144 Mr Thomas Stevenson's Description of 



In the present case, then, the following i*equisites were to 

 be provided for. In the founding of the talus-wall, all that 

 was required was some method which would enable the found- 

 stones to be laid as deep in the sand as possible, for which 

 purpose the dam did not require to be absolutely water-tight, 

 provided it were capable of excluding from the inside the 

 sand which was so liable to replace what was removed from 

 the interior. For the excavation of the rock, on the other 

 hand, it was necessary that the dam should be water-tight, 

 and suitable for taking out all the partitions ; and both situ- 

 ations required piles for fitting close to the iiTegular bottom, 

 and those piles needed some support other than the soil into 

 which they wei'e to be driven. 



To effect such objects, it was clear that the means to be 

 adopted must be at once easily managed and efficient. For 

 although, where there is time for their employment, many 

 complicated and troublesome refinements of construction are 

 forced to answer purposes which might have been attained by 

 simpler means, or by less cumbrous arrangements, yet I was 

 well aware that in the hurry and bustle attending tidal opera- 

 tions and night-work, nothing can be tolerated but what is in 

 every respect easily managed and truly efficient. 



In the accompanying Diagrams, A G (Plate III.), re- 

 presents a frame of double waling pieces connected at the 

 angles by the uprights I I, and bound together by the long 

 bolts L, with forelocks and washers, while E F shews si- 

 milar double-framed walings for the inside of the dam, and 

 of smaller dimensions, with their uprights D, and connect- 

 ing bolts K. These frames being placed in the required 

 position, the one frame inside of the other, the piles C, are 

 driven doAvn between them with heavy malls. 



The dam was 12 feet long by 10 feet broad inside, so that 

 five men were able to work in the interior.* If it was to be 

 fixed within low water-mark, the two frames being placed in 



* Since this paper was printed, a Cofferdam on the same principle and 30 

 feet square, has been made for the Forth Navigation works, Stirling, where, in 

 the removal of the " fords," under my direction, much difficulty has hitherto 

 been experienced, from the constant flow of the river. 



