Ci»Ab3 X.] ME11Z* EQUATORIAL; BOND'S APPARATUS FOR OBSERVING- TRANSITS.

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sion is made equidistant from the two parallel wires.The axle of the telescope has a striding level for deter-mining its horizontally.

Troughton objected to repetition in large instruments.Zach and Bohnenbergev had shown that repetition intro-duced errors larger than those it was intended to correctin well-graduated vertical circles. Struve found that evenin the measurement of horizontal angles it was muchbetter to measure the angles on different parts of the arcthan to repeat. Accordingly, this instrument is con-structed to measure horizontal angles and double zenithdistances, but not to repeat. In the older instrumentsconstructed by Keichenbach and Ertel, the circles weremade to clamp at their circumference.

In a universal instrument having a horizontal circle of24 inches diameter (used by Struve as the principal in-strument for the triangulation for the Russian arc of themeridian passing through Dorpat), when clamped at thecircumference, on turning the slow r motion screw thespokes were found to head through an angle of nearly8" without moving the telescope. In order to avoid thissource of error, the clamps are applied to the axes of thevertical and horizontal circles of the instrument.

The heads of the foot screws are graduated; this facili-tates the operation of levelling the instrument, and alsoenables the observer to measure the angle correspondingto a division of either of the levels.

The axle clamps at the extremity most distant from theaxle have flat surfaces of polished steel. This surface ispressed by a spring against the point of the slow motionscrew. By this contrivance the axle is prevented fromrotating, without the danger of straining orbending it, orlifting a horizontal axis out of its Ys, which might happenif the outer extremity of the clamps were fixed, as in theaxle clamps that have been used in this country.

It is called a “ universal instrument” because it is atransit instrument, and serves also to measure zenithdistances and horizontal angles. It is the invention of thefirm now represented by Ertel.

The double wires (vertical for the telescope; probably20" distant from each other) were the line introduced byKeichenbach or Ertel. When a single line is made tobisect a very faint object, the object is extinguished.This is not the case when the object is made to bisect thespace between two wires 20' asunder, though it would beif the wires were much closer.

The telescope is diagonal, and consequently only one-half the usual length, the eye, in observation, beingalways at the axis; and by this means observations can bemade in the zenith.

There are on the inner portion of the horizontal circle10,800 lines; and on the outer portion 7,200, or alto-gether, 18,000 lines, as fine and distinct as on the largestastronomical instrument. This piece of work alone occu-pied a fortnight, working many hours daily; and it is amasterpiece of hand dividing.

M. Ertel’s objects have been, the combining of thegreatest possible simplicity with the greatest possiblefirmness.

This form of instrument, invented by M. Ertel, hasbeen used with great success in the measurement of thearc of the meridian, by Struve.*

The Jury voted to Ertel and Sou, a Council Medal forthis beautiful instrument, which award was sanctioned bythe Group ; but unfortunately did not pass the Council ofChairmen, who sanctioned the Prize Medal only.

Merz and Soxs (Bavaria , No. 30, p. 1100,) have exhi-bited an equatorial, with the polar axis adjustable withincertain limits, so as to adapt it both as a portable instru-ment, and applicable to different latitudes within thoselimits. The focal length of the telescope is 4 feet, and theobject-glass lias an aperture of 4 inches, a very unusuallylarge aperture for this size of telescope, admitting a largepencil of light, and thus adapting the telescope for faintobjects.

* For a description of this instrument, see 4i Descriptionde l’Observation Astronomique Central de Poulkova,” parF. G. W. Struve, and also the “Recueil des Aetcs,” pourl’annee 1831.

It is furnished with powers magnifying 20, 64, 96, and216 times. It has an hour circle, and a declinationcircle, both of 7 inches diameter.

The workmanship of this instrument is good, and fullysustains the justly celebrated name of the maker.

The telescope was tried twice; first, by Sir John Iler-schel, Lord Wrottcsley, and Mr. Glaisher , who reportedof the object-glass as being first-rate; and secondly, afterthe lapse of a month, by Sir David Brewster , M. Matthicu,and Mr. Glaisher , whose report was, that the secondaryspectrum w as not completely corrected, and this appear-ance was attributed by Mr. Glaisher to a disturbance ofthe lenses, since the first examination. At the time of thesecond examination, the image of the sun was good, andthe spots on his surface, with their penumbra, were wellshown.

A Council Medal was awarded to Messrs. Merz for thisinstrument.

AY. Bond and Sox (United States , No. 463, p. 1464.)exhibit an apparatus for observing transits, by means of agalvanic current. It consists of an electric break-circuitclock, a galvanic battery of a single grove’s cell; con-necting wires: a cylinder, around which paper is wrapped,and a spring governor, by which a uniform motion isgiven to the cylinder.

The clock is like those in common use for astronomicalpurposes. The pallets and the escapement-wheel areinsulated, both from the pendulum and from the otherwheels. When the battery is in connection, the circuit isbroken by the pallet leaving the tooth of the wheel, but isrestored at the instant of the beat of the clock, w hich is infact the sound produced by the completion of the contactrestoring the circuit; the passage of the current beingthrough the pallet and the escapement-wheel alone.

Two wires pass from the clock, one direct to thebattery, and the other through the break-circuit key usedby the observer, and through the recording magnet, backto the battery.

The electro-magnet, with a slight difference in the formof the armature, is the same as that of Morse’s telegraph.The armature carries a glass pen, supplied with ink froma small reservoir. Under this pen the paper revolves, onwhich the records are made. The breaking of the circuitby the clock, every second, is marked by an off-set madeby the pen; and the breaking of the circuit by theobserver, is similarly recorded, between the second marksof the clock. Unless a motion perfectly uniform is givento the cylinder, the second marks at the end of the hour,instead of being arranged in regular straight lines uponthe paper, will change their relative positions. Uniformmotion is given to the cylinder by means of an apparatuscalled a spring governor. The train of wheels whichcommunicate the motive power to the cylinder is con-nected with a small fly-wheel. This fly is for supplyingmomentum, and near it is placed a half-seconds pendulum,with a dead beat escapement. The connection betweenthe escapement-wheel and the fly' is through a shortspring. The elasticity of this spring allow s the motion ofthe escapement-wheel to be completely arrested at eaclivibration of the pendulum, while the momentum of thefly, acting for a small fraction of a second only on thespring, keeps up the motion of the cylinder.

The cylinder revolves in one minute; the second marksin a continuous spiral.

In observing, the observer, with the break circuit key atthe instant of the transit of a star over the wire of a tele-scope, touches the key, and the record is instantaneouslymade on the paper.

In the ordinary method of observation of transits, theobserver listens to, and counts the beats of a clock, whilsthis eye is directed to the object passing the wires; thushe combines the two senses, of hearing and seeing, in suchmanner as to be enabled to compute, mentally, the fractionof the second when the object passes every wire, the timeof which he then writes down in an observing-book, stilllistening and counting the beats of the clock, and so ontill the object has passed all the wires. In this newmethod, he observes with his eye the passing of theheavenly bodies, and at the same instant touches thebreak circuit key ; with this apparatus it is evident that a