EQUATORIAL INSTRUM 1 N I . 



K'jrATOKlAL IN'STUr.MKNT. 



320 



funued of two portion*, fattened together at the height of 44 feet from 

 the bottom of the baae by eight screw bolU and nut* painting through 

 flanges 3 inched in width from the shafts of the column, thus affording 

 the means of an approximate meridian adjustment ; the base of the 

 pillar was 9 feet in diameter. The polar axis was of cast iron, 6 inches 

 in diameter and 6 feet in length, connected to the declination axis by a 

 flange 18 inches in diameter. The inner male declination axis in 34 feet 

 long between iU bearings, and the outer or hollow axis wag 7 inches in 

 diameter, and both of cast iron. The inner axis and its flange formed 

 one casting with the central hollow cylinder, to the flanges of which 

 the corresponding guu-met.ol flanges of the telescope were bolted. The 

 tube was of copper. The fitting bearings of the declination axis were 

 cylindrical, and the axis was secured by a steel collar, 4 inches in length, 

 fastened by cross pins to the male centre, at the end to which the 

 circle was fixed. In the inner axis there was a counter-sunk cavity to 

 receire this collar, and a second similar cavity of large diameter to re- 

 ceive a steel plate fastened against the end of the steel collar by 

 eight steel screws, by which means the end-stroke of the axis was 

 adjusted. The declination circle was of gun-metal, regulated by an 

 endless screw with an excentrir lever attached to a dove-tail slide for 

 gearing, and a pair of bevelled wheels at either end for Hooks' joint 

 adjusting rods. There were also two other radial arms with clamp 

 screws for securing the telescope to the circle. The upper part of the 

 polar axis fitted into a coupling-block, with a hemispherical bottom, 

 supported by an angular projection from the top of the pillar, which 

 had a corresponding cavity, the whole being bound together by boll s 

 and nuts with spherical faces, bearing in corresponding cavities their 

 washers ; the bolts passing freely through the holes. The lower end of 

 the polar axis consisted of a hemisphere of hardened steel, bearing on a 

 hardened steel die, surrounded by an oil-cup attached to dove-tail 

 slides, with motions in rectangular directions, supported by a very 

 strong bracket projecting from the main column and applicable to the 

 final adjustment of the polar axis. The hour-circle was moved by a 

 weight and train of wheels, regulated by a Siemen's governor, the ball 

 being suspended by four springs. 



Mr. Simms exhibited an equatorial generally of the Fraunhofer form, 

 the only peculiarity being in the declination axis, which was not covered 

 up, but open and exposed between its two supports. The open part 

 was cylindrical, and could be set horizontal by a striding level, similar 

 to that of a transit instrument. Mr. Simms had also a small equa- 

 torial, adapted to the latitude of London, but without clock motion. 

 The instrument was said to be inexpensive, and, considering its size, 

 was effective. Mr. Dullond exhibited a portable equatorial supported 

 on a strong folding tripod with divided circles, a finder, levels, and eye- 

 pieces. The instrument was said to be well adapted for seeking for 

 comets. Merz and Sons (Bavaria) had an equatorial with the polar 

 axis adjustible within certain limits, so as to adapt it as a portable in- 

 strument, and to be applicable to different latitudes within those limits. 

 The focal length of the telescope was four feet, and the object-glass 

 had an aperture of four inches, unusually large for this size of tele- 

 scope, but, by admitting a large pencil of light, adapting it for faint 

 objects. 



In mounting a large telescope equatorially, it is not expedient 

 to rest the pivots on small superficial bearings, or to relieve the 

 weight by friction rollers, unless the axis of the rollers be made 

 large. It appeared, in Mr. Cooper's equatorial and elsewhere, that 

 when a heavy pressure was laid on a small bearing, the surfaces did 

 not slip freely, but clung together as it were, causing small oscillations 

 about the position of rest. Again, great care should be taken in 

 following Sisson's construction, or that which we have described as 

 Ramsden's, to provide against any twisting in the axis ; simple flexure 

 being of little importance. In Sisson's construction, the junction of 

 the telescope and its cross axis should be very firm ; in Ramsden's, the 

 union of the two sides of the polar axis with the base should be such 

 as to prevent all wriggling ; and in both, the telescope ought to be very 

 stiff, if it is likely to be roughly handled. To Ramsden's we should 

 apply steadying rods. It is also better to rest the pivots of the 

 declination axis in Ys than in collars ; but if a collar is preferred, it 

 should be formed of an upper and lower half, which can be adjusted 

 to clasp the pivot close when the collar wears loose. After all possible 

 precautions, accuracy is not to be expected from a large equatorial, 

 when used as an instrument measuring with its circles, compared with 

 those of moderate size. The equatorial by Trough tc m , at Armagh, 

 carries a telescope of only 42 inches focal length and 2| inches aperture, 

 with an hour circle of 6 feet, and yet the telescope is more powerful 

 than the circles used. Large equatorial* are required whenever optical 

 power is wanted, as in the examination of nebulrc, noting occultations, 

 Ac., where the micrometer is alone required for measurements, as in 

 observations of double stars, or determinations of the diameters and 

 forms and constitutions of planets, and investigation of the systems of 

 those which have rings, satellites, tc. In these respects Lord Rosse's 

 large reflector erected at Parsonstown Castle, Ireland, deserves especial 

 notice. The clear aperture is feet, and the magnitude of reflecting 

 surface is 2S'274 square feet. Though used as a Newtonian telescope, 

 the apparatus of suspension is to arranged that the instrument must 

 be worked as an equatorial. [TnEscopK.] 



The adjustments of an equatorial instrument are easy enough to a 

 person tolerably familiar with the management of other artronomienl 



instrument* ; and the corrections which are to be applied to obser- 

 vations made with an unadjusted equatorial, ought not to present any 

 difficulty to an astronomer acquainted with spherical trigonometry, 

 and with the ordinary rules for determining the value of the coefficients. 

 Still, as there are many persons who possess equatorially-mounted tele- 

 scopes, and have not the knowledge or even the leisure to understand 

 this subject thoroughly, we shall proceed to give directions which will 

 enable any one to adjust his instruments with more than sufficient 

 accuracy and without trouble. 



We suppose the latitude of the place and the direction of thr 

 meridian to be approximately known ; and we shall speak as if the 

 instrument showed north polar distance, and the hour circle, when 

 the sun is observed, read as an ordinary clock. Let the polar axis be 

 placed nearly in the direction of the poles of the heavens. The 

 adjustments proceed in the following order : 1st. The polar axis is 

 placed at the altitude of the pole. 2nd. The indices of the declination 

 circle are made to read 0, when the telescope points to the pole ; this is 

 sometimes called " correcting the colh'mation in declination," or " north 

 polar distance." 3rd. The pole of the instrument is brought into the 

 meridian, and as it has already been set at the proper altitude, it now 

 coincides with the pole of the heavens. 4th. The line of sight of 

 the telescope is made perpendicular to the declination axis ; this is 

 similar to the collimation adjustment in the transit. 5th. The decli- 

 nation axis is placed exactly _ at right angles with the polar axis, if the 

 means of adjustment are allowed. Oth. The hour circle ia made to 

 read O k , when the telescope is in the meridian of the place. 



1st. Observe any known star in north polar distance, and then 

 turning the polar axis half round, observe the same star again ; these 

 observations should be as near the meridian as possible ; and if the 

 instrument is much out of adjustment, the star should not be very near 

 the pole. Take the mean of the two observations, which is the dis- 

 tance of the star from the pole of the instrument, correct it for 

 refraction, and then compare the result with the true north polar 

 distance given by the Nautical Almanac, or computed from a standard 

 catalogue. If the star be abote the pole, and the instrumental north 

 polar distance be greater than the true north polar distance, it is clear 

 that the instrumental pole is farther from the star than is the pole of 

 the heavens, or that it is too lota ; but if the instrumental north polar 

 distance be lea than the true north polar distance, then the pole of the 

 instrument is too high. Correct this error by the proper screws 

 for raising or depressing the polar axis, which may be done at 

 once if the thread of the screw and the length of the polar axis 

 be known. 



2nd. Take half the difference of the above two observations ; thin is 

 the index error of the declination verniers or microscopes, and they must 

 be moved just so much in the proper direction by their adjusting 

 screws, and set, if there be more than one, at their proper distances. 

 The polar distance read off will now correspond with the true instru- 

 mental polar distance in every position of the instrument. Several 

 pairs of observations should be taken, in order to ascertain these two 

 errors with great accuracy before they are finally corrected and con- 

 sidered to be settled. 



3rd. Turn the instrument six hours from the meridian either way, 

 and observe the north polar distance of any known star not very near 

 the pole nor yet near the horizon. Correct this for refraction. \Ve 

 will suppose the star observed to the east of the meridian, and that 

 the observed distance exceeds that given by the Nautical Almanac or 

 the standard catalogue ; then the pole of the instrument is further 

 from the star than is the pole of the heavens, or is to the west of its 

 proper place ; hence the upper pivot must be shifted to the east, or 

 the lower pivot to the west, the proper quantity. In this, as in the 

 former case, several stars should be used for greater accuracy ; but 

 there is no necessity for reversed observations, as the index error in 

 already supposed to be corrected, or at least to be known, and there- 

 fore easily allowed for. The polar axis is now adjusted both in 

 altitude and azimuth. 



4th. Observe the transit of an equatorial star over the middle 

 vertical wire or mean of the wires, note the time, and read off the 

 verniers of the hour-circle. Turn the polar axis half round, and 

 observe the same star n second time exactly as before. Now if the 

 time between the two observations corresponds exactly to the differ- 

 ence between the two readings of the hour-circle, all is right; if not, 

 it is evident that one of the transits has been observed too early, and 

 the other too late, on account of the erroneous position of the wires. 

 If the time elapsed be greutrr, by 6" suppose, than the difference of 

 the hour-angles, the first transit lias taken place 3* too early and thr 

 second 3' too late. Set again upon the star and observe how far it 

 ap]>ears to travel in 3"; and then if the instrument is in tli> 

 position, move the wires this quantity in R.A. with the star, and 

 rice rertd if the instrument be in the second position. The rule 

 mutatu mutantlit will apply to any case, and where there are no 

 means of measurement and no mark, the adjustment must be made by 

 repeated trials. With a micrometer in R.A., or with a mark, it may 

 be performed with accuracy at once. 



6th. This adjustment may be performed in two ways, either astro- 

 nomically, or, when there is a level attached to the declination axis, 

 mechanically. In the first case observe the transit of a stir, H 

 than 45 from the equator, in reversed positions of the polur nxi-. 





