Class X.] SURVEYING AND LEVELLING INSTRUMENTS; ASTRONOMICAL COMPASS.

9SS

and an eye-piece, adjustable by screwing, similar to thoseof the levels. The telescope can be pointed to any object,from the zenith to a depression of 5U D or more below thehorizon, and can be clamped, at any altitude, to an armwhich is moved by a micrometer screw; and thus dis-tances can be measured from one station, by observingthe altitudes or depressions of two fixed marks on a staff.A level, like that of the transit instrument, is applied tothe axis of the telescope. (Prize Medal awarded.)

Llttig (Prussia, No. 81, p. 1053) exhibits a levellingprotractor, the divisions of which are good.

Kixzelbach (Bavaria , No. 26, p. 1115) has exhibited asurveying cross, being a cone of brass, with two pairs ofline slits, for sights, at right angles to each other, runningobliquely up its sides, and revolving on a base, the cir-cumference of which being graduated, is read off with avernier, the whole being screwed on the top of the sur-veyor’s rod.

Mr. Kinzelbacli lias also exhibited a diastimeter, ordistance-measuring instrument; being a telescope, pro-vided with a micrometer and divided scale, the micrometerbeing mounted with two parallel wires, moveable simul-taneously along a scale, graduated to minutes, so as toremain at equal distances from the centre of the field ofview, and thus to embrace the area of greatest distinctnessin all cases. Also an improved Wollaston's goniometer.

Imperial Polytechnic Institcteof Vienna ( Austria ,No. 130, p. 1014) has exhibited several beautiful instru-ments, constructed according to the plan suggested byProfessor Stampfer.

1. A large level.* The aperture of the object-glass is15 lines, and the power 20. The support of these levelsconsists of three feet, attached to the sides of a triangularprism by strong screws. The upper part of the prism is atruncated cone, fitting into a slightly conical socket, whichforms the lowest part of the brass-work of the level. Thissocket is clamped very firmly on the stand by a singleturn of a screw, so that the level can be readily attachedto, or removed from, its tripod. The lower part of thelevel consists of two circular plates, connected by a balland socket, as in the levels constructed in this country.But instead of four screws for making the upper platehorizontal, the Austrian level has two screws and twostrong springs, the screw being at one end, and the springsat the other end of each of two diameters of the plates, atright angles to each other. By this contrivance the uppercircle is more quickly and easily made horizontal than inmany other known instruments of this kind, without thedanger of either shaking or bending the plates, attendantupon the use of four screws.

The upper circle, which is about 4 h inches diameter, isgraduated, and has two verniers, reading to 30".

The greatest improvement introduced into this level isa micrometer screw, by which an angle in a vertical plane,if not more than 8^, can be determined to within 1" or 2"of the truth. This screw is of great use in making thetelescope very accurately level, when the instrument isemployed in the ordinary manner, and the situation of thelevelling staves so chosen that they are intersected, atsome point of their length, by a horizontal plane throughthe axis of the telescope. By the aid of the micrometricscrew, however, the difference to the altitude of twostations can be determined, when it greatly exceeds thelength of the levelling staff. For this purpose the staffcarries two marks, at a known invariable distance fromeach other. The angular altitudes or depressions of thesetwo marks, measured by the micrometer screw, serve todetermine, with great precision, not only the inner eleva-tions or depressions of either mark above or below theaxis of the telescope, but also its horizontal distance fromthe centre of the instrument. These distances, combinedwith the difference of azimuth of the stations observedwith the horizontal circle, afford data for constructing amap of the country passed over in levelling. The selectionof stations in this mode of levelling is limited only bythe condition that the elevations or depressions of themarks shall not exceed 8 D . By one observation, differencesof level of 100 feet between two stations may be measured:

thus the stations may be taken at much greater distancesfrom each other than in levelling by the old method.This advantage is particularly felt in levelling through ahilly country. By means of this instrument tiie altitudesof distant objects may be measured with great precision.

The telescope is adapted to the distance of the objectobserved by a sliding tube carrying the cross wires, move-able by a rack and pinion. This tube moves betweenthree bearings, attached to the inside of the outer tube, atequal distances from each other; one of these is a springwheel, which, by its constant pressure, prevents all shakeof the tube carrying the micrometer. The telescope isadapted to the focal length of the eye of the observer, by-having that lens of the eye-piece which is next the eye set! in a cap, which screws on the end of the tube, and byturning it is brought to the proper distance, to give dis-tinct vision of the cross wires.

2. Smaller levels, of similar construction, in one ofwhich the telescope has a power of 15. In another thepower equals 12. A fourth is similarly constructed, withthe exception that it has no horizontal graduated circle:the power equals 6.

3. Two pocket telescopic levels.

4. A ruler for plane table surveying, with a telescopeprovided with a micrometer screw, similar to that of thelarge levels for measuring distances.

5. A ruler for plane table surveying, with telescopicsight. The Jury awarded Honourable Mention for theseinstruments.

ScHRODTEit (Prussia, No. 484, p. 1078) exhibits a 6-inclitheodolite, of very good workmanship. The Jury awardedHonourable Mention to Mr. Schrddter.

Imperial Ijorsk Works (Russia , No. 169, p. 1372)exhibit a levelling instrument, mounted on a well-bracedand very firm tripod stand. The instrument is well madein every part, with good divisions, and reflects credit uponthe artist.

Burt (United States , No. 187, p. 1449) has exhibitedan “astronomical compass,” an instrument intended forthe survey of lauds, bays, &c., for the determination oflatitudes, apparent time, and the magnetic declination.

It consists of two plates of about 6^ inches in diameter:the upper revolves about the lower by means of a centre-piece, which, when fixed, leaves the under plate to revolvefreely. The two plates can be clamped together by meansof two clamps. The upper side of the lower plate isdivided, the graduations being seen through two openingsin the upper plate, and which are furnished with verniers.A small magnet is placed on the upper plate for deter-mining the declination. To the upper plate is fixed agrooved arc, a latitude arc, a declination arc, an hour arc,two spirit-levels, &c. The latitude arc is fitted to one end ofa curved bar, the other end of which is in connection withthe hour arc, and can be adjusted to the latitude of the place.The declination arc is placed upon a limb which revolvesequatorially upon a centre, and there is a second moveablelimb turning ou a pivot at one end, and furnished with avernier at the other, which moves on the declination arcand affords a means of clamping it to the sun’s declination.To each end of a moveable brass limb a small brass plateis attached at right angles; into the upper side of one,and to tlie lower side of the other, a small convex lens isinserted, fitted with a sliding shade, through which thereis a small hole; opposite to each lens is fixed a small silverplate; on these plates fine lines are drawn, sufficientlyseparated to include the image of the sun. The arcs oflatitude and declination have each a radius of 5 inches,and are graduated to 15’, and read by verniers to singleminutes. The hour circle has a radius of 2r} inches, andis divided to half degrees. The instrument is furnishedwith every means of adjustment and verification. It iswell adapted for surveys in new districts, and has renderedgood service in magnetic districts, where it is understoodthat instruments constructed with magnetic compass havefailed. The Jury voted a Prize Medal to Mr. Burt.

Standard Measures of Length.

Whitworth and Co. (Class VI., No. 201, p. 290) haveexhibited a standard bar measurer. This machine consistsof a metal frame, at each end of which is placed a micro-

* See the 20th volume of the Polytechnic Institute.