212.—It will be seen that by this means firm adjustment may be made either by raising or lowering one end of the telescope, as also by a lateral rocking motion should the web or bubble not be quite to position. This plan is certainly moderately solid, and little fault can be found with it, except that a little torsion may be put on the telescope by unequal screwing, and that it appears slovenly in leaving an open gap between the limb and block; therefore the author prefers in his own form of level, which will be presently described, that the block be solidly fitted down upon the limb, as is shown in the section Fig. 60, and the telescope be placed permanently exactly parallel with it. If the vertical axis be once fixed truly perpendicular to the axis of the telescope as solidly as possible there is very little risk of a bell-metal centre of ¾ inch or so diameter being bent; therefore all parts may be closely fitted between the axis and the telescope. Some makers, instead of screwing down at both ends of the limb, make one end a rocking centre and adjust only by screw at the other end. This plan lacks a little of the stability looked for in the dumpy system. The general construction of the vertical axis is the same as that of the Y-level already described. The parallel plates, tripod head, and tripod are also the same, [art. 193, Fig. 55].
213.—As the telescope of the dumpy level does not possess any simple means of determining the accuracy of the fitting of its sliding tube, it is a very important point in these levels that this fitting should be good, so that the object-glass does not droop when extended. For this reason the inner sliding tube of the telescope should be as long as possible, and its adjustment by the rack sufficient to bring an object in focus at 15 to 20 feet distance. This point is sometimes neglected. The author was once amused by a young surveyor bringing him an invention, which was to fix two points by the side of the telescope to enable him to read at short distances. It was seen on examination of his own level that his telescope, a badly-fitted one, would not read at half a chain, hence the ingenuity of his invention. In some cheaply made levels the solid ring fitting to the telescope, above described, which connects the limb firmly with the bubble tube, is replaced by blocks soldered on the telescope with soft solder: the method is very unsound from risk of imperfect soldering. The blocks are very liable to become loosened by a jar.
214.—The diaphragm of the dumpy level is generally webbed with two vertical webs and one horizontal. In use the image of the staff is brought between the vertical webs, which indicate whether it is held upright. The upper margin of the portion of the horizontal web between the two vertical ones is the index of level to which all readings are made, either for adjustment or for reading the levelling staff in the field. The somewhat loose and slovenly four-screw adjustment for a level diaphragm used in rough work with capstan-head screws, shown [Fig. 23, p. 50], which is necessary for the adjustment of the telescope in Y's, has been abandoned for many years in the better-constructed dumpy levels by all good makers, and the more solid construction, shown below, Fig. 61, used in the place thereof. In this plan there is no lateral adjustment: the diaphragm is carried as a frame in a dovetail slide, and is adjustable by vertical screws only. The figure shows the face of diaphragm:—BB′ slide pieces, A slide moved by capstan-head screws.
Fig. 61.—Diaphragm of dumpy level with webbed stop.
Fig. 62.—Same, with stadia webs.
Larger image
215.—Subtense or Stadia Webs.—It is very advisable in all levels to have two extra webs, or lines cut on glass, placed one on each side of the central horizontal web or line, fixed at such a distance apart that the image of 10 feet of the staff when placed at 10 chains distance may exactly cut the inner space between the lines. These webs or lines may be used as a means of measuring distances often more exactly than can be performed with the chain if the surface of the land is irregular; or, in any case, they form a good check upon chain measurement. If the webs or lines are separated so as to subtend an arc whose chord is 10 feet at 10 chains, it is easily seen that 1 foot of the staff will represent this chord at 1 chain, and that each ·01 of the foot on the staff will represent 1 link in distance. A diaphragm webbed or lined in the manner described is shown in Fig. 62. There is some difficulty in placing webs in exact position, and allowance should be made for the optical conditions by the addition of a plus factor. This important subject will be fully discussed hereafter in Chapter XII.
Fig. 63.—Tripod.