928 



EQUATORIAL INSTRUMENT. 



EQUATORIAL INSTRUMENT. 



926 



for forming an image of the sun the mode of observation then in 

 vogue or, with a telescope, in the modern manner for viewing the 

 moon, stars, and other phenomena. Gruenberger's equatorial is almost 

 identical with that described and figured by Cassini as his machine 

 paraliactique, 'Memoires de 1' Academic,' 1721, p. 18, and which is 

 also drawn and described under the same name by Bailly, ' Histoire 

 de 1' Astronomic Moderne,' vol. i. p. 601, plate v., fig. 38 ; and 

 by Lalande, 'Astronomic,' 1792, 2400, et seq., plate xxiv. In 

 1674 Hooke published his ' Animadversions ' on the first part of the 

 ' Machina Ccelestis' of Hevelius. In this remarkable tract he describes, 

 p. 67 et seq., tab. 2, fig. 15, the polar axis, on which he proposes to fix 

 his quadrant for measuring intermutual distances, and " the watch- 

 work, which is to make it move round in the same time with the 

 diurnal revolution of the earth." * The regulator of the watch-work is 

 a ball and string, describing a conical surface. When so near an ap- 

 proach had been made to this great desideratum in telescope-mounting, 

 it is somewhat remarkable that nearly 150 years should have elapsed 

 before it was realised. The astronomical sector with which Flamsteed 

 observed at Greenwich from 1676 to 1689 bears so strong a resemblance 

 to the quadrant and stand described in Hooke's tract, that we are in- 

 clined to attribute the merit of its contrivance to Hooke, particularly 

 .is he recommends the artificer Tompion as " having been imployed by 

 him to make that which he had." Flamsteed's instrument was a 

 sextant mounted upon a polar axis ; and a description of the instru- 

 ment, with a plate, will be found in the ' Historia Ccelestis,' vol. Hi., 

 p. 103. 



The next step in the construction of the equatorial was made by Romer, 

 to whom we owe the transit and circle, and whose merits as an astro- 

 nomical mechanic surpass even his great countryman Tycho. Ac- 

 cording to Horrebow, ' Basis Astronomic,' p. 39, tab. 1 , the equatorial 

 of Romer was erected about 1690. Here we have the second essential to 

 the instrument, a telescope mounted on a cross axis, which is placed 

 at right angles to the polar axis ; (this is sometimes called the decli- 

 ftofe'o* ajrit, from its carrying the declination circle, or transit <udt, from 

 its analogy to the transit). In this construction the telescope can be 

 pointed in every direction, and we have heard Romer 's general idea 

 recommended by a very competent judge as that which he would now 

 follow in mounting a 20-foot telescope. Graham's differential sector 

 and Sisaon's equatorial are described in Vinoe's ' Practical Astronomy,' 

 136 and 140, with figure* ; but the author has committed an error in 

 his description. Graham's astronomical sector is supported like Hook's 

 quadrant and Flamsteed's sextant (see Smith's ' Optics' 885, book 3, 

 chap. 9) ; but, in explaining the instrument, Smith uses a figure for 

 illustration which Vince has copied as the actual instrument. His in 

 the Royal Observatory. 



A portable equatorial stand for carrying a reflecting telescope was 

 invented by Short, and is described and figured in the ' Phil. Trans.' 

 for 1749. This most have been a very ricketty affair, judging from 

 the plate. In 1771 Nairne gave a description and figure of a much 

 better instrument in the same work; and in 1772, or 1773, P. and J. 

 Dollond published an account of a " universal equatorial instrument, 

 or portable observatory," which in firmness and contrivance is very 

 similar to that of Nairne. This construction, with some slight varia- 

 tions, was followed by Ramsden, in a portable equatorial made for Sir 

 George Schuckburgh, in 1779. All these portable equatorial* labour 

 under this serious defect ; the telescope cannot be pointed on the same 

 itar in reversed positions, nor upon any star within 30* or 40 of the 

 p. !. Lalandc, in his ' Astronomic,' 2413, pi. xxvi., gives an account 

 of an equatorial made by Megnie", for the President de Saron, in which 

 this difficulty is overcome by fixing the telescope at the extremity of 

 the cross axis, so as to overhang the hour circle. A very beautiful 

 universal equatorial, in which the same position of the telescope is 

 adopted, wan constructed by Troughton, a figure and description of 

 which will be given at the latter end of this article. There are some 

 equatoriala by Nairne and Adams which have a telescope at one end 

 of the cross axis, but we are not aware that they have been figured or 

 described. 



A new era in the equatorial commences with the construction 

 by Ramsden for Sir George Shuckburgh in 1791, which is elabo- 

 rately described with plates in the ' Phil. Trans.' for 1793. In this 

 instrument the telescope and the declination circle form a complete and 

 symmetrical transit circle, which is supported at its extremities by two 

 assemblages each of three long parallel brass tubes forming the polar 

 axis. The upper ends of these tubes are set into a circular open 

 frame, from the centre of which the top pivot rises, and their lower 

 ends rest on the base of an inverted cone, which has the bottom pivot 

 at its vertex. The polar axis is so long as to allow the telescope to 

 revolve completely, and the object-glass is large enough to give a sight 

 of the pole through the upper circle. The general form of this instru- 

 ment is followed, with some variations, in almost every considerable 



* Hooke aiwcrts that a clock with circular pendulum was Invented by him 

 la 1689, and complaint that Huygbens had published* description of this (In 

 1674) " without naming him at all u concerned therein." Huyghens says that 

 he inrrnted the clock with a circular pendulum about the same time that he 

 applied the oaeillatUg pendulum, that la, sixteen years before the date of his 

 book, and that Mreral clock* of thia description had been been made, " non sine 

 raceeara," in th mean time. Both Hooke and Haygben* promised to give 

 complete description of this clock at mother opportunity. 



English instrument which has been since made with the view of being 

 used to measure with the circles. All have had a long polar axis sup- 

 ported at the extremities, and divided in the direction of its length 

 into two limbs or cheeks, within which the declination axis is supported 

 as a transit between its piers. Of this kind are the equatorials of Cam- 

 bridge and Brussels, as well as of Armagh, described in Rees'a ' Cyclo- 

 paedia,' and of Camden Hill, described by Troughton, and figured in 

 the ' Phil. Trans.' for 1825, in the Preface to Herschel and South's 

 Observations of Double Stars. 



After Fraunhofer had succeeded in forming larger and better achro- 

 matic object-glasses than had hitherto been thought possible, he chose 

 for a stand one which is in principle identical with the Machine 

 Parallatique, only having the telescope hung on one side, exactly as in 

 the portable equatorial of Megni^. To these he applied a very in- 

 genious clock movement, which greatly facilitated the optical use of the 

 instrument, and rendered it a better micrometrical measurer than had 

 previously existed. To prevent the telescope from bending, Fraunhofer 

 applied a system of balance weights, which were, we believe, invented 

 or brought into extensive use by Reichenbach. It would probably 

 have been better if he had made the telescope stiff, by bracing a form, 

 and had omitted the balance weights for the telescope altogether. It 

 is evident, from their construction, that the least jar or roughness in 

 handling the telescope must set the weights in motion, and consequently 

 ruin the action of the object-glass until they come to rest ; and it is 

 not worth while to purchase a little useless accuracy in the measure- 

 ment with the circles at such a cost. There is considerable inconve- 

 nience in this construction ; for example, in many positions a star cannot 

 be followed from one side of the meridian to the other without bringing 

 the telescope over, that is, turning the polar axis and the telescope 

 each of them half round. This is particularly disagreeable in the 

 measurement of double stars, which ought, if possible, to be observed 

 near the meridian. There is, perhaps, a little more inconvenience in 

 giving small motions to the telescope ; but neither this nor the impossi- 

 bility of using reversed observations in a majority of instances is of 

 much consequence. On the other liand, when the workmanship and 

 material and engineering are good, an instrument of this form may be 

 expected to stand better in adjustment, and to admit of larger dimen- 

 sions, and to require less room, than any other. 



The most celebrated of Fraunhofer's equatorials is one which is 

 erected at Dorpat, and has been described by Professor Struve. 

 (' Beschreibung des grossen Refractors von Fraunhofer,' Dorpat, 1825, 

 folio, with plates.) 



Some enormous telescopes have been mounted on this principle : 

 one carrying the largest achromatic telescope we believe in existence, 

 of 14 inches aperture and 25 feet focal length, was erected at Markree 

 Castle, county of Sligo, for E. J. Cooper, Esq., by Mr. Grubb, of 

 Dublin. The tube of the telescope was constructed under the 

 direction of Mr. Cubitt, and is a capital step in the art of applying 

 large telescopes. The form is that of a very long barrel. T bars 

 of wrought iron extend longitudinally from the object to the eye 

 end, the edge bars being directed towards the centre, and these, after 

 they are connected crosswise by strong rings, are covered with iron 

 plate, which is } inch thick in the centre and -fa inch at the extre- 

 mities. The stiffness of this tube is more perfect than would be 

 readily believed without actual examination, and we see no reason to 

 doubt that a telescope, of at least twice these dimensions, may be 

 mounted with success, so far as the tube is concerned, whenever such 

 an object-glass can be procured. In most of the other large equa- 

 torials which we have seen, the telescope is the weakest part; and 

 as it is the habit of many observers to move their instruments by 

 laying hold of the eye end, it is desirable, for this and for some 

 better reasons, to have it as stiff as possible. The equatorial erected 

 in 1838 at the Royal Observatory, Greenwich, was also mounted by 

 Mr. Grubb. The object-glass, the definition of which is very good, 

 was made by M. Cauchoix of Paris, and was presented to the Observa- 

 tory by the late Rev. R. Sheepshanks. It is nearly 7 inches in clear 

 diameter, and of 8 feet focal length. The telescope is on one side of 

 the axis, and is counterpoised by weights on the other, as in Fraun- 

 hofer's method. The hour circle and declination circle are of small 

 size and are chiefly used for setting the instrument. In an equatorial 

 made to carry a 20-foot telescope, constructed for the observatory of 

 Cambridge, at the expense of his grace the Duke of Northumberland, 

 and of Ramsden's form, though with many peculiarities and improve- 

 ments, Mr. Airy has employed a square wooden tube. Mr. Dollond 

 has made several equatorials on Sisson's construction, which answer 

 very well. The polar axis in these is square, and composed of four 

 strong slabs of wood, making a sort of tube, a little tapered towards 

 the extremities. This is found to be abundantly stiff and firm, at 

 least up to the dimensions which have been tried, namely, for telescopes 

 of from 10 to 12 feet focal length. 



Among the astronomical instruments in the Great Exhibition of 

 1851 were several equatorials, which are minutely dewribed in Mr. 

 Olaisher's masterly report (Class X.). We select a few particulars. In 

 Mr. Ross's large equatorial, the telescope had an aperture of 11 4 inches, 

 and a focal length of 18 feet. The hour circle was 2 feet 3 inches in 

 diameter, and the declination circle 2 feet 8 inches. Theinntrumentwas 

 supported on a round cast-iron pillar, 10 feet 9 inches in height, 2 feet 

 3 inches in diameter at the bottom, decreasing to 1 foot at the top, and 



