It would be impossible to over-rate the importance of a constant faithful supervision of the building of the lining, especially the arching. The work has to be carried on by workmen in cramped positions, with imperfect light, and surrounded by all kinds of obstacles and inconveniences, and unless a detailed inspection be rigidly maintained, a carelessness in the selection of the materials, and a laxity in the workmanship, will be the inevitable result. Figs. 204 to 219 are sections of tunnels which have been constructed for double and single line railways. The sections give the normal form and dimensions adopted in each case, although there may have been many portions of the work where unfavourable or treacherous material necessitated an increase in the thickness of the side walls, or of the arching, or of both. The types vary in accordance with the opinions of the designers as to the most suitable section for the purpose, and range from the comparatively thin lining and vertical side walls shown on Fig. 207, to the almost circular form and very thick lining shown on Fig. 216. The latter is the section which experience has proved to be the best to sustain the enormous all-round pressure exerted by certain descriptions of swelling clay. Careful judgment will be required to decide which parts of a rock tunnel may be left unlined. The apparently solid-looking portions are oftentimes deceptive, and numbers of instances are on record of large pieces of rock falling down in tunnels which for many years had been considered as thoroughly secure. Where there is any doubt it is better and safer to put in a lining, even if only to the extent of an arching springing from side walls of solid rock, as shown on Fig. 206. A moderate additional expenditure at the time of construction may prevent a serious catastrophe afterwards. [Illustration: Fig. 220, 221, 222, 223] The faces or entrances to tunnels may be constructed with curved wing 178 walls, as in Fig. 220, or with straight wing walls, as in Figs. 221, 222, and 223. Where the approach cutting is in rock, the latter form is generally adopted.