Chap. VIII.

GEOMETRY.

8*15

attached to the instrument, for the purpose of taking observations with the sun, andmoderating intense light. Before used, the index and horizon glasses must be so adjustedthat they are perpendicular to the plane of the instrument, as well as parallel with eachother; when the index division of the vernier is at 0 on the arc, and the optical axis of sightor telescope, parallel to the plane of the instrument.

The Index Glass is adjusted by moving the index to the middle of the limb, and holdingthe instrument in a horizontal position witli the divided limb away from the observer, andthe glass to the eye ; look obliquely down the glass, so as to see the circular arc, bydirect view and by reflection in the glass at the same time, and if they appear to formone continued arc of a circle, the index glass is in adjustment.

The Horizon Glass is in its right position, when by a sweep with the index the reflectedimage of any object passes over or covers its image when seen directly ; any error in thiseffect may be rectified by turning a screw, placed at the lower end of the frame for thispurpose.

The Parallelism of the planes of the two glasses, when the index is at zero, may bereadily ascertained, by setting the zero on the index at the zero on the limb ; when, ontaking an observation, it is observed that the object seen by direct vision and that reflectedcoincide and appear as one, the glasses are parallel to each other : any deviation is calledthe index error, which does not often occur in a well-made instrument.

Hadley's Quadrant is often made use of in the same manner for determining the time, lati-tude and longitude of a place, and is as useful to the land-surveyor as the navigator : its arcis graduated to minutes of a degree, and the vernier at the extremity of the movable limbinto seconds, which can be seen by the aid of a magnifier attached.

The telescope at the sides can be moved upwards or downwards by a milled screwbeneath it: and it can be so placed that the field of view is bisected by the line that sepa-rates the silvered from the unsilvered part on the horizon glass: it is adjusted bv sliding the

Fig. 1070.

tube at the eye end of the telescope, so as to make the locus suit the eye of the observer.'I he inverting telescope is furnished with two wires parallel to each other, between whichthe observations are to be made, the wires being first brought parallel.

A more simple instrument is made on the continent, con-sisting of two side branches or radii, at right angles, asat A, having the limb E B divided into 90°, and each sub-divided into 60'. G F is the telescope, which is rectifiedby the frames II, II. At P is a joint into which the qua-drant is screwed ; S is the foot or stem, 5 feet in length,which lias a screw at T to secure it.

AM and IK represent bars of brass, on which the instru-ment traverses : V, V, are four screws by which the stand canbe elevated.

Vi hen this quadrant is used for taking heights, after theobservation is made by the telescope, the angle is ascer-tained upon the quarter circle by a plummet or bob at-tached to a fine silk, which works in the box B; thishanging always perpendicularly, the number of degreesis counted up to it.

Fig. 1071.

'The Barometer is frequently used for measuring the heights of mountains, it being foundthat the mercurial column invariably depends on, and is in proportion to, the atmospheric