SCIENCE AND PRACTICE. 397 



self-adjusting. This will be better understood by reference 

 to Fig. 180. D is the drum upon which the strap a, s, s, s, b 

 presses. It is hinged at the end a, on which the greatest 

 strain com.es, at a greater distance from the fulcrum c of the 

 lever bar a c than the other end b. The difference between 

 the distance c b and c a, along the bar, is very little at 

 most 2 inches. When the lever is deflected towards the left 

 hand by a force, w, it is evident that the strain of the strap 

 upon the circumference of the drum will be increased, and 

 that this increase will be exactly proportional to the force 

 exerted, and to the difference of the strains upon a b, or to 



the length . Suppose the drum to turn in the direction 



shown by the arrow at a moment when the force w was just 

 sufficient to balance the friction of the break, if by any acci- 

 dental cause such as dirt, or want of any lubrication the 

 coefficient of friction between the surfaces suddenly increased, 

 the strain becoming greater on a than on b, the lever would 

 be proportionally deflected towards the right ; and, owing to 

 the eccentric position of its bearing in turning, would 

 slightly loosen the break-strap and thus reduce the friction. 

 The reverse occurs, of course, when the coefficient of friction 

 is lessened. 



The adaptation of hydraulic pressure to the apparatus for 

 supplying the balancing force w is novel. On the right side 

 of each of the breaks is a cylinder, connected with a feed- 

 pipe, at top and bottom, attached to a force-pump kept work- 

 ing during the paying-out, for moderately lubricating the 

 break, to prevent it getting too hot by the friction. The 

 connecting rod attached to the piston working in the 

 cylinder is coupled to the point a of the bar a c. A. four- way 

 tap enables the water pressure to be put upon the upper or 

 lower surfaces of the piston in the cylinder, and the blocks 

 on that side are lifted or depressed according to requirement. 

 When the pressure is underneath the piston, the latter is 

 forced up and, with it, the connecting-rod, while the blocks 

 are turned from right to left round the break-wheel, carrying 

 the end of the lever bars with them, and separating the ends 



