Chap. VIII.

GEOMETRY.

841

verniers; with these horizontalplates* and their divisions, allangles in a plane parallel tothe horizon are taken in de-grees and minutes, and theyare adjusted by four parallelplate screws, g , set in pairs op-posite to each other; thesescrews are to be moved ac-cording as the two spirit levelsplaced at right angles a in-dicate, and the whole broughtto a perfect level. Thesespirit levels are usually filledwith spirits of wine, and arehermetically sealed ; the tubeis slightly curved, and isplaced with its convex sideupwards, so that the air bub-ble, when level, occupies thehighest central part. Thesetwo plates, called the upperand lower horizontal, are,when adjusted, retained intheir position by clamp screws,It. On the upper plate rests aframe, on which are the angularreceptacles called Y’s, intowhich the telescope is placed;and they are so fixed, thatby turning the telescope to ob-serve an object, the horizontalmotion is communicated to oneor both of the circular plates.The telescope has in the focusof the eye-piece and object-glass three lines formed ofvery fine wire, one of whichis horizontal, the others cross-ing its middle or central pointdiagonally, so as to divide theglass into an hexagon. Bymeans of these wires an objector any part of it may bepointed at. From the low’erpart of this telescope is sus-pended a spirit level, whichbeing more delicate than thosefixed on the vernier plate isused finally to adjust the cir-cular plates, and to bringthem to a true horizontal po-sition. The under part of thetelescope has a vertical semi-circular arc e, for the purposeof taking altitudes, the axis ofwhich rests on two points inthe frame which supports it,

A, B, the horizontal limbs.

A, the vernier plate, which turns.

C, the vertical axis.

1), the ball or socket movement.dd, spirit levels.

K, a magnifier to read off the degrees;

F & G. Plates are held together by the ball D.

//, is a screw to adjust the level or line ofcollimation.b, b , are milled screws to adjust the instrument, and set in level.p. is a screw to adjust the telescope laterally.

H, a clamping screw, by which means the collar c may be tightenedto the axis C, and kept from moving.

J, the magnet box.

I, is a slow-motion screw, by which the instrument is moved moreexactly than could be done by the hand.

i, t\ clips, to reverse the telescope by screws j,j.

K & L are frames into which the pivots are placed, on which thevertical arc M is turned round, and on which the telescope is fixed.N is a microscope for reading off the degrees.

O is a clamp screw.

P a slow-motion screw, by which the vertical arc and telescope aremoved.

Q, a milled screw for moving the object-glass of the telescope.

so that when the upper plateis horizontal, the semicirculararc attached to the telescope is in a vertical plane; the angle of inclination being indicatedby a fixed index and vernier attached to the upper plate. This vertical arc is adjustedand retained at any required angle by means of a clamp and tangent screw. One sideof the arc is divided into degrees and parts, and the other shows the difference between anhypotenuse of 100 units, and the base in right-angled triangles calculated to degrees ofinclination of the hypotenuse, from O to 45. To the upper plate is attached a compass,which serves to notice the bearings of the different stations, and also is a check to the angles