SCIENCE AND PRACTICE. 395 



cable by the break, and indicated by the dynamometer, in 

 order to lay out the line under any desired conditions. 



"When a cable is being paid out two forces act simultaneously 

 upon it : 1st, gravitation, by which it falls to the bottom 

 this force acting perpendicularly ; secondly, friction of the 

 water, which, by reason of its position, lying upon an inclined 

 plane, gives it a tendency to slide down in a line from the ship 

 to the point where it touches the bottom. The angle formed 

 by the cable in going down depends obviously only upon the 

 relation between the speed of the ship and the velocity with 

 which the cable falls freely in water, and is independent of 

 the tension with which it is laid. The force, however, with 

 which the cable tends to slide down the inclined plane is very 

 nearly equal to the weight of a length of cable reaching from 

 the point where it touches the bottom perpendicularly to the 

 surface of the water. This sliding tendency is of great use 

 in successfully submerging a cable, as it enables the engineer 

 to lay out his line with sufficient sliding, or slack, to cause it 

 to fall into all the irregularities of the bottom without span- 

 ning any of them. If the bottom were quite level a cable 

 could without danger be laid out without any slack, in which 

 case it is evident that the engineer would have to oppose just 

 so much resistance or break-power as would balance the ten- 

 dency to slide or to take slack. 



This balancing or break power would be exactly the weight 

 of a length of cable in water equal to the depth, if the resist- 

 ance of the water against the surface of the cable did not 

 retard the inclination to slide in a slight degree, and thus 

 to lessen the force necessary to be applied in retaining it. 



The bottom being, however, irregular, the engineer has to 

 be supplied with soundings which instruct him at each point 

 of the course what depth of water he is in. He knows, also, 

 the angle which the cable makes in the water, and therefore 

 he can tell easily under what depth of water the point of the 

 cable is which at any moment touches the bottom, and ac- 

 cording to this depth he has to regulate his break-power. 

 Were he to regulate the power according to the plumb-line 

 from the ship to the bottom, at any moment he might fall 



