20 THE ADDRESSES, LECTURES, ETC., OF 



drive it down, which is its own absolute weight in sea-water, and 

 is balanced by the resistance to lateral displacement offered by the 

 water ; and the second force tending to make the cable slide, by 

 virtue of its own weight in sea-water, down the inclined plane 

 which is produced by its position in the water, and which has to 

 be balanced by a retaining force applied to the brake-wheel on 

 board ship. If this cable was left at any one moment to itself it 

 would slide, as you may observe in the case of a stick which is a 

 little heavier than the water in which it descends in a slanting 

 direction. The strain to be applied at the ship must be equal to 

 the tendency of the cable to sliding, and this force can be accu- 

 rately determined if the depth and the specific gravity of the cable 

 are known. This question has been ably treated in a paper read 

 by Messrs. Longridge and Brooke before the Institution of Civil 

 Engineers. The amount of retarding force to be applied to a 

 cable in paying it out has to be equal to the weight in sea water 

 of the cable in question, reaching from the ship down to the 

 bottom of the ocean, in order to prevent sliding in either direc- 

 tion, but this amount of retarding force has to be varied according 

 to the amount of slack which it is intended to give to the cable 

 when laid. If, for instance, you have a cable of double the specific 

 gravity of water, weighing in water one ton per mile, and pay it 

 out into a depth of two miles, you will have to apply a brake-force 

 equal to two tons, or the cable will begin to run overboard with a 

 velocity exceeding that of the vessel through the water. 



The Chairman: Do you know the strength of the Atlantic 

 cable ? 



Mr. Siemens : I do not know exactly the weight in tons. I 

 believe it is equal to supporting 9 or 10 miles of its own weight 

 in water, which is amply sufficient for all purposes. 



The angle of descent of a cable is simply the result of the two 

 velocities to which I have referred. If the rate of progress of 

 the ship is four times as great as the rate of descent, there will be 

 an angle of 1 in 4, or 22| ; but if the rates are equal, there will 

 be an angle of 1 in 1, or 45 ; or if the ship's velocity is constant, 

 it will be determined wholly by the specific gravity of the cable 

 itself, and the nature of its surface, which will determine its rate 

 of descent laterally through the water. If a cable is simply covered 

 with hemp, its resistance is excessive, as may be easily proved by 



