for its objeft-glafs ; but 

 at the requeft of the late Dr. Mafkelyne, Mr. Dollond fub- 



ftituted 



