162 TUNNELLING THROUGH VAllTOUS STRATA. 



delay in driving (lowii-liill, wliicli can iiovor bo satisfactorily 

 obviated. The autlior knows that it is at times practically 

 impossible to have a gradient both ways in a tunnel, but he 

 also knows that this is not always the case, and considers 

 that it should be done in all cases where reasonably pos- 

 sible. It is, doubtless, always possible, in constructing on 

 a down gradient, to run the tunnel as far as it will go with 

 a slight rise, or on the level; but this moans afterwards a 

 lot of underpinning, and, as was said before, ho always feels 

 inclined to discourage that mode of working, except in cases 

 where it is absolutely unavoidable. 



Tho author now comes to the third portion of his subject, 

 viz., tiumelling through hard ground or rock. Although 

 under the two previous heads there might bo some amount 

 of rock to tunnel through, yet under tho namo of hard 

 ground tunnelling, is here meant groutid wliich cannot be 

 excavated without blasting, and such as would be met with 

 in tho MoiTiitaiu Tjimostono, Dcvonia.n, and chiefly amongst 

 the older formations. The usual way of beginning operations 

 woiild, no doubt, be in almost every instance by driving a 

 bottom heading; and, as a rule, tunnels through ground of 

 this nature rarely liave shafts, owing to the hardness of the 

 ground through which they would have to pass, and the 

 expense of sinking ; besides, in many cases, owing to their 

 great depth, tunnels of any length in such strata being 

 generally through hills of considerable height, and probably 

 inaccessible for shafts. The author considers it quite an open 

 question as to whether hand labour is not still tho cheapest 

 mode of drilling rock (although, of course, it cannot compare 

 with machine drills in point of speed) when the cost of 

 compressing the air, the wear and tear, and breakdowns, are 

 taken into consideration, especially for widening tho heading 

 to the full size of the tunnel. His experience is— that 



