TUNNELLING THEOUGH VARIOUS STRATA. 



157 



possible to got him to alter a design after the work was 

 once begun ; and if a bridge foil down, lie would build it 

 again in exactly the sanio way. Doubtless, he was often 

 right, as a bridge would often fail through bad work or the 

 design not having been properly carried out; but, still, 

 perhaps a Ultle more pliability would have corrected what 

 ajipoarod to his greatest admirers one of the chief faults 

 in his character. 



The next portion of the author's subject ho pur])Osos deal- 

 ing with is, tunnelling through medium ground— that is, 

 ground which does not come under the head of being very 

 heavy, and requiring very heavy timbering, nor yet is it 

 any more fit to be classed as rock tunnelling. Such ground 

 as this would be chiefly through the bettor portions of the 

 coal measures, hard and compact sands, as are found in 

 the Oolitic formation, through the chalk, and, in fact, any 

 gi'ouiid which does not roqiuro a great amount of blasting, 

 or, on the other hand, heavy timbering. In such cases as 

 these, a bottom heading is, no doubt, the best form of begin- 

 ning operations, as these kinds of tunnels are not necessarily 

 free from water, and therefore the bottom heading is re- 

 quired for drainage purposes, if i'or no other. It is also 

 easier to form break-ups for the purpose of enlarging to tlio 

 fuU-sizod tunnel; and as the ground will in most cases stand 

 with little or no timber, it is the cheapest and most expedi- 

 tious mode of carryhig out the work ; and after the heading 

 is sufficiently advanced, any number of break-ups can be 

 formed, so as to expedite the work to the required degree. 

 To form a " break-up " is usually 'done by breaking down 

 over the roof of the heading, until sufficient space has been 

 made to erect a scaffolding, which then receives the dirt or 

 rock without blocking up the heading, and leaves it free for 

 the passage of wagons and materials from other parts of the 



