250

SIMMS’ ALTITUDE AXD AZIMUTH ; ERTEL’S UNIVERSAL INSTRUMENT. [Class X.

Simms (741, p. 476*). A portable altitude and azimuthinstrument, with circles of 15 inches in diameter, havinga telescope of 24 inches focal length, and of 1*9 inchaperture, -with two direct eye-pieces, of powers of 35 and60, also a diagonal eye-piece magnifying 40 times.

This instrument is of the most recent form. A strongbrass tripod supports the superior parts, to the centre of-which the azimuth axis is fixed, and the azimuth circle isscrewed upon its face.

A strong plate circulates upon the azimuth axis, carry-ing two pillars of massive proportions, by which thealtitude circle is supported. To these pillars the azimuthmicroscopes are fastened, and one of these pillars carriestwo ccuical branches at its upper end, for the purpose ofholding the microscopes by which the altitude circle isread, and a sensitive spirit-level is so placed as to detectany instability in these brandies, and to give its amountin arc. The telescope and axis are formed like those of atransit instrument, and two circles are on each side of thetelescope, braced together with it by intervening pillars.

The circles were graduated upon Mr. Simms’ self-actingengine, upon rings of silver, and the subdivision is madeto single seconds of an arc by means of two oppositemicrometer readings upon each circle. The microscopesof this instrument have achromatic object-glasses.

The transit axis is adjusted by the usual standing level,and each circle is fitted with a clamp and tangent screwmotion.

The novelty in this instrument is the introduction of acentral collimator, a ready means at all times of deter-mining the nadir point: this important addition is the in-vention of Mr. Simms.*

The invention consists in placing a small achromatictelescope in the centre of the azimuth axis, which is per-forated for its admission. It is furnished with a cross ofdelicate spider lines in the focus of its object-glass, andhas suitable adjustments.

If the principal telescope of the instrument be directedto the collimator, an image of the cross lines of the latterwill be seen upon the diaphragm of the former, and as thecollimator remains at rest, being in firm connection withthe tripod upon which the instrument stands, it followsthat if the superior parts of the instrument be turnedround in azimuth, the axis of the two telescopes may, byadjustment, be made to remain perfectly coincident duringan entire revolution.

If, in this state of things, the azimuth axis be set trulyvertical by means of the spirit-level, which is fixed uponthe instrument, the centre of the collimator becomes anadir point to which all observations may be referred.

This point lias additional uses; for instance, in perform-ing the collimation adjustment of the telescope, and insetting the altitude axis at right angles to the azimuthaxis, and would consequently supply the place of thetransit level, if by accident the latter were broken.

The importance of this invention, especially to thescientific traveller, is obvious; several such instrumentshave been made by the Exhibitor, which are in use inthe Boundary Survey of the United States , and on othersimilar services.

For the several inventions in these beautiful instru-ments, the Jury voted, unanimously, the Council Medalto Mr. Simms, which award was also passed unanimouslyby the Group, but was not confirmed by the Council ofChairmen , and therefore Mr. Simms will not receive thatkind of Medal to which the Jury considers him fullyentitled.

Crickett (267, p. 437) exhibits a model of a stand formounting an equatorial.

Ertel and Son* (2. Zollverein, Bavaria , No. 25, p.1099,) exhibit a portable universal instrument, with hori-zontal circle of 15 inches in diameter, and two circles ofaltitude of 10 inches in diameter. The horizontal circleis divided to two minutes of arc, and is read by twomicrometer microscopes to seconds; it is also furnishedwith four verniers, which read to two seconds. The divi-

* For a full description of this invention, see the 15thvolume of the “ Transactions of the Royal AstronomicalSociety .”

sions are fine, clear, and distinct. The circles of altitudesare well divided to four seconds of arc. The two reflectorsfor illuminating the division under the microscope are somade as to receive light from all sides.

All the clamps are applied to the centre to prevent anybending of the spokes, or affecting the figure of thecircle,*

On fixing the lower clamp, the instrument was found tobe absolutely fixed; and in this state was capable of beingused as a transit circle. The level was made to rest onthree bearing points; it is good; and when placed on theinstrument no motion of the vertical circles affected thehorizontal fixity of the instrument, or vice versa.

The horizontal circle is, in fact, composed of two con-centric parts in the same plane;—an inner one, whichmoves on its own pivot, and revolves, without touchingthe outer part of the circle, and each moves indepen-dently of the other. The centring of these circles isbeautiful.

This is a telescope of security, and which has its ownmotion, and can he set upon any object; and the fixity ofthe instrument when thus tested was found to be quitesatisfactory.

At the intersection of the telescope with its horizontalaxis is placed a right-angled prism. By internal reflectionat the hypothenusal surface of the prism rays of lightcoming from the object-glass are reflected towards one endof the axis where the eye-piece is placed, so that in allpositions of the telescope the observer looks in a horizontaldirection; on the other end of the angle is a graduatedcircle of 10 inches in diameter, with four venders (in thesame plane as the graduated limb of the circle) eachreading to 4". The vernier circle carries a level moveableround the axis of the vernier circle, and capable of beingclamped to it in any position with the level fixed: adouble zenith distance of any object may be observed onone part of the limb. The level may then be fixed in anyother position, and the double zenith distance measuredon any other part of the limb.

The supports of the horizontal axle of the telescope risefrom a strong horizontal piece of metal, to which isattached a vertical steel axis, passing through a verticalcylinder—terminating, below, in a tripod with foot screws.The steel axis carries a horizontal circle, graduated by aline at every three minutes, and also two arms carryingmicrometer microscopes. The vernier circle (15 inches indiameter) has a hollow* axle which fits on the outside ofthe cylinder through which the axle of the vernier circlepasses, and is capable of being made to revolve in its ownplane. Attached to the end of the vernier circle is a cheektelescope with a vertical wire moveable by a micrometerscrew.

The vernier circle is graduated by lines at every twominutes. This graduation is read to seconds by each ofthe two microscopes. The vernier circle being capable ofrevolving in its own plane, horizontal angles may be mea-sured on different parts of the arcs, by both the verniersand the microscopes. The microscopes have two wiresvery close to each other parallel to the division in thecentre of the field. In observing, the image of the divi-

* The axle clamp, as used by Troughton. is essentiallydifferent from that employed by Ertel. AVitli Trouthtonit was a matter of convenience: with Ertel it is consideredindispensable in order to obtain accurate observation. Itis used for the horizontal circle of the Westbury circle,while the more important part, the vertical circle, has aclamp on its limb. In the AVestburv circle the extremityof the arm of the clamp is fixed in the direction of its lengthas well as in the direction perpendicular to its length.This, in the case of the ‘Westbury circle, strains thevertical axle. In the case of a transit instrument, it eitherbends the axle downwards, or lifts it out of the nearest Y,so that it touches one side of the Y only, or rocks fromside to side. In the Munich instrument the axle clamp hasa polished surface, which is made to press against the endof a screw by means of a spring or the tension of a springby which a weight is suspended, and is acted upon by noforce in the direction of the length of the arm. I findthese clamps recorded as new in notes made during a stayat Munich.-AY. II. M.