lSi7.] 



THE CIVIL ENGINEER AND ARCHITECrS JOURNAL. 



283 



beea carried up to the lerel of low tides, had entirely disappeared. The 

 sea had carried away from 15 to 18 feet of the summit, giving to the 



breakwater the following configuralioa : 



Base. Height. 



The interior slope, or that to the south.. 12 50 12 50 



Superior slope, north .. .. 47"50 6'20 



Inferior slope, north. . .. .. 9" 6"30 



Upon this base, thus fnrmed by the action of the sea, it was determined 

 (0 attempt the erection of an artificial island, exposed to the most violent 

 agitation of the waves. By the end of the year 1803, the central portion 

 of the work was completed to low-water mark of ordinary tides. The mo- 

 dification and changes which the original breakwater had undergone, left 

 no room to doubt that the small stones of which the then elevation was 

 composed, would not resist the winter gales ; that it would be necessary 

 to consolidate the whole by a superstructure on the south side, composed 

 of immense blocks of stone, to be raised to the height of the highest tides, 

 as tlie only means of preventing the smaller materials trom being washed 

 »way into the interior of the work by the north-east and north-west gales 

 of winter. As anticipated, this superstructure resisted the force of the 

 sea, prevented the washing away of the smaller materials, which, arrested 

 by this barrier, gradually augmented the height of the breakwater, forming 

 a solid and compact surface at a new slope, of which the base was about 

 quadruple the vertical height. 



It was, however, soon observed, that these small stones pressed up, and, 

 transported by the northerly winds to a level above that of the sea, equally 

 yielded to the same action in northeast and north-west gales. Under 

 these circumstances of weather, of frequent recurrence in the winter of 

 1803, a portion of the stones which had been thrown in on the sea side 

 became subject to a lateral displacement, and were deposited in great 

 abundance in a conical mound at each extremity of the recently elevated 

 central portion of the digue. 



It was remarked, that the winds and the currents had both tended to 

 give these mounds the precise configuration of the tracing of the proposed 

 batteries ; .ind thence it was naturally concluded, that the form proposed 

 was that best adapted to insure stability, since it agreed so perfectly with 

 that which the sea itself had assigned to the mass of stones set in motion 

 in bad weather. 



From motives of economy, the interior of the fort was composed of small 

 materials ; but the whole was necessarily revelled with large blocks, 

 capable of offering an adequate resistance to the action of the sea. This 

 was performed in a very simple and easy manner : in bad weather the 

 blacks were cast overboard from the stone vessels, above the places which 

 were dry at low water. In fine weather this was discontinued, and the 

 stone vessels were moored over the required spot, or were sunk, so as to 

 rest upon a berm or slip which had been left in the slope at the foot of the 

 revetments. From the vessels thus sunk or moored, the blocks of stone 

 were hoisted by their own tackle clear of their decks, when they were 

 transported by cranes, or derricks, disposed on the summit of the break- 

 water, and deposited in their proper places on the revetments. By these 

 aimple and expeditious means, the central portion of the breakwater having 

 been raised to the height of 2 metres 92 centimetres above the level of 

 high-water, spring-tides, upon an extent of 195 metres in length, and 19 

 metres 50 centimetres in breadth, a battery was established thereon, 

 armed, provisionally, with four 36-pounders and two heavy mortars. 



In the subsequent construction of the terreplein and part of the parapet 

 of the battery, owing to the bad weather and too great haste in construct- 

 ing it, the precautions necessary to its thorough solidity were neglected, 

 and a violent gale coming on ere the exterior revetment was completed, a 

 portion of the temporary parapet, which had been formed with small 

 materials not cemented, was destroyed. The mass of the battery suffered 

 no injury ; and the centre and the extremities of the terreplein resisted the 

 action of the sea without the least alteration, and, notwithstanding the 

 insufficient height and imperfect state of the exterior revetments, a garrison 

 of 60 men remained in the battery in perfect safety. 



In May 1805, the works were sufficiently advanced to arm the battery 

 with 20 pieces of heavy ordnance within 24 hours. 



The works suffered little material injury until the 12th February 1808, 

 when, during a dreadful gale from the north-west, the sea submerged the 

 battery, destroyed and upset the epaulements, and the wooden barrat:ks of 

 the garrison. 



The main effect of this violent gale was to put an end to any further 

 displacement of the materials ; and the work has ever since presented the 

 appearance of a natural slope of rocks, over which the ordinary "varechs" 

 cross, and are developed in an unbroken undisturbed mode which proves 

 the perfect stability of the work. 



The examination made at this time of the new configuration which had 

 been given to the mass of stones, confirmed and verified by subsequent 

 experience, has established the fact, that jetties thus constructed, and 

 exposed to the action of the sea, assume, between their summit and their 

 base, four slopes essentially different, and which vary according to the 

 violence of the waves, the duration of their action at the several parts in 

 the rise and fall of the tides. 



The situation of the exterior revetments of the battery being the effect 

 of the natural action of the sea in its greatest agitation, was considered as 

 that most suited to insure the permanency of the materials of which the 

 work was composed ; and accordingly, after the storm of 1808, new bar- 

 racks were constructed for 60 men ; a new parapet was raised to cover 



the artillery of the port, but without making any modification in the exte- 

 rior slopes, which have ever since maintained the exact degree which the 

 sea had assigned to them, though these slopes have, since that period, 

 never had the least care bestowed on them. 



The digue, thus completed in its central portion, remained armed with 

 20 pieces of artillery during the whole war. In the year 1811 it was 

 decreed to substitute for this temporary fort a permanent defence, thus 

 described in the decree dated 7th July of that year : 



The battery to be established upon the breakwater of Cherbourg will be 

 constructed in an elliptical tour of masonry, of dressed blocks of granite, 

 of which the great axis shall be 35 toises, and the smaller 19 toises, in 

 conformity with the plans and sections annexed to the present decree, and 

 to the following specifications. 



The foundations will be established on the rock-work at low-water 

 level. 



Upon this massive foundation, which will be 28 feet high, and at the 

 level of the terreplein of the existing battery^ will be placed a barrack, of 

 which the walls will be pierced with 78 loopholes, capable of containing a 

 garrison of 60 men, water-cisterns, and powder-magazine. 



The gorge of the battery will be defended by two flanks. 



A general platform upon the roof of the barracks, which will be bomb- 

 proof, will serve for the site of a casemated battery for nineteen 36-pound- 

 ers. The embrasures will be 30 feet above high-water mark. 



A second platform will be constructed above the casemates, to serve, in 

 case of necessity, for another tier of guns. 



That portion of the existing battery without the site of the tower will be 

 preserved, and the slopes towards the sea, which protect it, will be care- 

 fully kept in repair. 



BREAKWATERS OF CHERBOURG AND PLYMOUTH. 



Annex {S).— Report m'lde to the Academy of Sciences, Paris, by M. 

 GiRARD, in the name of the Commission composed of Messrs. Prony, 

 GiRARD, and Dupin, upon a Mfmoire by the Baron Cachin, Inspector- 

 general of Bridges and Roads, entitled, " Memoirs upon the Digue of Cher- 

 bourg, compared with the Jetty or Breakwater at Plymouth." 



The perpetual secretary of the Academy of the Mathematical Sciences 

 certifies, that the following are extacls from the proceedings reported in the 

 sitting of Monday, May 3, 1815. 



The Commission appointed in 1792 assured itself by an attentive obser- 

 vation of the effects of the sea upon the digue, that the materials of which 

 it was constructed had no stability until they were faced with large blocks 

 of from 15 to 20 feet cube at least; but the most important modification 

 which this Commission proposed in the construction of the work was, to 

 carry the summit to the height of three metres above the highest spring 

 tides, as the only means of guaranteeing the solidity of the work itself, 

 which was the main object it was intended to effect. 



The old digue or jetty, which had been provisionally carried up to the 

 level of low-water mark in the year 1784, had now offered an experience 

 of 20 years. It was ascertained that the storms during this interval of 

 time had lowered the summit from four to five metres ; the interior slope 

 had preserved the inclination which had been originally given to it of 45 

 degrees, but its exterior slope, which had been originally formed upon a 

 uniform inclination of three metres of base to one of height, was found to 

 be entirely changed, and presented two distinct slopes ; that of the lower 

 portion was of 9 metres of base to 6 metres 30 centimetres of vertical ele- 

 vation, whilst the inclination of the superior or upper slope had become 

 five times less ; that is to say, of 47 metres 50 centimetres base to metres 

 20 centimetres vertical height. 



These observations demonstrated what was the profile of the greatest 

 stability, which was the most important point to know ; and as it was 

 remarked that the principal effect of the action of the sea, in strong winds 

 from seaward, was to carry from the outside to the inside of the break- 

 water the materials of which it was composed, it became necessary, after 

 having opposed a sufficient obstacle to this displacement, to abandon to the 

 action of the sea itself the task of arranging the exterior surface which 

 was exposed to its force in the manner and in the slope most suited to 

 their object. 



Accordingly, towards the end of the year 1803, M. Cachin caused to be 

 raised on the top of that portion of the digue which had been already 

 elevated, a sort of parapet, built with very large blocks, of which the 

 summit was carried up to the level of the highest tides ; thus the smaller 

 stones which had been cast into the sea, by chance as it were, on the exte- 

 rior of the digue, at the depth of low water, were borne up by the force of 

 the waves to the foot of this parapet, and were there disposed on a regular 

 slope, which, offering the least resistance to the movement of the waves, 

 possessed the greatest stability. During the production of this effect, the 

 horizontal base of the exterior slope became about quadruple that of its 

 height. , , 



Besides the movement of the materials in a vertical direction, whenever 

 the wind blows hard from the north, or perpendicularly to the digue, these 

 materials receive an impulsion from the effect of the winds which blow 

 from the north-east aud north-west, and in consequence of this impulsion 

 two accumulations have been formed at the extremities of that central 

 portion of the digue destined to support the battery, in the shape of two 

 conical mounds, which serve for Epaulements for the work. 



This peculiar configuration, the effect of natural causes, is also found ♦« 



