of Forth, in the year 1807, he very clearly laid down the broad principles on which he held that such works should be designed: "Every harbour," he said, "should be so constructed as to have its mouth as much exposed as possible to the direction from whence vessels can most conveniently enter in stormy weather when they are least manageable; but the Heads should be made of such a form as to admit of the least sea entering it, or so as to occasion as little swell within the haven as possible. This cannot by any practicable construction be entirely avoided; but means should be provided within the harbour so as to reduce the recoil of the waves to a minimum, for it is the undertow or retiring sea, after the breaking of a wave, that renders vessels most unmanageable by making the helm lose its effect. At such a time the mariner is at a loss what to do, or how to manage his vessel; and for the want of due attention to these particulars many of the most considerable artificial harbours in the kingdom are exceedingly difficult of access, and some of them are most unsafe even when entered." The great point, he held, was not only to make a harbour to keep out the sea, but to do so in such a manner as not to render its entrance from the most exposed or dangerous quarter difficult in stormy weather, when its shelter was most needed; and while it must be so designed as to afford a safe shelter for shipping, it must also be easy to enter and easy to get out of. That so many modern harbours, constructed

1 From the following brief description it will be observed how skilfully he carried out these views in laying out the intended harbour at Charleston. He proposed to construct two great piers, one placed at the western extremity of the little inlet, to which a railway was being laid down-the straight part extending outwards about 154 yards, from which there were to be two kants of about 64 yards each, the last going 57 yards below low

water mark. From thence there was to be a return bend about 70 yards long, in a direction considerably to the north of east. At 50 yards from the extremity of this pier, another of the same length was proposed to be made, forming an angle with it of about 120 degrees, with two other kants similar to the former, and a larger one extending to the shore; the entrance being 50 yards wide, and the outer arm or kant of the cast pier making

at great cost, are found comparatively inaccessible in severe weather, is, we believe, to be accounted for mainly by the circumstance that they have been laid out after no definite rule or principle whatever. If they succeed, it is often the result of a happy accident; and if they prove failures, it is supposed that it could not have been helped. Even within the last twenty years several expensive havens have been constructed, which have proved to be so dangerous that they can scarcely be used. But by Mr. Rennie's forms of piers, vessels, if they have only steerage-way, must enter the harbour in safety. They cannot strike on the pier-heads, if the most ordinary care be used, the very recoil of the waves forcing them forward into port; and, as any swell which might enter would have ample space to expend itself, the ship could either be brought up, or take the beach without damage if necessary. Again, a sailing vessel, on leaving the harbour, supposing the wind to be blowing right in, could lie out upon either tack and make an offing, if it were prudent to put to sea at all. And although the narrowed distance between the two pier-heads might be termed the entrance,

an angle of 120 degrees with it, so that both the outer arms made similar kants with each other. A large space would thus be enclosed, which, he believed, would make a very commodious and capacious harbour. "By the above construction," he says in his report, "though it may seem that its exposure will admit of the swells from the south and south-west getting into the harbour, yet when it is considered that the angle at which a wave will strike the Heads will occasion a rebound in a similar angle to that in which it is struck, and as this will be the case from each Head, it follows that these reflected waves, meeting each other, will occasion a resistance which will have the effect of preventing a considerable part of the sea-wave from entering the harbour, and what does enter it will ex

pend its fury on the flat beach within and soon become quiet." This might, he added, be in a great measure prevented by extending the pier-heads further seaward, but which the large additional expense precluded him from recommending; and, indeed, there would always be abundant shelter for the shipping under one or other of the pier-heads. Besides, as the Frith was only about two miles wide at the place, the probability was that there would be no such heavy seas as to render so expensive a measure necessary. The plan was, however, carried out to only a limited extent, and we merely quote the report for the valuable principles to be observed in the construction of harbours, which are here so clearly enunciated.

yet in effect it is not so; for the moment the vessel gets within the outer angles of the two return arms or kants, she may be said to be in or out of the harbour, as the case might be. In this way the fullest width of entrance and the smallest space for the admission of swell are ingeniously and effectually secured.

Whilst occupied on the works of the Ramsgate Harbour, of which he was appointed engineer in 1807, Mr. Rennie made use of the diving-bell in a manner at once novel and ingenious. It will be remembered that Smeaton had employed this machine in the operations connected with the building of the harbour;' but his apparatus being of wood, was exceedingly clumsy, and very limited in its uses. In that state Mr. Rennie found it when he was employed to carry on the extensive repairs of 1813. The east pier-head was gradually giving way and falling into the sea at its most advanced and important point. No time was to be lost in setting about its repair; but from the peculiarly exposed and difficult nature of the situation, this was no easy matter. The depth at the pier-head was from 10 to 16 feet at low water of spring tides; besides, there was a rise of 15 feet at spring and 10 feet at neap tides, with a strong current of from two to three knots an hour setting past it both on the flood and at the ebb. The work was also frequently exposed to a heavy sea, as well as to the risk of vessels striking against it on entering or leaving the harbour. Mr. Rennie's first intention was to surround the pier-head by a dam; but the water was too deep and the situation too exposed to admit of this expedient. He then bethought him of employing the diving-bell; but in its then state he found it of very little use. No other mode of action, however, presenting itself, he turned his attention to its improvement as the only means of getting down to the work, the necessity for repairing

1 See Life of Smeaton, p. 69.

which had become more urgent than ever.

Without loss

of time he proceeded to design and construct a bell of cast iron, about 6 feet in height, 4 feet wide, and 6 feet long, having one end rather thicker and heavier than the other, that it might sink lower, and thus enable the exhausted or breathed air more readily to escape. At the top of the bell eight solid bull's-eyes of cast glass were fixed, well secured and made water-tight by means of leathern and copper collars covered with white lead, and firmly secured by copper screw bolts. To the top of the inside were attached two strong chains for the purpose of fastening to them any materials that might be required for the work, and flanges were cast along the sides of the bell, on which two seats were placed, with footboards, for the use of the men while working. In the centre of the top was a circular hole, to which a brass-screwed lining was firmly fixed, and into this a brass nozzle was screwed, having a leathern water-tight hose fastened to it, 23 inches in diameter. The hose was in lengths of about 8 feet, with brass-screwed nozzles at each end, so that it could be lengthened or shortened at pleasure, according to the depth of water at which the men in the bell were working. For the purpose of duly supplying the machine with air, a double air-pump was provided, which was worked by a sufficient number of men. The air-pump was connected with the hose referred to, and was either placed on the platform above or in a boat which constantly attended the bell while under water. Two stout wrought iron rings were fixed on the top of the machine, to which ropes or chains were attached for the purpose of lowering or raising it. The whole weighed about five tons; and it was attached to a circular framework of timber, strengthened by iron, erected over where the intended new circular pier-head was to be built, and so fixed to a pivot near the centre of the work, that it was enabled easily to traverse its outer limits. On the top of the framework was a truck, made to move

backwards or forwards by means of a rack on the frame, and a corresponding wheel provided with teeth, worked by a handle and pinion. On the truck were placed two powerful double-purchase crabs or windlasses, one for working the diving-bell suspended from it, and the other for lowering stone blocks or other materials required for carrying on the operations at the bottom of the sea. By these ingenious expedients the building

apparatus was so contrived as to move all round the new work, backwards and forwards, upwards and downwards, so that every part of the wall could be approached and handled by the workmen, no matter at what depth, whilst the engineer stationed on the pier-head above could at any time ascertain, without descending, whether the builders were proceeding in the right direction, as well as the precise place at which they were at work.