see at once the object, and the intersection of the wires, clearly and sharply defined before it. THE VERNIER. The vernier is a contrivance for subdividing, to any extent, the smallest division in a graduated scale. In the theodolite the lower circle is divided into half degrees or 30 minutes; and the vernier so arranged as to read off to one minute. On inspection of the theodolite, it will be found that the degrees are marked with a longer line than the half degrees. Should the broad arrow of the vernier fall within any one of these half degrees, then it becomes necessary to measure this quantity, which, by means of the vernier, can be done to one minute. Hence to ascertain the number of degrees and minutes contained in a given angle, observe where the broad arrow of the vernier is; if, between a full degree and a half degree, so many degrees and as many minutes, as are denoted by the number of the first division line of the vernier (reading onwards as the degrees number), that coincides with the corresponding division in the limb; or, if between a half degree and a whole one, so many degrees and 30 minutes, plus the same number of broken minutes, as is denoted by the coincidence of the corresponding lines of the vernier and limb. Thus, if the broad arrow of the vernier point between 265° and 265° 30', look along the vernier from right to left till you find a line on the vernier coinciding with, or as it were, forming one line with one of the graduated divisions of the circle underneath-suppose this line to be 10' from the broad arrow-the angle will then be 265° 10'; suppose it 20', and the angle will be 265° 20', and so on. CHAP. II. THE METHOD OF USING THE THEODOLITE. There are two methods of using this instrument generally adopted; the first by the needle: the second by the back angle. The first (by the needle) I will briefly describe.—The broad arrow of the vernier, and the zero point of the horizontal limb, are, by means of the adjusting-screw made carefully to coincide, always with the magnifying glass; the needle is then released, and allowed freely to play upon its agate; and the whole instrument, with the two circles, firmly clasped together, turned round until the north end of the needle coincides, as nearly as the eye can tell, to the north point or zero in the graduated circle in the compass box. The whole is then clamped, and if in clamping any error has arisen, it is carefully corrected by the large adjusting-screw (T). Now, if the two plates be detached, and the vernier plate turned round to the object, the angle read by the vernier, will be the angle made at the station, between the first object and the north end of the needle. If the vernier plate be again unclamped, and turned round to the second object, the vernier will, in this case, also denote the angle made between this second station and the same north point. The second method, by the back angle. In this case the instrument is placed at the second station, the bearing between the first and second being assumed as a base line, determined in position, and the angles are all based upon that line; thus, supposing the starting-point to be A, the first station B, the base line AB, and the several stations C, D, E, F, &c., then the angle at B is that between A and C; the angle at C, that between B and D, and so on. THE PURPOSES FOR WHICH IT IS USED. There are two cases in which the theodolite is found invaluable, and to which it is principally confined. First. That of the survey of a road or river, by measuring along it, and taking the angles of its several bends, which is called traversing. Secondly. By determining the position of several new points, in relation to the position of some known line, by means of the angles made between these new points and the old ones. This is called surveying by "two stations." |