of the tunnel is usually approximately the centre line of the street or streets. Cases, however, often arise when it is either not possible or inconvenient to follow the line of the streets. When the line follows the approximate centre of the streets, boreholes may be put down from the surface from time to time. The underground line may then be checked and corrected by these, and in this manner a line having curves in it may be driven. For very long distances between shafts, however, such as across wide rivers, on which no intermediate boreholes can be put down as checks, the centre line is usually made straight. When buildings, large warehouses, &c., intervene as obstacles, they must be carefully traversed round, and in all cases there should be a check traverse made.

Instruments for Setting out Tunnels.-As regards the instruments to be used, all the setting out of some of the largest tunnels in this country has been done with a 6 in. theodolite reading to 20 seconds, but for ordinary tunnel work a 7 or 8 in. instrument reading to 5 or 10 seconds is more suitable. Fig. 196 shows Troughton & Simms' 7 and 8 in. instrument. With micrometer microscopes 1 second may be obtained by estimation on these instruments. Micrometers are shown in illustration of 12 in. theodolite in Chapter XII.

Setting out Curves in Tunnels. When curves occur in tunnels the curve itself is not ranged out on the surface; the intersection angle only is measured and the distances to intersection point. If the intersection point is inaccessible, measurements are taken from which the intersection angle and distances to intersection point are calculated. From these data the position of the tangent points is calculated, and the curve set out underground in the usual manner (see Chapter V.). In sharp curves the difference between the chord and the arc is to be allowed for.

Taking Measurements. — All measurements should be taken with the steel tape, and it will be found best to measure along the surface of the streets, afterwards taking levels to reduce the measurements to the horizontal. The best way to mark the end of the tape is to make a mark with a piece of chalk, and then accurately mark on this the measurement with a fine-pointed lead pencil. All linear measurements should be repeated, and should agree to a small fraction of an inch.

Observing Angles.-Angles should be observed three times, all three verniers of the instrument being read at each observation. This is generally sufficient, but in special cases it may be necessary to repeat and check observations oftener.

Marking Centre Line on Surface. The centre line is to be marked on the surface when in towns by driving spikes or wedges of iron between the setts of the roadway, the centre line being marked on these with a steel punch. Measurements are taken from the corners of buildings to locate these points as described in Chapter II., Fig. 84.

In the country the surface line may be marked by stout pegs

having brass nails driven into them, on which punch marks are made. In special cases brick piers may be built round the pegs.

The theodolite should be kept in very accurate adjustment, and "both faces" (see Chapter II.) should be used whenever possible.

It is perhaps needless to say that in busy towns on account of the traffic nearly all the surface work must be done during the night with the aid of lamps.

Traversing between Shafts.-Suppose, for instance, that obstacles as shown intervene between shafts A and B, Fig. 197. In this case the line may be carried round by the traverse shown. Thus the angles at C, D, E having been measured, and also the distances CD, DE, the total distance CF may be calculated, as also the distance EF and the angle EFC; from these the point F may

be located, and the line laid out over the second shaft B. The point F should be checked by a similar traverse along the other street as shown by the dotted lines. In many cases when tunnelling through towns, where the line does not follow the streets, it is possible to range out the line directly along the centre line by erecting temporary platforms on the roofs of the houses, on which the instrument may be set up. This is usually more accurate for the alignment than any traverse unless in situations very much exposed to wind and vibration.

Transferring Surface Line down Shafts.-Points having been conveniently fixed on the centre line at each shaft, the next

Fig. 198.-Arrangement for Transferring Line down Shafts.

operation is to transfer the line down below. This is usually effected by means of plumb bobs suspended by fine steel wire and accurately ranged in on the centre line. The arrangements used on the Totley Tunnel have already been described. Fig. 198 shows a very handy stand for suspending the plumb lines. The gun-metal stand abcd is about 3 or 4 in. high by about 6 in. long, and is fastened by four screws to the timber a. This is a stout timber placed across the shaft at right angles to the centre line of the tunnel, and must be strong enough, and fixed so as to be perfectly steady and free from vibration. The line is first roughly ranged over the shaft, and the hole B is cut out of the

timber. The bottom of the stand has also an oval hole in it, and it is screwed down to the timber centrally over the hole B.

The next operation is to fix the wire to the clip arrangement C. This piece is in two parts, and fits round the head be of the stand, and is screwed to fit the shaft E which is turned by the handle H. The clip c is first loosely put on, and the wire inserted from underneath. It is then tightened up by the small bolt and nut K so as to grip the wire firmly. The plumb bob may now be suspended from the bottom of the wire. Then on turning the handle H, the rotation of the screw shaft E causes the clip c, together with the wire and plumb bob, to travel in either direction along E. The wire w being viewed by the telescope of the theodolite, by means of this arrangement may be brought to accurately coincide with the cross hairs of the telescope. The arrangement permits of very exact adjustment of the wires, and is simple and quick. The preliminary work of fixing the timber and boring the hole in it may be done at any time during ordinary working hours, and the stand also placed on it and screwed down. The stand is then taken off, and when the actual plumbing down of the centre line is done (usually on a Sunday) it is only necessary to fit the stand over the holes and screw it down to the timber. The wire then being inserted in the clip, the whole arrangement is ready for the theodolite in a very few minutes.

A similar arrangement is fixed on each side of the shaft, the stands being placed as far apart as possible, consistently with ensuring that the wires do not come in contact with the sides of the shaft, slides, &c. This is of course to get as long a base line as the diameter or width of the shaft will permit.

As a rule in an ordinary shaft it is not possible to get more than about 6 ft. of base between the plumb lines.

The plumb bobs are usually of lead, and 35 to 40 lbs. weight. They are suspended in buckets of water placed at the bottom of the shaft in order to diminish the oscillations and bring them to rest as soon as possible.

Setting out Underground Line.—Having ranged in the plumb lines by setting up the instrument above ground accurately in the centre line and as close as it is possible to see the wires, the wires should then be carefully examined by travelling slowly down the shaft in the cage or by the ladder, inspecting them by