146 REPORT— 1866. 



which waa considered safe. It was further necessary to he ready, at an instant's 

 warning, to stop the engine in case the cable fouled any part of the ship ; and the 

 author had seen a cable broken owing to the impossibilty of stopping the engine 

 soon enough. Moreover, even when the above precautions were taken, it was 

 impossible to avoid a considerable variation of .strain, due to the pitching of the 

 ship, which alternately slackened and lengthened the cable as it hung vertically ; 

 and in most cases in the author's experience cables, while being picked up in great 

 depths, liad broken from this cause. All these dangers were avoided by the ma- 

 chinery invented by the author, of which two models were shown. These two 

 forms were identical in principle. A spur-wheel, fast on a main shaft, driven by 

 the engine, geared into another spur-wheel centered in the periphery of a brake- 

 drum, loose on the main shaft, and restrained from turning by an Appold's brake ; 

 the second spur-wheel in one form geared directly into an internal-toothed wheel 

 bolted on the picking-up drum, which was also loose on the main shaft above 

 mentioned. When the brake-drimi was stationary, the engine simply drove the 

 brake-drum through the spiu'- wheels in the ordinary manner ; but when the strain 

 on the cable reached the amoimt corresponding to that given by the weight 

 restraining the brake-drum, the picking-up drum ceased to revolve, because the 

 brake-di-um turned instead, carr3ang round the second or intermediate spur-wheel, 

 which rolled inside the internal-toothed wheel, instead of driving it ; the centre 

 on which this intermediate spur-wheel worked might be looked on as a fidcrum, 

 and the wheel itself as a lever, by which the engine pushed roimd the picking-up 

 drum : if the fulcrum yielded, the weight could not be lifted. The second form 

 of model was exactly similar in principle. A second intermediate wheel, of dif- 

 ferent diameter, fast on the same shaft as the first, geared into an external-toothed 

 spur-wheel connected with the picking-up drum. The action was identical with 

 that already described. If the strain increased beyond that required to stop the 

 picking-up drum, it would turn in the other direction, and the cable would be paid 

 out instead of picked up, although the engine would continue to run. in the same 

 direction as before, and exerted the same power. In practice, as was shown by 

 the models, the engine might be driven at any speed ; the cable would only be 

 subject to the strain chosen, which might be increased or diminished at will; it 

 would come up quicker or slower as the ship fell or rose ; it woidd stop whoUy if 

 the cable fouled ; it would be paid out if, ft'om inattention, the ship drifted out of 

 position, or from any other cause the strain increased on the cable. More than 

 this, the cable might actually be paid out as the ship rose, and picked up as it fell, 

 and the whole would take place with perfect smoothness and constancy of strain. 

 The Appold brake gave a constant restraining power to the bralce-drum, whatever 

 the coefficient of friction might be. The gear exhibited formed at once a paying- 

 out and picking-up machine. It might be termed an accm-ate slip-coupling, and 

 could be applied to many purposes — as, for instance, to the measiu-ement of steam 

 power let out. With one of these couplings on the transmitting shaft, it would 

 be impossible to overload the shaft. Similarly, the coupling would serve to pre- 

 vent a break-down in cases where the machinery was liable to sudden starts or 

 stoppages. It woidd prevent undue strains on the ropes of collieries and lifts, and 

 other applications would readily occm- to mechanical men. 



On Zinc Sheathinc/ for /S7»jjs. By Samuel J. Mackie, F.G.S. 



Iron ships are subjected to a great amount of corrosion, and are so liable to 

 foul, that sailing-ships of iron cannot be sent on long A'oyages. Copper sheathing, 

 or Muntz's metal, cannot be applied to iron ships as it is to wooden ones, because 

 the iron being positive to copper, electrical action would be set up, by which the 

 iron woidd be destroyed at a greatly increased rate. If, then, a metal were found 

 which should be positive to iron, when the two metals were in contact in sea- 

 water, the conditions of the voltaic battery formed by the iron ship and its 

 sheathing would be reversed, and the sheathing would be destroyed while the 

 iron would be preserved. A further condition was required to be satisfied, namely, 

 that the metal forming the sheathing should not be destroyed too quickly, but 

 only sufficiently to prevent the growth of animal and vegetable parasites by the 

 slow but constant scaling of the surface. Such a metal was zinc, the cost of which 



