240 APPLIED SCIENCE 



Alexandria Cable, of little more than two-thirds the length. 

 Here, then, is at least as great an advance as in the other 

 branches of submarine telegraphy. In future long cables the 

 speed may be calculated on this new basis, which has long been 

 advocated by a few, but which had not received practical con- 

 firmation till now. The speed at which messages can be sent 

 through a given length of cable is simply proportional to the 

 quantity of copper and gutta-percha used, provided the rela- 

 tive proportions of these materials remain unchanged, as is now 

 practically true in most cases. The new experiment would 

 therefore allow the engineer to adopt a core of one-eighth the 

 weight which he could have adopted upon the old system of 

 telegraphy to obtain the same speed ; or if he be not bold 

 enough to adopt so small a core as this would sometimes lead to, 

 he may at least choose the smallest core which on mechanical 

 grounds he thinks safe to adopt. Here we may catch a glimpse of 

 possible cheap cables hereafter. 



So long as the cable was coiled on board the l Great Eastern ' 

 it was not possible to transmit more than five or six words per 

 minute through it, even with the best appliances. The differ- 

 ence between a coiled cable and a straight one, as a means of 

 signalling, has long been known, and that difference appeared 

 from Mr. Jenkin's experiment on the Red Sea Cable (' Trans- 

 actions of the Royal Society, 1862 ') to be possibly great enough 

 to halve the speed, or even reduce it to a still smaller fraction of 

 that obtained on the straight cable ; but crucial experiments on 

 this point were wanting. The extra retardation is produced 

 partly by the induction of the current on various parts of itself 

 in neighbouring coils, and partly by the magnetisation and de- 

 magnetisation of the iron sheathing, which forms a sort of huge 

 electro-magnet. The effect produced by the coiling is analogous 

 to giving the electric fluid an inertia, and consequent momentum, 

 an analogy long since pointed out by Sir William Thomson, in 

 a paper by him and Mr. Jenkin on the discharge from a coiled 

 cable, published in ' Phil. Mag.,' 1861. A new illustration of tins 

 analogy was discovered on board the ' Great Eastern ' by Sir 

 William Thomson, in the fact of an oscillating current flowing 

 in and out of the insulated cable when first charged. Tliis 

 phenomenon was described by Mr. Varley in a paper read at, 



