Class X.'
SURVEYING AND LEVELLING .INSTRUMENTS—THEODOLITES.
that of the tube suspended within it, the air with whichit is filled will not be compressed sufficiently to admitwater into the glass tube connected with it until the leadshall have descended to a depth of 25 fathoms. This tubeis therefore well adapted for the measurement of deepsoundings, whilst the other registers small depths, whichotherwise could not be indicated. This instrument in itsuse is attended with less loss of time than the casting ofthe ordinary deep-sea lead.
Mr. Ericsson lias exhibited an instrument for measuringdistances at sea. This instrument is intended chiefly forthe use of naval officers, to enable them to determine, bymeans of a single observation taken at sight, the distanceof a vessel, either advancing to or receding from theobserver.
If the eye be placed at a certain height above the levelof the sea (say on the main-top of a ship), the verticalangle, formed by a line passing from the eye to any fixedpoint in the horizon, and a line from the eye to the waterline of the hull of a vessel in the distance, will becomegreater as the vessel approaches to, and will become lessas the vessel recedes from, the spectator; to measure thisangle and to determine the distance of the ship, is thechief purpose of this instrument, which is composed of thefollowing parts: —
An ordinary reflector, an object-glass, and a sight formeasuring angles. The reflector is attached to the end ofa spindle which is turned by means of a lever. At thelower end of this lever, in a slot, a sliding nut movesfreely up and down, in which a thumb-screw is made towork. A pinion on this screw works into cogs cut in thecircumference of a graduated index-plate, which is sup-ported upon a frame, and revolves by means of a socketin a centre piece. The scale of the index-plate is gra-duated into yards for every variation of distance of 10yards, from the horizon to 400 yards distance from theship, as viewed from an elevation of 100 feet. For theseand the other beautiful instruments exhibited by Mr.Ericsson, a Prize Medal was awarded to him.
Keller (France, No. 280, p. 1190) exhibits a doubleplanisphere, designed to assist navigators in great-circlesailing. It consists of two concentric circles, the one fixedand transparent, the other moveable.
St. John (United States , No. 95, p. 1439) has exhibiteda very ingenious self-detector compass. In appearance itis very similar to the ordinary box-compass, and differschiefly in the following particulars:—upon the compasscard, and attached to it by pins, are two small needles,called satellites; to the centre of each a brass indicator isfixed; on the face of the card are engraved two semi-circular scales, or arcs of circles, so placed that theircentres are in the centre of the compass card, and in aline joining the centres of the satellites: both these scalesare graduated to degrees, the numbering of the graduationproceeding on either side from the central point of eachscale. The satellites are balanced upon pins, move freely,and thus permit the brass indicators to move easily overthe graduated arcs, according to the amount of disturbance.The satellites and the main needle being equally mag-netized, remain stationary when there is no cause of dis-turbance in the magnetic meridian, and the brass indicatorspoint to their respective centres; but, should there be anycause of disturbance, the indicators move simultaneouslyin opposite directions, thus indicating its amount, and if thecourse of the disturbance be local, the indicator attachedto that needle, the nearest to the seat of disturbance, willpass through the greater angle, and thus the direction ofthe disturbing cause is shown.
Mr. St. John has also exhibited an aquatic velocimeter.This instrument is designed to show upon a dial-face thedistance traversed by a vessel in a given time, and itsvelocity. Upon the dial are three circles, each providedwith an index, and so graduated that the first performsone revolution in a mile, the second in 100 miles, and thethird in 1,000 miles. The motion is communicated to theclock mechanism by means of several mechanical adjust-ments, within a tube, which is inserted in the bottom ofthe vessel, and protected from injury by a coating ofIndia-rubber, and a wooden box, by which it is completelyenclosed The chief part of the mechanism consists of a
revolving shaft, which receives its motion trom a paddle-wheel attached to it. The water acting upon its fanscauses it to turn round, the shaft in connexion with itperforming one revolution in a distance of 4 feet. Thearrangement also includes a metal frame, and pistons toobviate any danger which might arise from the positionof the pipe within the vessel; there are also facilities fordisconnecting the apparatus from the clock-work, for thepurpose of setting the index to any time or point desired.This instrument is new, and seems to be well adapted forits work. The Jury consider much merit due to Mr. St.John for these inventions, and award the Prize Medal tohim.
Surveying Instruments.
The instruments exhibited in the British portion of thissection are few in number, and, with the exception ofthose by Simms, Dollond , Marratt, and Yeates, arc, forthe most part, of an ordinary kind, there being neitherimprovement nor attempt at such in their construction.
In the Foreign Department it is otherwise. In Belgium may be found Beaulieu, worthily following in the steps ofhis celebrated master, Gambey. In Germany , Breithaupt’sadmirable method of covering the divisions of the circleby a thin circular plate seems to be generally adopted, andcommands attention. The further improvements whichhe has introduced in his surveying instruments will bedetailed presently.
The Polytechnic Institute of Vienna has exhibitedvarious instruments for levelling, which present manyimprovements, the result of more than an ordinary degreeof attention. Mr. Burt, of America , has exhibited a com-pass well adapted for surveying.
Surveying and Levelling Instruments.
Simms (No. 741, p. 476) exhibits a transit theodolite.This kind of instrument was introduced a few years sinceby Mr. Simms; it is intended especially for the use of thescientific traveller and civil engineer. It is, in fact, anextremely portable altitude and azimuth instrument. Theordinary vertical arc of the theodolite is extended to acomplete circle, and is read by two opposite verniers.The range of the telescope is unlimited, like that of atransit instrument, and by means of a diagonal eye-piece,observations can be made in the zenith. The axis is per-forated for illumination of the field of view. The instru-ment is 8 inches in diameter; it is graduated in silver to10 ,/ of arc; its readings are by means of verniers. Byestimation the angle can be read to less than 5''.
The telescope is 13 inches in focal length, and 1} inchaperture, and is furnished with magnifying powers ofabout 25 and 40 times. The magnetic needle, spirit level,lamps, and tangent screws, and other fittings, do not differmaterially from those usually adapted to theodolites ofthe best construction.
Dollond (No. 145, pp. 426—428). A transit theodolite.The circles are 12 inches in diameter, each reading by'verniers to every 10" of arc. The telescope fixed to thealtitude circle is 20 inches focal length, and 1*6 inchesaperture, furnished with direct and inverting eye-pieces,and is supported on Ys, on two strong cones resting onthe azimuth circle. The compass is 5 inches in diameterand divided to every degree. The spider lines are illumi-nated in the usual manner through the axis of the altitudecircle.
Yeates (No. 332, p. 446) exhibits a very portable smalltheodolite, with good divisions, well made, and stronglyput together. It is furnished with one large spirit level.Also a 4-inch theodolite, displaying good workmanship;a prismatic compass with a spirit level attached, and anoptic square, intended for use in determining distances.All these instruments are well made, and deserving ofHonourable Mention.
Marratt (No. 409, p. 554) exhibits a 7-inch theodolite,which reads to 15". It is furnished with a locking-plateand tripod. The lower portion consists of a massivetripod with foot-screws, to which is fixed the centre onwhich the instrument revolves. The lower limb is in onepiece, and is furnished with tangent-screws and threeverniers. The exterior centre and Ys for the support of