796

THEORY AND PRACTICE OF ENGINEERING.

Book II.

appears in the centre of the telescope. The spirit level must then be adjusted in amanner that it shall be exactly parallel with the axis of the telescope, or what is called itsline of collimation. The end of the level next the eye-glass has a screw by which it maybe elevated or depressed at pleasure; having by means of this screw got the bubble in itsright place, the telescope must be reversed again and again, until it is seen that the bubblesteadily maintains its position. To take an observation, the three legs of the stand areopened and placed firmly on the ground, with the lower plate of the instrument as nearlylevel as the eye directs; then by turning the various screws the level can be soon broughtinto its proper position: three adjustments are always required for the Y spirit level : thefirst regards the wires of the telescope, which should be made to coincide very exactly withthe axis of the rings on which the telescope turns; the second adjustment brings the levelparallel to this axis; and the third sets the telescope perpendicular to the vertical axis, sothat the level may preserve its position when the instrument is turned round upon the staves.

SPIRIT LEVEL.

A A, the ends of the telescope.

C, screw for adjusting the telescope.

E, screws for elevating the instrument.G, the frame.

I, screw to secure the instrument.

L, rack for side motion.

B, the screw.

D, the spirit level.

F, the Y’s which support.

H, the compass.

K, screw to elevate the instrument.

To adjust the line of collimation, the eye-piece being drawn out, you should direct thetelescope to some fixed object, care being taken that you get a distinct view of the crosswires, and that you notice where their intersection cuts : after this, by turning the telescoperound on its axis, you must observe whether the wires cut the same object in the sameplace: should they do so, the instrument is fit for observation; but if this is not the case,the wires must be moved by turning the small screws near the eye-end of the telescope untilhalf the quantity of error is got over, and then trials must be made again, till the adjust-ment is correct. To place the level parallel to the line of collimation, the telescope mustbe moved till it lies in the direction of two of the parallel plate screws, and by then movingthe screws the air-bubble is brought to the middle of the glass tube: the telescope shouldthen be reversed endwise, that is to say, one end brought in the place of the other, andthen the air-bubble examined again; if it be not in its proper place, the parallel plate-screwsmust be used to make it: repeated trials are needed to effect this properly. To set thetelescope in a perpendicular position with its vertical axis, it should be first placed overtwo of the parallel plate-screws; then by unscrewing one, and screwing up the other, theair-bubble may be brought to the centre of the tube ; then the instrument must be turnedhalf round, and if not correct, then again adjusted; then turn the telescope one quarterround, and by repeated trials its adjustment may be completed.

Troughtoris improved Level is preferred by many to that of the Y, in consequence of itsadjustments not being so likely, after they are once perfected, to derangement. Thetelescope rests at once upon a horizontal bar made to turn round upon the head of thestaves which support it, in the same manner as in the theodolite. The spirit level isplaced on the top of the telescope, and over it the compass-box : the wire plate has