Chap. VIII.

GEOMETRY.

795

with the instrument. Then the telescope mustbe turned half round, or the other side up-wards, in order to see if the thread cuts the samepoint; if it does, it is sufficient proof that thetelescope is properly centred, and has its sightparallel to the points of support; but if thispoint of sight or visual ray cuts above or below,then the screw must be turned which elevatesor depresses the object glass, until the visual ray coincides with an intermediate point,and then again reverse the telescope, to see if it is properly centred.

To rectify the Level, or to make the spirit level attached agree with the telescope, twopiquets, as G and II, are planted not more than 300 feet apart; then from the station Gobserve the piquet H, the spirit level being adjusted until the bubble of air is in its centre,as at K; the card I must then be raised or lowered until it coincides with the visual ray ofthe observer at G. The observer at G mustthen place against his piquet another card K, atthe height of his eye ; when looking at the cardI, he must then move the level to the piquet H,and place it horizontally at the same heightas the centre of the card I, to observe the pi-quet G ; then if its visual ray coincides with thecentre of the card K, it is a proof that the visualray is parallel to the horizon, and that the levelis rectified. But if it happen that the visual raycuts above or below the centre of the card K, as in the figure shown at L, then keepingthe eye at the same height, lower the telescope until the visual ray cuts an intermediatepoint, as M; keep the telescope in this position, and adjust the spirit level until thebubble is in the centre, which is done by means of adjusting screws. Then proceeding tothe piquet G, keeping the level at the same height as the centre of the card M, and bonethe middle of the card I, which if the visual ray cuts in the centre, it proves that thevisual ray is parallel to the horizon, and consequently that the visual ray of the telescopeattached to this level agrees with the spirit level.

To rectify the Level by a single Station. —

Having centred the telescope of the spirit level,place it on its stand ; then knowing two pointswhich are on a true level, and at a shortdistance apart, place the eye-piece of thetelescope at the height of one of these points, sothat the air bubble may be in the middle of theglass tube ; then if on boning the second pointit cuts the thread of the telescope, it is a proofthat the level is properly rectified and fit foruse.

It is known that the point A at the cornerof the house B is on a true level with thecill of the window of the inn D at C: centrethe telescope to that level in such a mannerthat the thread always cuts the same point Fw. 917 .when turned either way.

The Spirit Level in general use, of the most convenient and improved form, has a telescopeab, supported at each end, and a spirit level below it, inclosed in a brass tube, with a pieceof glass let in at the upper side, to observe the position of the air bubble : the instrumenthas a compass usually attached to it. By means of the socket F, and the screw below, thetelescope may be raised or lowered at pleasure ; on the plate M are four levelling screws,which pass through the upper plate, and by which it may be adjusted. The cross hairs inthe telescope are fixed to a brass ring placed within the tube, and are kept in their positionby four small screws, as seen at o on. The eye tube at a is to be drawn out until thesehairs appear exactly in the focus of the glass, and are clearly seen. The telescope is thendirected to some vertical object or piquet, and by turning the screw g it is raised orlowered to the required point. Clear and distinct vision of an object is obtained byturning the screw B, which by its connection with a rack and pinion contained in thetube •which carries the object-glass, either lengthens or shortens it: when this has beendone so that the horizontal hair is observed to be in contact with the object, turn thetelescope half round, so that the spirit level is above, and see if the same point ofcontact is preserved; should it not be so, then the screw oo must be turned until itis right in both positions: the horizontal hair being adjusted, the vertical one mustalso be treated in the same manner; when both are properly adjusted, their intersection

Fig.9IG.

Fig. 915.