the light of a good lamp to see that they are everywhere free from contact.

The theodolite is now brought down to the bottom of the shaft and set up in line with the two wires. In doing this the theodolite should be set up as near as it is possible to focus the telescope on to the nearest wire. The writer has seen the theodolite set up 2 or 3 chains back from the wires and there got into line with them. This is a great mistake, as an error in bisecting the wires with the cross hairs which would be imperceptible on the wires at that distance would mean a not inconsiderable error in the posi tion of the instrument itself. On the other hand, if the theodolite is set up as near as possible to the wires, they may be bisected by the cross hairs of the telescope with as great accuracy as is possible with the power of telescope used, and the error in the position of the instrument itself will consequently be the least which the circumstances permit of. The instrument should have adjusting screws for getting it into line, see m' and n', Fig. 51. These are most useful in tunnel work, and should always be provided.

Marks for fixing Underground Line.-The line is now to be permanently fixed underground. A common way of doing this is to mark it on byats of timber fixed across the tunnel so as to be clear of the horses, &c. Nails, dogs, or spikes are driven into these, and the line marked on them by a steel punch mark or a file mark. A steel punch makes a more accurate mark than can be made by filing. In iron-lined tunnels, wooden wedges may be driven into the joints between the segments in the roof, and nails then driven into the wedges. The wedges are, however, marks or punch marks on

apt to get loose and come out, and file the iron segments themselves are better. A good way of fixing the line is to drive stout pegs 3 in. square and about 3 ft. long

into the formation or invert of the tunnel. These are surrounded by brickwork about 3 ft. square, parged over with cement flush with the top of the peg. After the cement is set, brass nails may be driven into the pegs, and punch marks made on them. The point made by a punch is readily seen when brass nails are used. When the line can be fixed in this way it is more convenient for setting up the instrument on it and for fixing the lights to be sighted on than when it is marked on the roof or on byats placed across the tunnel.

Transferring Surface Line down Shafts when Plumb Bobs cannot be used.—When on account of wetness of the shaft, vibration and jar from pumping engines, &c., it is not possible to get plumb lines satisfactorily down the shaft, the method adopted on the Severn Tunnel may be employed. In this case the heading was 2 miles long, and the base line 12 ft. A large transit instrument was set up at the top of the shaft and accurately in the centre line. The heading having been driven for a short distance on approximate lines, a wire 100 yds. long was stretched in the bottom, one end being at the side of the shaft, and the other at a point 100 yds. up the heading. The ends of the wire were stretched over the threads of horizontal screws, and the wire was stretched by means of weights hung from the ends. By this means on turning either screw lateral motion was given to the wire. When the wire was lighted by electric light a length of 14 ft. was seen from the transit instrument at the top of the shaft. The furthest points visible on either side of the shaft were carefully adjusted to the line by the transit instrument, and by rotating the screws; a base 100 yds. long was thus secured. When junction was effected the headings met without error.

Underground Sights and Lamps.-A very good form of sight is a large tin lamp about 12 in. square, having the front made of ground glass. If the handle when let down projects slightly beyond the face of the lamp, it is very convenient for attaching the plumb bob, the string of which is distinctly visible against the illuminated ground glass front of the lamp. The arrangement is as indicated in Fig. 199, and the lamp being placed on the brickwork behind the peg and the plumb bob attached to it, it is then adjusted by moving the lamp until the plumb bob is exactly over the punch mark on the nail.

A piece of oiled paper or tracing paper held behind a plumb line and illuminated by candles or a lamp placed behind the paper is a good sight for short distances. If the light is held in front of a paper the shadow of the plumb line may lead to error.

For greater distances at which a plumb line is not visible a lamp or carriage candle must be used similar to those shown in Figs. 194 and 195.

The theodolite then having been got into line with the wires suspended down the shaft, plumb lines illuminated as above

described or for greater distances lamps or carriage candles, are carefully ranged in. The points of suspension of the plumb lines or lamps are then marked, and these points mark the centre line of the tunnel permanently underground. From these the brickwork or iron lining of the tunnel is ranged in as the work proceeds.

The operation of transferring the line down the shaft to permanent marks underground should be repeated until results are obtained agreeing sufficiently closely to guarantee the meeting of the lines from each end of the tunnel or between shafts, to the required degree of accuracy.

More accuracy will be attained by making the sights underground of moderate length than if very long sights are used ;at least for fore sights.

Illuminating Cross Hairs of Telescope.-In sighting on to distant points the cross hairs will not be visible unless illuminated. In some instruments this is effected by having the horizontal axis of the telescope hollow, the end being closed by a piece of glass. On the upright supports of the horizontal axis of the telescope there is attached a special lamp provided with the instrument, for illuminating the cross hairs. The writer's experience is that this is not a satisfactory arrangement, as too much light on the cross hairs makes the distant light's image very faint, while with too little light the cross hairs cannot be distinctly seen. As the lamp lighting up the cross hairs through the axis of the telescope does not permit of ready adjustment of the amount of light thrown in on the cross hairs, a candle or lamp simply held in the hand in front of the object glass and slightly to one side of it is more satisfactory. For reading the vernier also a candle or lamp must be used.

Arrangement for Sighting on Wires Suspended in Shaft.* There is sometimes a difficulty in distinguishing the two plumb line wires down the shaft. To obviate this, Mr F. W. Watkins devised the following illuminated slip apparatus for use in the Croton Aqueduct, New York. Two vertical strips of brass 3 in. long are attached to separate horizontal bars moving in guides and provided with a tangent screw motion, by means of which either one or both strips are moved so as to make the vertical aperture

* Minutes Proceedings Inst. C. E., vol. cxvi.

between them as small as required. One of these instruments is screwed to a plank bracket close behind each plumb line, and so placed that the further can be seen through the telescope just above the other. When the slits are adjusted exactly to the plumb line wires the latter are removed and lights are placed behind the slits. Thus two fixed illuminated lines are substituted for the wires. The results of the alignment (see page 303) showed great accuracy with the use of this apparatus.

White Reflectors to Verniers.-American theodolites for ordinary field use generally have a piece of white celluloid or ivory fixed above the vernier to reflect the light on to it. This will be found most useful for underground work especially. Pieces

of white glazed notepaper will do if nothing else is at hand. If the light is not vertically over the vernier, errors of reading will be caused, without these shades.

Alignment of the Totley Tunnel.-See page 286.

Alignment of the Mersey Tunnel (Fig. 200).—The distance across the river was about of a mile. The base line obtainable between the plumb lines down each shaft was 6 ft. On the Liverpool side the shaft was on the centre line, but large warehouses and other buildings intervened between A and B. On the Birkenhead side the shaft was off the centre line as shown. It was therefore necessary to measure DC and DCA on the surface, transfer the line down below, range out DC, measure DC, and set off DCA down below.

All the alignment was done with a 6 in. Cooke theodolite reading to 20". When the headings met near the centre of the river

the alignment was found to be correct to a fraction of an inch, both for centre line and level.

Alignment of Dunmail Rise Tunnel on Thirlmere Aqueduct * (Fig. 201). The base line. in the shaft was 6 ft. 9 in. between the plumb line. wires. The length driven from the shaft to point of junction was 1,518 yds. or 675 times the base. From the south end nearly a mile -1,715 yds.—was driven, then the angle at A was turned, then nearly another

mile was driven to point of junction. To fix the position of A, 1,715 yds. had to be measured on the surface, which fell 250 ft. in this distance. This measurement was made

with rods 10 ft. long, set horizontal with a hand level, and a plumb rule similar to a mason's, specially made for the purpose.

80

An error of in. in the base was sufficient to account for an error of 8 in. at the point of junction. The error at the point of junction was 8 in. in line and 24 in. in level. The total length of this tunnel was nearly 3 miles, the section being 8 ft. 6 in. × 7 ft.

Examples of Errors in Alignment of Long Tunnels. -The following shows the results of the alignment in some of the longest tunnels constructed.

Minutes Proceedings Inst. C. E., vol. cxvi.