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THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. ON THE CONSTRUCTION OF THE MOLE DES NOIRES, WHICH SHELTERS THE FRONT HARBOUR AND ENTRANCE OF THE GATES OF ST. MALO. (With an Engraving, Plate I.) ( Translated from the French of M. Girarcl de Caudemberg, Engineer- in-Chief of Roads and Bridges.) The Mole des Noires, forming part of the general plan of a floating basin which is to be common to St. Malo ana St. Servan, has been in progress for the last two years, and is situated as pointed out in fig. 1, stretching from A to B. When the wind bows from S.W. to N.W., it is very much exposed to the action of the sea, and was consequently during its construction exposed to all the most unfavourable contin gencies, by which works in direct contact with the sea are affected. For the purpose of opposing this action, a form lias been given to the mole of an arc of a circle of 095 feet (212 met.) radius. The breadth of the top, including the parapet, is 19 feet (5-80 met.), which is strictly necessary for preserving a free passage for warping to the upper part, and for giving to the works the stability and resistance ; necessary to support the difference of pressure resulting from the maximum of the simultaneous elevation and depression of the waves on the two opposite faces. The dimensions of the mole are given in the section, fig. 2, in which are also shown the high and low water marks at spring and neap tides, which sufficiently justify the great elevation given to the work. This section also shows the great aque duct or interior tunnel, and of the channels communicating with it. The aqueduct extends the whole length of the mole up to the head, and the upper and lower channels or pipes are made at every 65 feet distance. The lower inclined pipes end in a number of sluices, which are for the purpose of clearing away the silt in the front harbour. The opinion of M. de Caudemberg was that this silt was little to be feared, but as the commissioners appointed by the Minister of Marine, insisted upon having an aqueduct which could work the sluices, in the front harbour, M. de Caudemberg suggested the plan now in execution. The aqueduct is 1978 feet (603 met.) in length and 7 feet 2 inches (2-20 met.) diameter, and is carried through the mass of the quay of the front harbour, crosses the gates of the inner harbour, and takes its rise in the floating i asin. It is constructed throughout of an annular No. <0.—Yon. IV..—January, 18-11. form. The pipes carrying the water to the sluices are inclined as re presented in the section, and are of cast iron, their inner diameter is about 10 inches (0-40 met.), and the different parts which compose them are secured by a simple joint with resinous mastic. The upper part is terminated by a hemispherical cup 18 inches diameter, with a j ball acting as a bomb valve. This valve is for the purpose of prevent- I ing the water introduced into the great aqueduct returning back again when the tide falls. Where however the sluices are intended to be ] worked the valve is lifted up, by means of a chain communicating with : the upper surface of the quay. The water which flows from the aque duct, through the inclined pipes, with the velocity of a column of water 24 feet high is carried through an opening 3 feet 3 inches broad, and of a mean height of 5 inches, so as to cause a stream of water to sweep away the silt. It should he observed that the large vertical pipes 19 inches (1 met.) diameter, serve for the evacuation of the air, and as manholes for cleansing and repairing the aqueduct. The parts marked ab cm tig. 2, are for the purpose of preventing the water at high tides from getting through the sluices, and causing an inverse pressure on the great aqueduct. The engineer found considerable difficulty while constructing the mole, on account of the position of the great aqueduct, which as it was necessarily built upon a centering, would in case of wood being employed, have been soon blown up by the waves and destroyed, or at least have had the mortar forced out and the work to begin over again. To prevent this M. de Caudemberg directed his attention to the con struction of a peculiar centering or shield. This centering was of cast iron, in moveable pieces, so that it should be readily managed in the progress of the works. On the outside was fixed an arm to break the power of the wave at the period of the shock, while at the same time the specific, weight of the centering prevented it from being carried away. It is formed of panels weighing about a hundred weight each, so that they could be easily moved. The whole shield was 26 feet (8 met.) long, and divided into 16 rings. M. De Caudemberg found that though by these means he broke the shock of the wave, that the works were still liable to suffer on account of the oscillations, particularly when the weather was rough, when masses of compressed air were forced into the great aqueduct, and so up the vertical manholes, causing spouts of water 30 or 40 feet high. These manholes however served greatly to modify the effects. As Ii
