18 48.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL, 



231 



the arms p, /, g, h ; and by turninc; the instrument or the paper 

 round, tlie other half may be drawn with equal facility. 



By constructina: the instrument so that the arms could be ad- 

 justed to any radius at pleasure, we sliouhl tlien be enabled to 

 represent a circle of any diameter within its ranjje. Ellipses, also, 

 could be represented in this kind of perspective, by its means. If 

 it were required to show the semi-ellipse a, i/, c, d, the only ad- 

 justment necessary would 

 , be to set the radius of /and 



A to the line a, 6, and that of 

 e and g to the line b, d, — 

 and the curve produced 

 would be the true figure, ac- 

 cording to the rules of the art. Moreover if the angles formed 

 by the arms could be altered at pleasure, we should have at com- 

 mand an infinite variety of simple curves, embracing all that are 

 derived from the isometrical projection of regular figures in any 

 plane. 



The form of the instrument may differ materially from that 

 represented here — it may be made to describe the whole figure 

 without being moved from its first position ; but our sketch and 

 description will, no doubt, be sufficiently suggestive of all that is 

 required to render it efficient. 



R. B. C. 



PORTABLE COFFERDAM,* 



SPECIALLY ADAPTED FOR HARBOUR AND OTHER MARINE WORKS. 



By Thomas Stevenson, F.R.S.E., Civil Engineer, Edinburgh. 

 (Read before the Royal Scottish Society of Arts, January 10, 1848.^ 



When it is necessary, in the execution of marine works, to carry 

 on founding or excavation in exposed situations within the high- 

 water mark, cofl"erdams of the common description are not found 

 to be answerable. Many circumstances conspire 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 wUl, in many cases even in the course of a few 

 hours, either entirely break up a well-constructed cofferdam, or 

 render it leaky and unserviceable. Again, wliere 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 fnmi the waves, to have 

 no stability, and to fall inwards as tlie sand is removed from the 

 interior, although every care be taken to support tliem with shores 

 or struts. 



Tlie temporary dams which are generally employed in the execu- 

 tion of tide-works are of a very simple construction, and are in- 

 tended 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 driven 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 inconvenience 

 still remains, for, as the stuff is removed from the interior, there is 

 nothing left but tlie single waling to resist 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. 



* Aa abstract of this paper was givea iu the Journal of February last, p. ii. 



At Hynish harbour, Argyllshire, in 1843, I had a tains-wall to 

 found on sand, whicli covered a rocky beach to the depth of from 

 two to tliree 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- 

 basin, were to be excavated to the level of the low -water springs. 

 The shores also were frequently subject, even during the summer 

 months, to a very heavy surf. 



The excavation of the tide-basin, which formed the landward 

 part of the work, was effected by means of a series of dams, con- 

 sisting of walls, built of pozzolano rubble. These were found to 

 be quite water-tight, and to answer remarkably well in every 

 respect ; but they required, for their protection against the waves, 

 a considerable bulwark or breakwater of Pierres perdues to shelter 

 them from the waves. 



In the excavation, however, which had to be undertaken seaward 

 of the breakwater of Pierres perdues, any attempt to exclude tlie 

 water during tlie wbole of the tide, was what I never considered 

 practicable. A trial was accordingly made to effect the excavation 

 by means of a low wall, composed of a clay-rubble, resembling in 

 its object those low dams consisting of logs of wood bedded in 

 clay, which are often adopted in harbour-works, and wliich are 

 only intended to keep out tlie tide during the first part of the 

 flood, and to be pumped dry before the operations of the next tide 

 are begun. But after many attempts with this clay-wall, it became 

 quite evident that it would not be possible, with its assistance, to 

 carry the excavations to near the level of low-water springs, which 

 was due principally to two causes. First, because sand and shingle 

 were, during almost every tide, washed in large quantities over 

 the top of the wall into our excavation pit ; and, secondly, because 

 the waves washed out the clay from among the stones, so as to 

 render the barrier no longer water-tight. 



Being now compelled to set about some other way of carrying on 

 the work, I had recourse to tlie simple method shortly to be ex- 

 plained, and which more than realised my expectations. Before 

 giving a description of this method, however, it will be interesting, 

 as well as still farther explanatory of the required objects, to quote 

 a few lines relating to somewhat similar difficulties, from a Report 

 upon the Harbour of Peterhead, which was drawn up in the year 

 1806 by the late Mr. John Rennie : — " The next material object of 

 consideration," says the Report, " is that of deepening the harbour, 

 which at present cannot well accommodate vessels drawing more 

 than 12 feet of water in the spring-tides, but in neaps is not suffi- 

 cient. To render this harbour more extensively useful, it would 

 be advisable to have 17 or 18 feet of water over the greatest part 

 of its bottom, and particularly 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 

 surface, and continues so for some time, may be done by blasting, 

 or by loosening the stones with quarrying tools in the usual man- 

 ner ; 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 

 working at all times of tide by quarrying 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 harbour. In such situations perhaps the simplest and 

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

 feet diameter, having strong 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 oute- 

 cylinder might be sunk thereon, to keep them in their situations." 



The cylinders proposed by Mr. Rennie were, no doubt, quite 

 adequate to tlie special purpose 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 disadvantages which precluded their 

 adoption in tlie present case. Those objections were the limited 

 area, the weight and unwieldiness of such cylinders, their inflexible 

 nature and unalterable form, as affording no means in themselves 

 of adaptation to the very irregular rocky bottom which was to be 

 excavated, and what was of as much consequence, the difficulty 

 which must ha\e attended the removal of the partitions of rock, or 

 those parts which would necessarily be left between the different 

 compartments of tlie cutting. The last two objections, it may be 

 remarked, refer equally to wooden caissons, or other contrivances 

 on the same principle. 



In the present case, then, the following requisites were to be 

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

 quired 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 



