Ferguson — On a Mode of Sub-agueons Tunnelling. 



79 



I 



sake of adhesion, without -wliicli the tube, on being pumped out, 

 would rise by flotation. It is conceived, however, that a level bed, 

 once obtained, could be effectually kept from being silted up, pending 

 the deposit, by several methods, and that the weight of the tube on a 

 soft bed would ensure such an amount of adhesion as, even without 

 that of the lateral and superincumbent material, would effectually 

 counteract the tendency to rise ; and this envelope would consolidate 

 with time. The point at which the floating action of muddy water 

 ceases, and the adhesive and repressive action of watery mud or 

 silt begins, has not, I imagine, been made the subject of accurate 

 investigation. Water acting on large surfaces, although admitted 

 through narrow orifices, exerts pressure proportionate to its head ; 

 but the limit of tenuity where percolation ceases to carry with 

 it the action of a continuous column of fluid, appears to be un- 

 determined. It is conceived, however, that it would be found at a 

 point short of that whers the mixture reaches a pasty consistence. 

 The preparation of recesses for guiding the descent of the tube 

 would entail the employment of cofferdams, within which sills should 

 be laid level with the floor of the trench. The sides of the recesses 

 should be carried back to the cross walls, 

 and, at a point under the level of high 

 water and lower than the draft of the 

 tunnel when floated, the alternate check 

 at either side should be retired, to admit 

 of the reception of the ends of the tube, 

 necessarily longer than the breadth of the 

 waterway. 



The construction of the tubular tunnel itself might either be in a 

 dry dock specially prepared for it, which would be hard to procure, or 

 on a raft, or by sections of a convenient length built on land and 

 launched, connected together and fitted with sides and roof in a wet dock 

 or other still water. Such sections should constitute shallow, open ves- 

 sels of no greater freeboard than would ensure their floating, so that the 

 subsequent work on the platform so formed should be continuous and 



TTharf. 

 High Water. 



Eiver bottom. 



rigid. The ballast necessary to keep the tube upright would be sup- 

 plied by the metalling required for the roadway, or, if this were not 

 sufficient, by admission of water. To secure such a tube against the 

 action of salt water it should be bronze-plated externally, and its 



