But although the screw-propeller has ousted the paddle-wheel in very many instances, yet this has been by no means universal. The advantage which the older method possesses is that it can work in less water than the screw needs for its revolutions. In certain harbours, for instance, and shallow rivers--especially in those extreme cases where it is weedy--the paddle-wheel steamer is still pursuing its useful work. It is therefore not unnatural that the tug should in many cases be paddle-driven. The illustration facing page 240 shows one of these paddle-tugs of a fairly modern date. She is owned by the British Admiralty. The _Dromedary_, as she is called, is well known among the Portsmouth craft, and just as the tug is employed for helping liners out of port, so the Admiralty use the _Dromedary_ for assisting such leviathans as the modern _Dreadnoughts_ out of Portsmouth harbour, and rendering assistance in berthing in a harbour where the tides are very strong and the water is considerably crowded. We referred in a preceding chapter to the serious difficulty which, owing to the gradual increase of the modern steamships, is felt in certain ports. New York harbour had to be dredged before it could accommodate the _Mauretania_ and _Lusitania_ with safety. Liverpool’s depth of water is such that the two Cunarders can only enter during twelve hours out of the twenty-four. Fishguard has had to be dredged, whilst Southampton has been, and will need it again. In a smaller degree most ports need constant dredging, otherwise local conditions combine to silt up the navigation channel. Now all this is carried out by specially designed steamships, which, like other vessels, have gradually been increasing to enormous sizes. We might divide dredgers into two classes--the “bucket” dredger, and the “suction” dredger. The illustration facing page 240 gives an excellent idea of the former. This is the _Peluse_, the largest sea-going bucket-dredger in the world. She was built by Messrs. Lobnitz and Company, Limited, of Renfrew, for employment on the Suez Canal. There is nothing in the least beautiful about this type of steamship. Ugly to look upon, splashed all over with mud and sand, covered with machinery and unsightly erections, they are sisters of toil to the ships of beauty. They “bring up” in a harbour or channel, and set their series of buckets dredging away to increase the depth. These buckets are readily seen coming down from a height in the centre of the ship. They are revolved by an endless chain, and the ship is cut open longitudinally to allow them to work.