Fig. 54.—Surveyor's Y-level.
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192.—The Y's are erected upon the limb, to which they are each fixed firmly by a clamping nut R at one end, and a milled head clamp at M. The telescope is held down by strap pieces, each of which has a joint at one end and a loose pin at the other, PP. The pin is secured from dropping when out of use by a piece of cord attached to a part of the instrument and to a loop through its head. At the top of the inner side of the strap-piece under YY″ a piece of cork is inserted in a cave. The cork by its elasticity keeps an equal but light pressure upon the collar of the telescope. It will be seen that by the above plan of holding the telescope, it is so far free that it may be revolved on its axis, by which perfect adjustment of the diaphragm to the axis of the instrument may be made in any direction.
Fig. 55.—Section of parallel plate and vertical axis—arrangement of Y and other levels.
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193.—Parallel Plates as a mode of adjustment of the vertical axis will be first described, as they present the oldest form of setting up adjustment. The vertical axis of the Y-level was formerly carried tapering downwards, and the upper parallel plate was placed at about the centre of the socket. Under this construction the socket was more liable to strain from the use of the parallel plate screws. It is more general now to construct the axis as represented in the illustration above, Fig. 55, for Y and other levels with parallel plates. This construction also renders the instrument more portable, as the parallel plates and axis may be detached and lie closer in the case; the plan is nevertheless open to many risks, which will be referred to in discussing a three-screw arrangement. The general construction is shown in the figure, of which the left-hand side is a half-section. A is a screw by which the parallel plates are attached to the limb of the instrument; M a large milled head, by means of which the screw can be brought up firmly to its collar; SS′ the socket which is ground to fit the cone C; C forms a part of the upper parallel plate UP; B a ball pin which screws firmly into C; LP lower parallel plate, part of which forms the ball socket, so that the whole instrument rocks about the ball B as a centre, by the action of the parallel plate screws PS; B′ female screw for fixing this part, which is called altogether the parallel plates, to the tripod head. A clamping screw is sometimes placed upon the axis for slow motion. The parallel plate screws are tapped, that is, have female threads cut into the upper plate UP, and their points press the lower parallel plate LP at certain points, there being a stop-piece placed round the point of one screw to prevent rotation. The pressure upon the screws can be increased as desired by means of the milled heads, and the instrument made rigid in proportion; but it is very undesirable that the pressure should be greater than that just necessary to support the instrument firmly, as it is easy by the power of the screws to disturb the figure of the axis and thereby derange it.
The diaphragm of the telescope of the Y-level is generally webbed with plain cross webs. The diaphragm and webs were described [arts. 99 to 106].
194.—The Setting-up of the Y-Level is necessary to be understood before the instrument can be adjusted. The same description which answers for the setting up for adjustment will also answer for the setting-up of the instrument in the field for actual work. In this description it will be convenient, therefore, to consider the instrument as being in this case in adjustment as it leaves the hand of the maker. The after adjustments will be presently taken as from the original state of the instrument, as the maker has to do them in the first instance. Practically, the civil engineer has only to make slight differential adjustments at any time, as an instrument, by the solidity of its construction, will retain the general adjustment nearly, upon which further adjustments take more the nature of final corrections, which become necessary only from accidental causes.