THE HYDRAULIC BRAKE 783 



may be adjusted. In its simplest form the brake is extensively used as a 

 dashpot for damping the vibrations of governing mechanism and the like. 



When used as a buffer stop, the body whose kinetic energy is to be 

 absorbed forces in the piston rod and produces a flow of liquid at high 

 velocity through the connecting orifices. The energy of the body is thus 

 partly transformed into kinetic energy of the liquid, which is dissipated 

 in eddy formation, and partly expended in overcoming the frictional 

 resistances of the connecting passages, together with the mechanical 

 friction of the brake. The whole of the energy is thus ultimately 

 transformed into heat. Since the energy absorbed by the brake is 

 constant for a given mass moving with a given velocity, and is equal to 

 the mean resistance of the brake multiplied by the length of its stroke, it 

 is evident that the pressure in the brake cylinder will have its least 

 maximum value when this pressure, and therefore the resistance, is 

 uniform throughout the stroke, and when in consequence the pressure- 

 displacement diagram forms a rectangle. The brake is therefore prefer- 

 ably designed so as to give as nearly as possible uniform resistance, and 

 since the resistance varies as the square of the velocity of the liquid 

 through the connecting orifices (very nearly), while the velocity of the 

 moving body, and therefore of the piston, varies from a maximum at the 

 instant of impact to zero at the end of the stroke, it is necessaiy 

 either to make the connecting passages of diminishing area towards the 

 end of the stroke so that the velocity of efflux may remain constant, or to 

 discharge from one side of the piston to the other through a spring- 

 loaded valve set to open at the required pressure. 



Where passages of constant area are used it is evident that the resist- 

 ance falls off very rapidly as the velocity diminishes and, e.g., has only 

 one-quarter of its initial value when the velocity is reduced to one-half. 

 In fact, if the resistance were solely measured by the production of 

 kinetic energy in the contained fluid, the body would never be brought 

 absolutely to rest. Actually, however, the additional resistance in the 

 shape of solid friction at the cup leathers, &c., together with that of 

 returning springs or balance weights, prevent this state of affairs being 

 realised in practice. 



The area of the connecting passage may be varied by forming it as a 

 circular orifice through the piston, and allowing this to work over a taper 

 circular spindle fixed longitudinally in the cylinder, the available passage 

 area varying with the diameter of the spindle. A somewhat similar 

 device is applied to a type of buffer stop adopted by Mr. Langley, two 

 rectangular longitudinal slots cut in the piston body working over two 



