158 



TUNNELLING THEOUOH VARIOUS STRATA. 



tunnol, or from other break-ups. Tlio breaking down on to 

 staging can, where the ground will stand, then bo carried 

 forward for a long distance, without arching the lengths as 

 you go along, and saves a great deal of time. It is usual 

 for the ground to be broken down for about the width of 

 the heading right up to the top of tho tunnel, as ground 

 which would not stand if widened out at the sides, will 

 often stand in this way for any length of time, and it serves 

 to expedite the progress of the work. Walking along the 

 bottom heading whilst this work is in progress, a person 

 unacquainted with the work would think that nothing but 

 the heading had been done; and yet, at the same time, nearly 

 two-thirds of the tunnel excavation may have been com- 

 pleted, and, therefore, in measuring up work for payment 

 on account, a good practical knowledge is really required to 

 toll the real value of the work and to estimate its true value. 

 A few days afterwards, perhaps, the staging may bo taken 

 down, and then the real extent of the progress is visible, 

 and to any one not in tho secret, would seem magical, as 

 from the mere heading the tunnel would have boon suddenly 

 transformed into nearly its full size. It has often seemed 

 to the author that the remarks of tho uninitiated arc not 

 always so foolish as they appear, when one sometimes over- 

 hears them ; and that the apparent change from chaos to 

 perfect order must appear rather wonderful to those persons. 

 One method of tunnelling ground of this nature, is to take 

 out the whole arch of tho tunnel in tho manner before 

 mentioned, and then turn in brickwork or masonry, as the 

 case may bo. Afterwards, when tlio lower part of the 

 excavation is completed, underpinning the arch, and thus 

 completing the side walls ; but the autlior docs not like this 

 mode, as, although it is without doubt very expeditious, ho 

 docs not consider that the underpinning can be done so as 



