TUNNELLING THKOUGH VARIOUS STRATA. 161 



coal is in most of tliese cases mucli thinner than eitlier the 

 shale or the clay ; and where thq strata is dipping at all 

 steeply, the shale and coal often slip off the fireclay, 

 especially where a spring of water occurs, making the clay 

 wet and slippery. Caution should be exercised in cases 

 where these occur, as the ground, when excavated, often 

 looks veT-y sound and strong, and, from appearance only, 

 would load one to think that it would stand without timber- 

 ing, especially in the sides of the tunnel; but narrow escapes 

 have often taken place from this cause, one or two of which 

 have come under the author's observation, and which he has 

 witnessed as a rather too near spectator. In ground of this 

 kind, the timbering should be carried at least below the 

 springing of the arch, and not, as is too often the case, 

 merely round the crown and haunches of the arch. The 

 ground which is referred to is more often met with in 

 tunnels through the coal measures, chiefly through thick 

 beds of Clift and Pennant rock, which require little or no 

 timbering, and, therefore, there is often a tendency to risk 

 a length or two of these beds with insufficient timbering. 



There is no doubt that the reason of so many of the dis- 

 torted and sunken arches in many tunnels,—- and some of 

 them are very bad, — is owing to the want of a little 

 foresight, and to the careless way in which the timbering 

 has been done, very often by men of experience. You will 

 often find that practical miners, used to timbering heavy 

 ground, will over and over again make the same mistakes, 

 and will make the tunnel too large or too small in exactly 

 the same places, although admonished time after time. 



Tunnels are nearly always taken out too small at the 

 crown, and too largo at the haunches, the direct opposite 

 of which would be the better way ; but it is almost im- 

 possible to make miners alter their ways in this respect, 



