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Proceedings of the Royal Irish Academy. 



thickness should be such as would resist the pressure due to the 

 depth of the trench and of the supernatant water. Or it might he 

 constructed with double sides and roof, between which a coating 

 of concrete might be introduced from above. 



The floating of the tube into position should be accomplished at 

 high water, when, its ends being brought home to the down-stream 

 check of one recess and the up-stream check of the other, with such 

 arrangements to prevent jamming as might be necessary, the water 

 would be admitted through its length by valves, and it would sink 

 evenly down its guide-chambers to the bottom. This accomplished, 



Appro ach. 

 Cross Vail 



the process of filling in and making good the bed of the river would 

 take place. 



The .luting of the ends of the tube would be effected by puddling 

 between it and the masonry of the recesses, and the process of pump- 

 ing out would be proceeded with. 



To complete the connexions, the cross walls at the ends of the 

 approaches should be cut through, and the ends of the tube being 

 removed, the way would be open from end to end. 



In the case supposed the tube is straight ; but there is no reason 

 why such a tube should not be measured and fitted to the contour of 

 the bottom, and sunk into its bed by a similar process ; the only differ- 

 ence being that the concave of the bow would be already submerged on 

 being floated into position. In the supposed case, also, the force of 

 flotation was intercepted by the adhesion of the floor and sides of the 

 tube to the trench bottom and to the filling-in stuff thrown back from 

 above, as well as counteracted by the weight of the replaced river- 

 bottom. The adhesion, however, due to a perfect contact of so large 

 a surface as the floor of such a tube with a soft river- or sea-bottom 

 would, theoretically, exclude flotation, even though the top and sides 

 of the tube were surrounded by free water. Assuming perfect contact 



