ZENITH. 



as to iti tXreaion, when viewed by a fpeAator ; yet as it has 

 reference to the apparent place of a heavenly body, it is not 

 fixed, becaufe the earth's motion is continually carrying the 

 fpe^ator's eye in a circle that produces the optical effea of 

 an apparent circumpolar revolution of the celeftial bodies. 

 Whatever point of the hemifphere is at any moment vertical 

 to an obferver on any part of the globe, that point is the 

 zenith for the time then pajtng, and will again be the zenith, 

 or very nearly fo, after an interval of one complete rotation 

 of the earth ; and, therefore, ftriftly fpeaking, in every place 

 there are as many fuccefTive zeniths in this period, as there 

 are appreciable points in the circle generated in the heaveas 

 by the fuperior end of the vertical line carried round by the 

 globe in rotation. 



Becaufe every point of the horizon is juft 90° diftant from 

 the exifting zenith, in every place, and at all times, it will 

 be readily apprehended, that the complement of the altitude 

 of any heavenly body will, at any inftant, be the angular 

 diftance of that body from the zenith of the place of obferva- 

 tion ; but as this diftance varies inverfely with the altitude, 

 it is more properly called the co-altitude than the zenith- 

 SJiance, the former being a variable, and the latter a limited 

 quantity. The zenith-diftance of any ftar is properly the 

 complement of its meridian or greatejl altitude in any given 

 place, and as the latitude of the place varies, fo will the 

 zenith-diftance of the fame body, but inverfely ; the diftance 

 of the zenith to the pole being always the complement of 

 the latitude. Hence it is obvious, that when the zenith- 

 diftance of a ftar is obferved by any inftrument that meafurcs 

 it accurately, the latitude of the place may be inferred from 

 the tabular polar diftance of that ftar, as well as from the 

 declination, vv'hich is its complement ; and, therefore, it is a 

 matter of no importance in theory, whether the altitude or 

 zenith-diftance of a body tranfiting the meridian be taken, 

 for the purpofe of afcertaining its place in the heavens, when 

 the latitude of the obferver is known, or for the purpofe of 

 determining the latitude of the place, when the declination, 

 or polar diftance of the body, is known. Accordingly, in 

 the circular inftruments that have been recently introduced 

 into obfcrvatories, and that will reverfe in pofition, it is 

 ufual to number the divifions and fubdivifions fo, that when 

 they read altitudes with the graduated plane facing the eaft, 

 they read zenith -diftances (on the meridian) when the fame 

 is made to face the weft, and vice •verfd ; fo that not only 

 are the oppofite errors of collimation of the telefcope and 

 of the bubble or plumb-hne thus correfted, but when 

 atmofpheric refraftion is allowed for, the fum of the two 

 readings, or of the averages of feveral, will be exaftly 90°, 

 or otherwife 180°, if the obfervations be truly taken, and 

 the inftrument duly adjuftedfor zero and collimation ; which 

 check is of great praftical importance in the delicate opera- 

 tions of the aftronomer. 



ZENiTH-S'eSor is an agronomical inftrument, by means 

 of which the angular diftance of a ftar is accurately mea- 

 fured from the zenith point of any given place towards the 

 north or fouth. The firft inftrument made ufe of for this 

 purpofe was contrived by Dr. Hooke, with a view of 

 determining the annual parallax of a iixed ftar, agreeably 

 to the fuggeftion of Galileo. The telefcope, which Dr. 

 Hooke, in the year 1669, made the eflential part of his 

 inftrument, was thirty-fix feet long, the principle of achro- 

 matifm not being at that time praftically applied, fo as to 

 allow of confiderable power with a ihort focal diftance of 

 the objeft-glafs ; but the length of the radius of his arc of 

 meafurement promifed advantages over every other inftru- 

 ment, which juftified the conception and execution of the 



plan, though its accomplishment failed of correfponding' 

 fuccefs. Indeed, the nice arts of conftrufting achromatic 

 telefcopes, and of dividing the arc of a circle with extreme 

 precifion, had neither of them yet been perfefted. From 

 the Cutlerian Leftures we learn, that the firft obfervatiou 

 with this inftrument was made on the 6th of July of the 

 above-mentioned year, on the ftar denominated y Draconis, 

 which, on that evening, was found to pafs at the diftance 

 of 2' 12" to the north of the zenith of Greftiam college ; 

 which was alfo found to be the cafe on the 9th of the fame 

 month : but on the 6th of Auguft next following, the 

 diftance was only 2' 6" ; and on the 2ift of October after 

 only i' 48", or l' 50"; whence it was concluded, that the 

 meafurement of a zenith-diftance taken by this inftrument ' 

 was liable to an error of 24", or perhaps more ; and it was 

 confidered, therefore, that the inftrument was quite incom- 

 petent to the purpofe for which it was intended. But an 

 original idea once fuggefted, as the bafis of ufeful fpeculation, 

 is not readily abandoned, even under an apparent want of fuc- 

 cefsful application. The Hon. Samuel Molyneux afterwards 

 availed himfelf of the manual Ikill of the ingenious Graham, 

 and by the afliftance of Dr. Bradley put up a zenith-feftor 

 at Kew, in the year 1725, which turned out to be much 

 more accurate than its predecefTor, though the focal length 

 of the objeft-glafs of its telefcope was only 24^ feet. With 

 this inftrument, and with one of about one-half its focal 

 length, were made two of the moft important difcoveries in 

 aftionomy that have graced the annals of this fcience ; viz. 

 the nutation of the earth's axis, and the aberration of light 

 in its paft^age from the heavenly bodies. As the hiftory of 

 aftronomical difcoveries, and that of aftronomical inftruments, 

 are mutually illuftrative of each other, and as a detail of the 

 minutiie is always interefting, that conneft great refults 

 with primary meafures, that might otherwife be confidered 

 as infignificant, we will make no apology for introducing 

 here Dr. Bradley's own account of his proceedings, as in- 

 ferted in the Philofopliical Tranfadlions of London, N°4o6. 

 p. 149 of the Abridg. 



" The following obfervations," fays the author, " were 

 begun by the honourable Samuel Molyneux at Kew, con- 

 tinued and repeated by myfelf at Kew and Wanftead, in 

 hopes of verifying thole that Dr. Hooke formerly commu- 

 nicated to the public, concerning the parallax of the Jixed 

 flars. (London, 1674.) Therefore the fame ftar was made 

 choice of by Mr. Molyneux, almoft the fame method fol- 

 lowed, and his inftrument conftrufted upon principles nearly 

 the fame, but greatly exceeding the doftor's in exaftnefs, 

 which was chiefly owing to our curious member (of the 

 Royal Society ) Mr. George Graham, to whom the lovers 

 of attronomy are alfo indebted for feveral other exaft and 

 well-conftru£led inftruments. Mr. Molyneux's apparatus 

 was completed and fitted for obferving about the end of 

 November, 1725 ; and on the 3d day of December follow- 

 ing, the bright ftar in the head of Draco (marked y by 

 Bayer) was for the firft time obferved, as it pafTed near the 

 zenith, and its fituation carefully taken with the inftrument. 

 The like obfervations were made on the jth, nth, and 

 I2tli days of the fame month ; and there appearing no mate- 

 rial difference in the place of the ftar, a farther repetition 

 of them at this feafon feemed needlefs, it being a part of 

 the year wherein no fenfible alteration of parallax in this 

 ftar could foon be expcfted. It was chiefly, therefore, 

 curiofity that tempted me (being then at Kew, where 

 the inltrument was fixed) to prepare for obferving the ftar 

 on Dec. 17th, when, having adjufted the inftrument as 

 ufual, I perceived that it pafTed a little more foutherly this 

 2 day 



