Class X.]

EQUATORIAL, AXD ALTITUDE AXD AZIMUTH INSTRUMENTS.

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faces, -which bear m corresponding cavities in their washers;the bolts pass freely through the holes.

The lower end of the polar axis is a hemisphere ofhardened steel, bearing in a hardened steel dye, surroundedby an oil-cup, which is attached to dovetail slides havingmotions in rectangular directions, which are supported bya very strong bracket, projecting from the main column,and are applicable to the final adjustment of the polaraxis.

The hour circle is moved by a weight and train ofwheels, regulated by a Siemen’s governor, the ball beingsuspended by four springs.

This instrument reflects high credit on its maker; it isdistinguished by solidity of structure, good mechanism,and distribution of strength.

Also, an astronomical telescope of inches in diameter,

42 inches focal length, mounted equatorial])'. This tele-scope, on examination, was found to be a very superiorinstrument, and is the best in the Exhibition for its size.

Also, an astronomical telescope 2£ inches in diameter,and 30 inches focal length, mounted on a pillar and clawstand.

Simms (741, p. 475*), an equatorial, generally of theFrauenhofer form, with the polar axis elevated for lati-tude 25°, furnished with a clock motion for counteractingthe effect of the earth’s rotation. The focal length of thetelescope is 7 feet, and its aperture is 4*9 inches: it isfurnished with illuminating apparatus, a set of negativeeye-pieces, magnifying from 60 to 450 times; a parallelline position micrometer with position circle, reading toone minute, and a transit eye-piece, the use of which is tofacilitate the general adjustment of the instrument, andalso for making observations for time, in the absence of atransit instrument.

The hour circle is IS inches in diameter, and is read toone second of time by opposite verniers. The edge ofthis circle is ratched to fit the teeth of a screw, which isin connection with the clock before mentioned.

The declination circle is 18 inches in diameter, and is ;read by opposite verniers to 5" of arc. The slow motionis by an ordinary clamp and tangent screen.

The action of the clock is under the government of arotatory pendulum, similar to the governor upon a steam-engine.

The only peculiarity in this instrument is in the de-clination axis, which is not covered up, but open, andexposed between its two supports. The open part isturned cylindrical, and can be set horizontal by a stridinglevel, similar to that of a transit instrument. This con-struction has several advantages over that in common use;it simplifies the adjustment and the rectification to themeridian, and adapts the instrument for use in the takingof transits, where extreme accuracy is not needed, andgreatly adds to the value of the instrument.

Mr. Simms also exhibits a small equatorial, adapted forthe latitude of London , but without clock motion. Thisinstrument is upon the same principle as the preceding;it is understood to be inexpensive, but as far as its sizepermits, it is an effective instrument. The focal lengthof the telescope is 4G inches, and its aperture 3£ inches;it carries a parallel line micrometer, with powers of 50,90, and 150, and the telescope is furnished with an illumi-nating apparatus. The hour and declination circles are6 inches in diameter; the former is read by vernier to onesecond of time, and the latter to one minute of arc, withclamps and tangent motions.

The instrument surmounts an iron pillar, with a tripodstand, and there are screw adjustments, both for elevatingthe polar axis to the latitude, and for correction to themeridian.

The open declination axis and riding level occur in thisinstrument as in the former.

Dolloxd (145, pp. 475—477), a portable equatorial,made for the late Capt. Kater, F.R.S. The object-glassof the telescope, which is 30 inches focus, and 4 inchesaperture, is worked from a formula of Sir John Herschel’s jand performs extremely well. The instrument is sup-ported upon a strong folding tripod, is furnished with !divided circles, a finder, levels, and a complete set of eye- !pieces. :

The instrument is well adapted for seeking for comets.

Simms (741, pp. 475, 476). An altitude and azimuth in-strument, known as the Westbury Circle.* The diameterof the altitude circles is 30 inches, and that of the azimuthcircle is 24 inches. The divisions are cut upon a band ofsilver fitted into each circle, and situated near the circum-ference. The method adopted in graduating these circleswas that invented by the late Edward Troughton, Esq.,F.K.S., and described by him in the “ PhilosophicalTransactions, ” t as a method of dividing by the eye, todistinguish it from the old method of dividing by thehand.

The circles are divided into 360 degrees, each degreeinto twelve parts or spaces, and these are again subdividedto single seconds by two micrometer microscopes placeddiametrically opposite each other upon both circles.

The microscopes for reading the altitude circle arefixed to the opposite ends of an arm which revolves aboutan axis concentric with, but perfectly independent of, theaxis of the circle, which arm can be fixed in any positionat pleasure. This contrivance, which, in effect, is thesame as the power of changing the position of the telescopeupon the mural circle was, we believe, first applied tothis instrument.

The microscopes for reading the azimuth circle arefixed immoveably.

The azimuth axis descends within the pier to a depthof nearly three feet, and turns upon a steel point within aconical hole: the upper end of this axis is supportedwithin a rectangular Y by openings opposite the pointsof bearing, and the axis can be corrected in regard tovertieality by adjusting screws, which act upon the framein which the Y is fixed.

The azimuth circle is screwed to conical radii whichspring from the azimuth axis near its upper end.

The altitude axis resembles that of a transit instrument,and the diaphragm is illuminated through one of thepivots in the usual manner. From the central zone ofthe axis two sets of radiating cones diverge, and to theends of these cones the circles are screwed, one on eitherside of the telescope. These circles are connected by crosspieces around the circumference, where they also lay holdof the telescope, whicli is thus secured against any consi-derable flexure, and the cones are braced together by twoinscribed squares of brass, one upon each face of theinstrument.

The object-glass was made by the late Charles Tulley :the diameter of its aperture is 2*7 inches, and the focallength is 36 inches. At the eye-end there is a diaphragmof the ordinary kind, with five vertical and five horizontalspider lines. The eye-pieces consist of both direct anddiagonal, furnished with magnifying pow ers varying from60 to 150.

The transit axis is adjusted by means of a spirit-level;there is also a spirit-level applicable to the telescope forgiving the horizontal point, and a plumb-line apparatus isfixed to the column by which the micrometer support iscarried, and serves as a watch upon the stability of thisimportant part of the instrument. The construction ofthis instrument is light and elegant, and, as experiencehas shown, it is capable of giving results of a high degreeof accuracy; but the modern plan of forming astronomicalinstruments of as few pieces as possible, of w hich Mr.Simms has given specimens in the alt-azimuth and greattransit circle at the Royal Observatory at Greenwich , isfast superseding the more elaborate, and if not less accu-rate, certainly less durable construction.

This beautiful instrument was made by the late Mr.Edward Troughton, at about the beginning of the presentcentury, for Mr. Pond, the late Astronomer Royal, who,by the observations he made with it at'Westbury, demon-strated the change of figure of the great mural quadrantthen in use at Greenwich . After long exposure to the in-fluence of the atmosphere, and consequent decay, it wasin the year 1823 repaired and re-graduated by Mr. Simms,the Exhibitor.

* For a full description of this instrument, see Fearson’sAstronomy,” vol. ii., pp. 419-434.f See the volume for the year 1809.