ZENITH. 



firft idea of a zenith-mierometer occurred to Mr. Trough- 

 ton, and certain that he was the firft who executed one, 

 and gave it its name. It was in i8o5 that he propofed 

 it to Dr. Mafkelyne, and in 1812 that he erefted it at 

 the Royal Obfervatory, as an appendage to the mural 

 circle. 



The telefcope of this inftrument is a Newtonian refleftor, 

 the tube of which forms the vertical axis, through which 

 axis a plumb-line pafTes centrically from end to end ; and, 

 therefore, is not made to vibrate by turning. The tube or 

 axis, 10 feet long, and 5 inches diameter, at the lower end 

 terminates in a pivot, while the upper end is received by a 

 right angle, againft the fides of which it is gently preftd 

 by two fprings ; contrivances which produce free and 

 fteady azimuthal motion. 



All the zenith inftruments require for collimation two 

 pofitions, which in the fefiors are called yac^" eajl 2lX\A face 

 ivejl ; two at iSo° diftance from each other are equally ne- 

 ceflary for the micrometer, and in that under confideration 

 are indicated by the contaft of flops, without regarding any 

 divifions. 



The large mirror at the lower end, ss well as the pivot, 

 are perforated, in order that the plumb-line may pafs freely 

 through them, the latter fuftaining the plummet in a water 

 veffel feveral inches below. The axis of the mirror is fome- 

 what inchned to that of the tube, for the purpofe of placing 

 the plane mirror out of the way of the plumb-line, which 

 latter, as before obferved, occupies the centre. The rays 

 of light, thus rather obliquely reflefted up the tube, are 

 turned into a horizontal direftion by the plane mirror, and 

 come to a focus a little beyond the fide of the tube, where, 

 with appropriate adjuftments, is fixed a double micrometer 

 for meafuring zenith-diftances. 



When the inftrument is flopped with the micrometer 

 toward the fouth, one of the moveable wires is made to bi- 

 feft a flar a little before it comes to the centre of the field 

 of view ; then the inftrument is turned to the oppofite flop 

 with the micrometer towards the north, which may be done 

 in three feconds, and then the other moveable wire is made to 

 bifeft the ftar. It is evident now, that the opening be- 

 tween the wires is double the zenith-diftance of the ftar, and 

 if the axis was truly vertical, the middle between them is 

 the point zenith. The revolutions of the fcrews are 

 counted in the field of view, and the parts to the third 

 decimal figure read off upon the micrometer heads, which 

 revolutions and parts are to be reduced to angular 

 meafure. 



The plumb-line is fufpended from a fixed point at the 

 upper end, and near the lower end is a reftangular arrange- 

 ment of microfcopes with adjuftments, which, together with 

 the adjuftments of the pivot, accomplifti the means of bring- 

 ing the axis and plumb-line coincident with each other, and 

 of affuring the vertical pofition of the former. 



The conception of this inftrument was not confined to a 

 reflefting telefcope, it was feen that a refrafting one would 

 fucceed equally well : in the latter cafe, the pivot at the 

 lower end muft be perforated to receive the eye-glafs, and 

 the plumb-line (hould be exterior to the main tube, but in- 

 clofed in a fmaller one to proteft it from the adlion of the 

 wind. In the cafe of the Greenwich inftrument, of which 

 the foregoing is a defcription, the refleftor was preferred 

 on account of its allowing the central pofition of the plumb- 

 line, which, being free from vibration, ftiortens the time re- 

 quired for adjuftment ; a confideration indeed of very httle 

 value; for it is nov known (but was not then) that in- 

 ftruments properly conftrufted, and ufed with care, which 

 they now are at our national eftablifliments, feldom or nevtr 



want re-adjuftment. A better caufe for adopting the rs- 

 fleftor was, that the horizontal view is more eafy than the 

 vertical one, efpecially as in the former the hands and whole 

 body are unembarraffed, and fit for aftion. But wliichevsr 

 conftruftion of the telefcope is employed, we know that 

 Troughton confiders the zenith-micrometer as one of the 

 moft elegant of his inventions. 



Zenith-Micrometer by Dollond. — The zenith-microme- 

 ter which is reprefented in Plate XXXIII. hy fgs. 3, 4, 

 5, and 6, has great advantages in the fimplicity of the con- 

 ftruftion and ufe, that former inftruments for the fame pur- 

 pofe do not poffefs, and is portable : the plan was propofed 

 by Mr. Pond, the aftronomer royal, and the inftrument ex- 

 ecuted by Mr. Dollond for tranfatlantic meafurements. It 

 is drawn from a fcale of two inches to the foot, and ia re- 

 prefented without a ftand or fupport. It can be applied to 

 either, as occafion may require. The component parts are 

 thefe ; -viz. an achromatic refrafting telefcope A A, 

 ifs,'- 3) 4- ) °^ 42 inches focal length, with an aperture of 

 2| inches ; a micrometer B, with two fcrews, each moving 

 a feparate wire through the field of view, which is extended 

 to two degrees. The tranfverfe axis C is 18 Laches long, 

 and glazed for a telefcope, with crofs wires that adjuft, fo 

 that when the principal telefcope A is taken out, (and for- 

 which there is a provifion,) the line of collimation may be 

 truly adjufted to the pivots, and when replaced will be 

 correftly at right angles to the principsl telefcope. The 

 ufe of the axis being formed into a telefcope, is, for the 

 purpofe of placing the inftrument correftly in the meridian, 

 by a mark that mud be found to the eaft, and alfo by 

 another to the weft, fo that when the ftar has been obferved 

 with one of the micrometer wires, and the inftrument is 

 changed for the purpofe of obferving the fame ftar with the 

 other micrometer wire, it may be correftly replaced, the 

 mean of the two obfervations being the correft zenith-dif- 

 tance. The inftrument is alfo furniftied with a plumb-line 

 D, and with a dot at E, the image of which is brought to 

 the place of the plumb-line by a lens ; this is known by the 

 name of the ghoji adjuftment. The plumb-line is fufpended 

 from a piece at F. The two fcrews G G are ufed for 

 bringing the dot carried by the telefcope, to coincide with 

 the plumb-line ; and is obferved to be fo by the lens at H. 

 This contrivance affords one of the greatetl advantages of 

 the inftrument, as it enables the obferver, at the moment 

 before he obferves the conjunftion of the flar with the 

 micrometer wire, to afcertain the correft pofition of the 

 inftrument by the plumb-line ; for the correftnefs of the 

 obfervation will depend on the accuracy with which the 

 plumb-line is made to coincide with the dot ; and by this 

 method the error of divifion is done aivay. The level [Jig. 6. ) 

 is ufed for leveUing the axis, and the circular piece [Jig. 5.^ 

 with the Ys a a attached, in which the pivots of the tranf- 

 verfe axis reft, may be applied to a mahogany-framed ftand, 

 or on a ftone pier, or bracket : it is reprefented as it was 

 made for a mahogany ft;and ( which had adjufting fcrews at 

 the feet) ; it confifts of two ftrong brafs plates, and is fur- 

 nifhed with a circular motion for bringing the inftrument 

 into the meridian, which motion is given by a pinion b; and 

 it is made faft by the four fcrews c, c, c, c. The two ad- 

 jufting fcrews G G, Jig. 3. muft alfo be attached to a 

 bracket or framed piece in the ftand ; and the fupport for 

 the water, in which the plummet is immerfed, may alfo be 

 applied to this bracket. The wires are illuminated through 

 the axis in the fame manner as in the tranCt-inftrument. The 

 value of the micrometer-fcrews is to be found in the ufual 

 manner, and reduced into feconds. 



The foregoing are the diftinguiftiing properties of this 



inftrument, 



