Description of a Portable Cofferdam. 141 



the high water-mark, cofFerdams of the common description 

 are not found to be answerable. Many circumstances con- 

 spire in rendering such erections inapplicable in situations 

 where they are required to stand for several tides. The 

 waves occasioned by a very moderate breeze of wind will, 

 in many cases, even in the course of a few hours, either en- 

 tirely break up a well-constructed cofferdam, or render it 

 leaky and unserviceable. Again, where there happens to be 

 a covering of a few feet of sand above a rocky bottom, the 

 piles will be found, even where there is shelter from the 

 waves, to have no stability, and to fall inwards as the sand 

 is removed from the interior, although every care be taken 

 to support them with shores or struts. 



The temporary dams which are generally employed in the 

 execution of tide-works are of a very simple construction, 

 and are intended to be serviceable during only one or two 

 tides. They consist of a row of short piles which are driven 

 in the line of a runner or waling-piece, and as the excavation 

 proceeds, the piles are from time to time dinven farther 

 down. But this kind of erection is very unsatisfactory, and 

 in many situations, and for a variety of purposes, it is in 

 fact quite useless ; for I have always found that it was 

 impossible with this dam to drive the piles straight, from 

 there being only one waling piece to direct them. But even 

 although they could be driven, a farther source of inconve- 

 nience still remains, for, as the stuff is removed from the 

 interior, there is nothing left but the single waling to re- 

 sist the pressure from the outside, and the bottoms of the 

 piles being speedily forced inwards, all attempts to carry the 

 excavation farther must necessarily be abandoned. 



At Hynish harbour, Ai'gyllshire, in 1843, I had a talus- 

 wall to found on sand, which covered a rocky beach to the 

 depth of from two to three feet. At another place, the 

 rock was not only to be bared, but a navigable channel, 

 twenty feet wide, and in some places as deep as eight feet 

 in the rock, together with a small tide-bason, were to be ex- 

 cavated to the level of the low-water springs. The shores 

 also were frequently subject, even during the summer months, 

 to a very heavy surf. 



