Chav. VIII. GEOMETRY. 797

three threads, two of which are vertical and the third horizontal; there is also some-times a micrometer scale, fixed perpendicularly on the diaphragm in lieu of the wires :one edge of a fine slip of pearl with straight edges is applied for this purpose, which isdivided into hundredth parts of an inch, and again subdivided into lesser quantities: in thefixing of this pearl micrometer the divided edge is placed so that it intersects the line ofcollimation, the central division indicating the point upon the staff where the level falls.The telescope generally shows the object inverted, consequently fewer glasses are required;and in this as in the Y level, the line of collimation and the level must be made parallelwith each other, and the telescope brought into a position exactly perpendicular to thevertical axis, and so that the air-bubble when turned round horizontally should alwayspreserve its position in the middle. This kind of spirit level being firmly fixed in its cell,the line of collimation has its adjustment given by the aid of two screws near the eye-end

Fig.919.

ricoroirro's’s improved level.

AB, the telescope. CD, horizontal bar.

F F, the spirit level. G, the compass-box.

ej , capstan screws for adjusting the level.

of the telescope. A bench mark made against a wall should be every now and thenexamined by the instrument placed at the same height from the ground, and any error inits collimation then would be readily discovered.

Mr. Gravatt's Level is another modification of the above, and has an object glass of largeraperture and shorter focal length; it also has a diaphragm with cross wires, and the spiritlevel is placed above the telescope; there is also a small mirror so fixed on a hinge that theposition of the air-bubble can be seen at all times by the observer; at the same time hereads oft' the staff.

Levels were formerly constructed with two telescopes, as A 13 and C D, each about 20inches in length, placed in such a manner that when the eye-glass of A B was at A, thatof C D was at D : each of these was so con-trived that they could be elevated or depressedat pleasure, by screws placed at F and E.

GI and H K were pivots placed at right angleswith the bottom, and were the points on whichthe levels were supported: by turning therule E F round on its pivots, the two tele-scopes were made to change their positions.

To the support of the level I K is attached aweight at X, of a square form, and weighing3 or 4 pounds, which is placed there to maintainthe telescopes in a state of equilibrium, but soas not to prevent their motion on their axis atQ. At R is attached three rings which holdthe eye or handle V: by means of the handleat T, which passes through the upper part of the box at S, the levels are raised or de-pressed when required ; the box Z, which contains this spirit level, is made of mahogany,and is furnished with a screw at Y to adjust the level.

Such a box placed on its stand was much in use in France , and its rectification waseffected in the ordinary way, by centring the two telescopes in such a manner as to maketheir visual rays always in a parallel with the pivots on which the circular motion of thetelescope was made: the level being mounted on its stand at the same height as theobject to be observed, the weight was permitted to remain at the bottom of the box, sothat the telescopes were not subject to motion or to oscillate: then looking through the

v

Fig. 920.