=Presence of Water.=--An idea of the likelihood of finding water in the tunnel may be obtained by studying the hydrographic basin of the locality. From it the source and direction of the springs, creeks, ravines, etc., can be traced, and from the geological map it can be seen where the strata bearing these waters meet the center line. Not only ought the surface water to be attentively studied, but underground springs, which are frequently encountered in the excavation of tunnels, require careful attention. Both the surface and underground waters follow the pervious strata, and are diverted by impervious strata. Rocks generally may be classed as impervious; but they contain crevices and faults, which often allow water to pass through them; and it is, therefore, not uncommon to encounter large quantities of water in excavating tunnels through rock. As a rule, water will be found under high mountains, which comes from the melted ice and snow percolating through the rock crevices. Some detrital soils, like gravel and sand, are pervious, and others, like clay and shale, are impervious. Detrital soils lying above clay are almost certain to carry water just above the clay stratum. In tunnel work, therefore, when the excavation keeps well within the clay stratum, little trouble is likely to be had from water; should, however, the excavation cut the clay surface and enter the pervious material above, water is quite certain to be encountered. The quantity of water encountered in any case depends upon the presence of high mountains near by, and upon other circumstances which will attract the attention of the engineer. A knowledge of the pressure of the water is desirable. This may be obtained by observing closely its source and the character of the strata through which it passes. Water coming to the excavation through rock crevices will lose little of its pressure by friction, while that which has passed some distance through sand will have lost a great deal of its pressure by friction. Water bearing sand, and, in fact, any water bearing detrital material, has its fluidity increased by water pressure; and when this reaches the point where flow results, trouble ensues. The streams of water met in the construction of the St. Gothard tunnel had sufficient pressure to carry away timber and materials. CHAPTER II. METHODS OF DETERMINING THE CENTER LINE AND FORMS AND DIMENSIONS OF CROSS-SECTION. DETERMINING THE CENTER LINE. Tunnels may be either curvilinear or rectilinear, but the latter form is the more common. In either case the first task of the engineer, after the ends of the tunnel have been definitely fixed, is to locate the center line exactly. This is done on the surface of the ground; and its purpose is to find the exact length of the tunnel, and to furnish a reference line by which the excavation is directed.