larging, or reducing plans by simply going over the lines with a tracing point. It is shown in Fig. 50.

Fig. 50.-Pantagraph.

The Eidograph.-This is a similar instrument, and was invented by Professor Wallace to remedy the unsteadiness of the pantagraph, which arises from its numerous joints and supports the paper.

upon

As a rule, neither the pantagraph nor the eidograph is a satisfactory instrument. They require a great deal of room on the drawing table, and for copying, enlarging, or reducing plans, one of the methods already described is recommended. Full descriptions of both the pantagraph and the eidograph are to be found in Heather's "Mathematical Instruments.'

Lithographing Plans When a great many copies of plans are required, it is usual to have them lithographed. The process of lithographing consists of first making a tracing of the plan in specially prepared lithographic ink. This is laid face downwards. on the stone, and the copy transferred to it. From the stone as many copies as are required are printed off. As the tracing is damped before it is laid on the stone, it expands, and sometimes gets stretched unequally. The impression is therefore usually larger than the original. The paper on which the copies are taken off the

*To get really satisfactory results with the pantagraph and eidograph, very well made and expensive instruments are necessary, and an extra large table to use them on.

stone being also damp during the process, shrinks after drying, which to some extent counteracts the expansion and stretching of the tracing. As a rule, however, lithographic copies are not to be depended on when much accuracy is required. All drawings to be lithographed should have a scale drawn on them so that it may be affected by the expansion and shrinkage along with the drawing. Even then the lithographs will often be found not to scale.

Reducing Plans by Photography.-The 6 in. Ordnance Sheets are reduced from the 25 in. sheets by photography. The details are then traced from the photograph, and the tracing is placed on a copper plate. The lines are then gone over with a sharp instrument which cuts through the tracing and indents the copper underneath it. When great accuracy is required, as in the case of the main stations, the points are plotted on the copper. Engravings from copper usually shrink, as they are taken off the copper when damp, and all measurements should therefore be taken with the scale drawn on the sheet. A process for photographing the details directly on to copper or zinc has also been introduced.

CHAPTER II.

SURVEYING WITH THE AID OF ANGULAR INSTRUMENTS.

Instruments: Theodolite.-By far the most useful instrument to the surveyor is the theodolite. The chief use of the theodolite is for measuring and setting out horizontal and vertical angles and for ranging out lines. Fig. 51 shows the instrument, as made by W. F. Stanley, Great Turnstile, Holborn, London. Fig. 52 shows Cooke's transit theodolite, as made by Messrs T. Cooke & Sons, York, and 8 Victoria Street, Westminster. Very good instruments are also made by Messrs Troughton & Simms, 138 Fleet Street, London (see Fig. 53). Each maker's instrument varies in small details of construction, but the main features in all are the same.

ab is the telescope, which is simply an ordinary telescope with the addition of a diaphragm c. The diaphragm is either a glass with fine lines etched on it, or a brass ring having fine spider's hairs or wires stretched across it, as shown in Fig. 54. Stadia hairs (see Chapter VI.) should be added as shown at s, s. The diaphragm is held in place by the four capstan screws d, d,d, d, Figs. 52 and 54, and is adjusted either horizontally or vertically by means of these screws. When properly adjusted, the intersection of the hairs at the centre of the diaphragm is in the axis or "collimation line" of the telescope. The telescope is focussed by moving the object glass at a out or in by means of a milled headed screw at the side of the telescope near the eyepiece (not shown in Fig. 52). When properly focussed, the image of the object observed coincides with the diaphragm. The image is viewed through the eyepiece b, which magnifies it. The eyepiece is adjusted by pulling it out or in until a clear view of the cross hairs of the diaphragm is obtained. By the addition of extra glasses an erect image in place of the usual inverted image may be obtained, but at the expense of the power of the telescope. There is no special advantage in having

an erect image, and one soon gets accustomed to the inverted image after a little practice.

The spirit level e is usually set on the top of the telescope, see Stanley's instrument (Fig. 51). In Cooke's instrument (Fig. 52) it is attached near the vertical circle. It is fixed by the capstan screws ff, by means of which it is made parallel to the collimation

line of the telescope, so that when the bubble is in the centre of its run the collimation line of the telescope is horizontal.

which

The vertical circle g is fixed to the horizontal axis upon the telescope revolves, and rotates with the telescope. It is usually divided into four quadrants, and is read by the verniers h, h. When the line of collimation is horizontal, the verniers