398 THE ELECTRIC TELEGRAPH. 



of the iron bands sufficiently to loosen the breaks. The 

 pressure being put upon the top of the piston, on the other 

 hand, draws the blocks and lever round in the other direction 

 and tightens the break. 



99. The Dynamometer. For the measurement of the ten- 

 sion under which the cable leaves the paying-out apparatus, 

 and also partly to modify the influence of the " pitching " 

 motion of the ship upon the cable, Dr. Siemens' plan of em- 

 ploying a dynamometer is universally adopted. 



Half-way between the pulley in front of the break drum 

 and the stern pulley, a weighted jockey- wheel is put upon the 

 cable, bearing it down in proportion to its slackness. This 

 wheel turns freely on an axle carried at the end of a long 

 lever. The relations between the strain upon the cable and 

 the position of the jockey- wheel will be easily understood by 



reference to Fig. 181. Let a represent the point where the 

 cable touches the stern-pulley, b the point when it leaves the 

 drum or the Y-wheel in front of the break, the straight line 

 drawn between a and b would be the position of the cable if 

 its own weight were nothing, and no dynamometer dragged 

 upon it. Let the jockey of the dynamometer be placed upon 

 it in the middle between a and b, the point on which it is 

 put will sink until the cable forms an angle with the line 

 a b; then, Q being the weight of the jockey, &c., and k the 

 tension of the line between a and Q, we have 



K sin. = JL 

 2 



or, 



K- Q ri 



2 sin. 



Further, if I is equal to half the distance between the points 

 a and b, and h the vertical distance through which the weight 

 Q has bent the cable, we have 



