=Rectilinear Tunnels.=--In short tunnels the center line may be accurately enough located for all practical purposes by means of a common theodolite. The work is performed on a calm, clear day, so as to have the instrument and observations subjected to as little atmospheric disturbance as possible. Wooden stakes are employed to mark the various located points of the center line temporarily. The observations are usually repeated once at least to check the errors, and the stakes are altered as the corrections dictate; and after the line is finally decided to be correctly fixed, they are replaced by permanent monuments of stone accurately marked. The method of checking the observations is described by Mr. W. D. Haskoll[2] as follows: “Let the theodolite be carefully set up over one of the stakes, with the nail driven into it, selecting one that will command the best position so as to range backwards and forwards over the whole length of line, and also obtain a view of the two distant points that range with the center line; this being done, let the centers of every stake ... be carefully verified. If this be carefully done, and the centers be found correct, and thoroughly in one visual line as seen through the telescope, there will be no fear but that a perfectly straight line has been obtained.” [2] “Practical Tunneling,” by F. W. Simms. [Illustration: FIG. 1.--Diagram Showing Manner of Lining in Rectilinear Tunnels.] The center line which has thus been located on the ground surface has to be transposed to the inside of the tunnel to direct the excavation. To do this let _A_ and _B_ be the entrances and _a_ and _b_ be the two distinct fixed points which have been ranged in with the center line located on the ground surface over the hill _A f B_, Fig. 1. The instrument is set up at _V_, any point on the line _A a_ produced, and a bearing secured by observation on the center line marked on the surface. This bearing is then carried into the tunnel by plunging the telescope, and setting pegs in the roof of the heading. Lamps hung from these pegs furnish the necessary sighting points. This same operation is repeated on the opposite side of the hill to direct the excavation from that end of the tunnel. These operations serve to locate only the first few points inside the tunnel. As the excavation penetrates farther into the hill, it becomes impossible to continue to locate the line from the outside point, and the line has to be run from the points marked on the roof of the heading. Great accuracy is required in all these observations, since a very small error at the beginning becomes greater and greater as the excavation advances. To facilitate the accurate location of points on the roof of the tunnel, a simple device was designed by Mr. Beverley R. Value, shown in Fig. 2. Two iron spikes, each having a small hole in the flat end, are driven into the rock about 9 ins. apart. A brass bar, 1 in. high, ¹⁄₄ in. thick and 10 ins.