214 APPLIED SCIENCE 



tional to the surface ; further, for the same reason, the waste 

 will be less the more bulky the cable and the rougher the 

 surface. With the old iron cables of small diameter and smooth 

 surface, very little advantage was gained by diminishing the 

 strain on the brakes below that due to the full depth of water ; 

 a very slight relief of strain was followed by a perfect rush of 

 cable out of the ship, and a loss of twenty or twenty-five per 

 cent, was followed by a comparatively small diminution in the 

 risk of fracture. In the cables of the Atlantic class, the bulk 

 relatively to the weight is very greatly increased by enveloping 

 each iron or steel wire in a separate covering of hemp before 

 laying them round the gutta-percha. These cables lie at a much 

 smaller angle with the horizon, they offer a much larger and 

 rougher surface than the simple iron cable, and consequently 

 the friction, as they run back on the inclined water plane, is 

 very much larger. With cables of that class it becomes prac- 

 ticable and desirable to diminish the strain produced by the 

 brake much below that due to the full depth of water. Slack 

 to the amount of twelve or fifteen per cent, diminishes the 

 necessary strain on the brakes by more than one-half, and the 

 importance of this relief can hardly be over-estimated. It 

 actually becomes practicable to disregard the depth over which 

 the ship is passing. The brakes may be set to give the strain 

 thought desirable, and the cable will then take care of itself. 

 In shallow water less slack will be paid out, in deeper water 

 more, but the amount is never excessive, and can at any time be 

 diminished by increasing the speed of the ship, which, by di- 

 minishing the angle at which the cable lies with the horizon, 

 augments the effect of the friction of the inclined water-plane. 

 This effect must not be confounded with the effect that would be 

 produced by a buoyant substance attached to the cable. The 

 hemp is no lighter than water, and does not tend by its buoyancy 

 to carry any part of the weight of the cable, but it increases 

 the bulk, and therefore increases the resistance of the water to 

 displacement, and both directly and indirectly increases the 

 surface friction. 



The strain on the new Atlantic Cables during submersion 

 was from 12 to 14 cwt. ; their strength is 150 or 160 cwt. Here 

 there is a co-efficient of safety of ten instead of two or four. The 



