Remoz-ing in Navigable certain cases where the free nriyigation of an important waterway is obstructed by masses of rock, it may be found expedient to remove them. To attain the de sired object, it is obvious that different methods from those described in previous portions of this section must be resorted to. The operation con sists, in general terms, in sinking a shaft from the surface of the rock-mass, or reef, to the depth it is intended the channel shall have, and in driving galleries in all directions from this shaft until the entire section of the reef to be removed has been honeycombed. A sufficient number of pillars are left to serve as a support for the roof of the rock, and in working the exca vation the utmost care must be exercised to avoid getting too near the bed of the river, and also to avoid shattering the roof. The removal of the rock in the galleries is effected in precisely the same manner—by the use of rock-drilling machines and explosives—as in tunnelling on land. (See p. 187.) When the top of the reef lies below the surface of the water, a coffer-dam must be sunk, to exclude the water.
When the rock-section to be removed has been honeycombed through out its entire extent, holes are drilled in the pillars and in the roof at regu lar distances apart; and when this work has been finished, the holes are charged witb explosive material. These charges are connected with one another by conducting-wires, in order that they may all be exploded simul taneously by electrical means from the surface. When the holes ate all properly charged and electrically connected, the water is permitted to enter the excavation until the entire work is flooded. The mine is then fired by the closing of an electric circuit, and the resulting explosion, if the work has been properly executed, shatters tbe roof and pillars to fragments. If necessary, the debris may be dredged up, in order to secure the required depth of channel.
//a//ct 's Point and Flood Rock most extensive works of this description which have ever been midertaken and successfully exe cuted were those involved in the removal of the reefs at Hallet's Point and at Flood Rock, two formidable obstacles to the free navigation of New York harbor, lying in the narrow channel leading from Long Island Sound to the East River. The removal of these reefs was undertaken by the national government, and NV aS successfully accomplished under the direc tion of General John Newton of the United States Engineer Corps. Fig ure (y5/. 55) gives a sectional view of the appearance of the submarine excavation at Flood Rock.
Conditions for Riz,er Navigation.—As a rule, important engineering works for the improvement of rivers are undertaken in the interest of navi gation. The term " navigable," as applied to a river, is a relative one. The principal conditions demanded are the following: A width of channel at least four times as great as that of the vessels intended to ply its section, and a minimum depth within this channel at least one-third greater than the draught of these vessels. Furthermore, the channel should have no very sharp curves; the current-velocity shonld not be too great; and if towage from the land is desired, it should be practicable to provide easy and safe tow-paths on the banks. Where the flow is rapid or irregular
over considerable distances, or Where the depth of water would otherwise be insufficient in dry seasons, the navigation may be improved by dividing the stream into sections or reaches, and by building across the stream at the end of each section a dam by which the water is held back. The in creased depth obtained in this manner will be governed by the height of the structures. The passag,,e of vessels is accomplished by the provision of side channels controlled by sluice-gates or locks. The constructions of this class will be more fully treated of under the head of Canals (p. 314). To pass by unfavorable reaches, it is often found necessary to build an artificial channel (canal), and to provide locks at suitable intervals to over come the fall.
Inlana' Transfiorlation: severe competition to which inland transportation by water-routes has been subjected by the introduction and development of railways has originated a number of sub stantial improvements in the methods of the former. The invention of steam-towage was a decided forward step, but its application was limited principally to streams of notable magnitude and depth, while for the comparatively shallow streams, especially with those having considerable current-velocity, its serviceability was restricted. One of the chief reasons to be assigned in explanation of this is the comparative disadvantage of applying power by either a paddle-wheel or a screw-propeller to a yielding medium like water, where the absence of a fixed resistance must be com pensated for by increasing the speed of rotation of the motive-power.
—These facts gave birth to the idea of causing a tow boat provided with suitable motive-power to propel itself, with its load, by traction upon a stationary chain or cable, the method being analog,ons in principle to the action of a locomotive drawing- itself and its attached train by the tractive force exerted upon the rails by the driving-wheels. The idea has been put in practice with highly-satisfactory results. The steam tow-boat is connected with the chain by one of the following methods: Either the chain is caused to make several MS around the circumference of a drum, or of several of them, placed in the lonkitudinal axis of the vessel and set in rotation by the vessel's engine (fi7. 39,/ig. 2o), or a "sproket-wheel" is employed—a wheel furnished on its periphery with pins spaced to con form to the distance apart of the links of the chain, so that the links will engage with the pins without slipping. With this device it is not neces sary that the chain should embrace more than one-half the circumference of the wheel. In all cases the rotation of the drum or wheel raises the chain from the river-bottom at the bow, and as the boat drags itself for ward upon it the slack is paid ont over the stern, falling- again to the bed of the stream. The steam tow-boats have engines of from fourteen to thirty horse-power.