Cable-lowage: Pulley.—Instead of chains, cables have been intro duced for the same purpose. In this case the cable may be made to take several turns about the circumference of a drum or drums, to prevent slip page, or the clip pulley (/5/. 31,_fig. 3), which is successfully applied for haulage and hoisting in mines, may be substituted for the drum. The face of the clip pulley is set over its entire periphery with a series of movable jaws of V-shape, each leg of the V being so pivoted that when any weight is brought into the base of the V it is firmly clutched and held immovably nntil by the rotation of the pulley the jaws gripping the cable are one after another released. For the same tractive power it is possible to use a cable much lighter than a chain, and it has the advantage, also, of being cheaper. The irregular jerking- movements caused by the winding and unwinding of the chain upon the drums, likewise, are obviated, and, filially, the cable is less liable to breakage.
Chain or Cable Towing-boals.—The hulls of these tow-boats (fii. 39, fic. 2o) are built of iron. They are flat-bottomed structures shaped alike at bow and stern, with a steering apparatus at both ends. At bow and stern is a projecting arm which is free to swivel in a horizontal plane through an angle of nearly go°, and which serves as a guide for the chain or cable. This device serves the useful purpose of allowing the course of the vessel to be altered, within certain limits, independently of the position of the chain, so that passing vessels may turn out for one another. This device likewise assists in the task of keeping the chain in mid-channel. It is found to have a tendency in curved sections of the channel to work over toward the convex bank; in consequence of which, after the passage of a number of tows, it will be found to have become slack. To remedy this, the towing-vessel is caused to pass alone from time to time without a tow, and by the forcible aid of its rudder the return of the chain to mid-channel is accomplished. The chain used for steam-towage on the Elbe is formed
of round iron links haying a diameter of .S6 inch, and a weight of twenty two and a half pounds per running yard.
From the data obtained in practice on the Upper Elbe, it is found that a chain towing-boat will draw a load of fifty thousand hundredweight (about twenty-five hundred tons) at the rate of two and a half to three miles per hour. The chain-boat requires less power than the tug, and con sumes, consequently, less fuel. The number of the crew may be reduced, the towed vessels can dispense entirely with the use of masts and tackle, and the forwarding of freight is done more rapidly and with greater regu larity and certainty as regards time of delivery.
Employment of Towage Syslems.—The earliest experiments with steam towage date from the year 182o. Chain-towagre, in its present perfected state, dates, however, from the year 1853, when it was first introduced upon the river Seine. In Germany the first chain was laid in the Elbe between Neustadt-Magcleburg and Buckau. The system has since been greatly extended on the Elbe, and has been applied to large sections of the Rhine, the Danube, the Oder, the Brahe, and other German streams. In France steam-towage has been applied in the Seine and the Rhone, and in numer ous canals. In Belgium and Holland it bas been extensively introduced, cables as well as chains being employed both in rivers and in canals. In other E,uropean countries less has been accomplished in this direction. In America chain- (or cable-) towing as above described does not appear to have received the attention it deserves. Towing by free self-propelling steam-tugs of special construction to reduce the danger of the washing away of the banks is in use upon the Erie Canal.