(3) This method, as its name implies, consists in taking out short lengths of the whole sectional profile before beginning the building of the masonry. In the English method the invert is built first, then the side walls, and finally the arch. The excavators and masons work alternately. The Austrian method differs in two particulars from the English: the length of section opened is made great enough to allow the excavators to continue work ahead of the masons, and the side walls and roof are built before the invert. In the American method the whole section of the tunnel is open at once: excavators and masons work simultaneously, but a very large quantity of timbering is required. (4) The Italian method is very seldom employed on account of its expensiveness, but it can often be used where the other methods fail. It consists in excavating the lower half of the section, and building the invert and side walls, and then filling the space between the walls in again except for a narrow passageway for the cars; next the upper part of the section is excavated, as in the Belgian method, and the arch is built; and finally the soil in the lower part is permanently removed. =Tunnels in Quicksand.=--Tunnels through quicksand are driven by one of the ordinary soft-ground methods after draining away the water, or else as submarine tunnels. =Open-Cut Tunnels.=--Open-cut tunnels are those driven at such a small depth under the surface that it is more convenient to excavate an open cut, build the tunnel masonry inside it, and then refill the open spaces, than it is to carry on the work entirely underground. In firm soils the usual mode of operation is to excavate first two parallel trenches for the side walls, then remove the core, and build the arch and the invert. In unstable soils, since the invert must be built first, it is usual to open up a single wide trench. In infrequent cases where a tunnel is desired in a place which is to be filled in, the masonry is built as a surface structure, which in due time is covered. =Submarine Tunnels.=--The mode of procedure followed in excavating submarine tunnels depends upon whether the material penetrated is pervious or impervious to water. In impervious material any of the ordinary methods of tunneling found suitable may be employed. In pervious material the excavation may be accomplished either by means of compressed air to keep the water out of the excavation, or by means of a shield closing the front of the excavation, or by a combination of these two methods. Tunnels on the river bed are built by means of coffer dams which inclose alternate portions of the work, by sinking a continuous series of pneumatic caissons and opening communication between them, and by sinking the tunnel in sections constructed on land.