THE CABLE SHIP ON REPAIRS. 243 



under steam before leaving makers' works by actual load lifted 

 to this power (170 B.H.P.). The wheels used to be of 

 good malleable cast-iron, but in modern sets they are made 

 of cast-steel, by which four times the strength, with only one- 

 twelfth additional weight, is obtained. Gear designed with cast- 

 steel wheels is, therefore, much lighter, because the parts are 

 much smaller for the same power. 



For convenience in dealing with work involving diflFerent 

 strains a single and double purchase is always fitted on 

 the gear. The single purchase, of course, runs the drum 

 at the quicker speed, and is used in the lighter strains ; 

 while the double purchase is used in the heaviest work, 

 and drives the drum at half the speed. In the gear 

 illustrated this is effected by the spur-wheels at A, B and 

 C. The pair of wheels at A on the first-motion shaft 

 are of different diameters, and are either bolted together or 

 cast in one. They can be shifted together along the shaft, so 

 that the large wheel engages with C, or the small wheel with 

 B, the latter position, as they are shown set in the illustration, 

 being that of slow speed. The wheels slide on a feather on the 

 shaft and are put in mesh as required by a suitable striking 

 lever or hand-wheel. 



By this arrangement the drum shaft, on which are the wheels 

 B and C, can be driven at two speeds, according to the work 

 required, while the engine speed is the same. In modern ships 

 sometimes three speeds are provided for. If the strain is 

 very great, as in heaving up cable from 2,000 fathoms, the 

 slow gear is put on ; a greater purchase is thereby attained, 

 and a greater load can be lifted by the engine, because it now 

 lifts it slower. This change from single to double purchase is, 

 of course, only made when the engine and gear are at rest. It 

 can generally be judged before starting which gear is best 

 suited to the work in hand ; but if while the gear is working it 

 is thought better to run on the other speed, it is a simple 

 matter to slow down the engine, apply the brake, shift over and 

 start up again. Sometimes, as in slacking or paying out, it is 

 necessary to let the gear run free of the engine altogether and 

 work on the brake, in which case the pair of wheels can be put 

 to the intermediate position between B and C, where they are 

 not in gear wirh either. 



r2 



