Objections to this system may, indeed, be raised on account of the damage sustained at Cherbourg and Plymouth as well as at Algiers, before the introduction of factitious blocks, but it is necessary to observe, that both at Cherbourg and Plymouth, the tidal current is exceedingly strong, its velocity being as much as 4 _met._ per second; that the sea at these points is very rough, and that there is reason to suppose that the damage would not have occurred had the blocks been rather larger, and the interstices well filled up. With regard to the roadstead at Algiers, it is, as is well known, constantly beaten over by heavy seas, no other point in the Mediterranean presenting such difficult conditions. We have in favour of our system most of the moles erected in the various ports of the Mediterranean, Genoa, Cannes, Barcelona, Valencia, Cadiz, &c. &c., all of which are constructed of natural blocks, the largest not exceeding 2 _m._, 50 cube, and which are nevertheless established at considerable depths of water. Finally, we have on our side the opinions of the most distinguished English engineers; opinions which have prevailed in Parliament, and in accordance with which, all the moles in the harbours of refuge in course of construction are being made, according to the system of natural blocks sunk into the sea, at certain slopes. The bottom of the beach, descending by a very gentle inclination, will, moreover, have the effect of abating the waves, and diminishing their action against the jetties. This is a well ascertained fact, and one which may, indeed, be easily conceived; for, supposing that the bottom of the sea, from a depth below the limit of the motion of the waves, rises by an extremely gentle slope, until it meets that limit; this meeting taking place at a very small angle, the bottom will be almost insensibly substituted for the limit of motion. At the point of this meeting the undulating motion is _nil_, it is very feeble at the adjacent points, and easily abated by the resistance and friction which the molecules experience against the bottom. The abatement will thus extend vertically up to the surface, and the waves will then gradually diminish in volume as they approach the shore. We have, therefore, adopted the system of loose stones for the jetties and for the mole, with but slight modifications suggested by our own experience, modifications which consist in making the jetty-heads in hydraulic masonry to a certain height, as well as the interior surface of the windward jetty, which is to serve for the towing of vessels.