a Portable Cofferdam. 143 



in neaps is not sufl&cient. To render this harbour more ex- 

 tensively useful, it would be advisable to have 17 or 18 feet 

 of water over the gi'eatest part of its bottom, and particu- 

 larly along the west quay. The mode of performing this 

 kind of work will be different, according to the difference of 

 situation. Those places where the tide ebbs from the sur- 

 face, and continues so for some time, may be done by blast- 

 ing, or by loosening the stones with quarrying tools in the 

 usual manner ; but in those parts where the tide seldom leaves 

 the bottom, and in others but for a short time, different methods 

 must be resorted to. The best of all would be enclosing large 

 spaces by cofferdams, and woi'king at all times of tide by 

 quaiTying tools or blasting, as might best suit ; but in some 

 situations this would be inconvenient, as the dams would be 

 in the way of vessels going into and coming out of the har- 

 bour. In such situations perhaps the simplest and most ex- 

 peditious mode would be to use cast-iron cylinders of 7 or 8 

 feet diameter, having sti'ong canvas fixed to the lower 

 flanch, which might be kept to the bottom by bags of sand in 

 places where there was but little agitation ; but where there 

 is much, an outer cylinder might be sunk thereon, to keep 

 them in their situations.'' 



The cylinders proposed by Mr Rennie were, no doubt, 

 quite adequate to the special purjiose and locality for which 

 they were designed, and they unquestionably possess some 

 advantages not to be gained by other means ; but, on the 

 other hand, they are attended with difficulties and disad- 

 vantages which precluded their adoption in the present case. 

 Those objections were the limited area, the weight and un- 

 wieldiness of such cylinders, their inflexible nature and unal- 

 terable form, as affording no means in themselves of adapta- 

 tion to the very irregular rocky bottom which was to be 

 excavated, and what was of as much consequence, the diffi- 

 ficulty which must have attended the removal of the partitions 

 of rock, or those parts which would necessarily be left be- 

 tween the different compartments of the cutting. The last 

 two objections, it may be remarked, refer equally to wooden 

 caissons, or other contrivances on the same principle. 



