Semi-automatic High-Pressure Installation. ' 159" 



from the open to the closed position that any such leakage 

 is possible. To close the valve, the needle N is opened and 

 the valve-stem S is thus free to rise and seat itself securely,, 

 the light spring R becoming compressed at the same time. 

 It will be noticed that the dermatine ring is so arranged 

 that it becomes automatically tighter under the pressure 

 which it has to hold. The screw plunger-pump can now be 

 refilled with oil, and, after closing N, the valve will auto- 

 matically reopen when the pump-pressure has been brought 

 to a value a little in excess of that in the remainder of the 

 pressure-system . 



Method of altering the pressure. — As already stated, in 

 much of the work for which this installation is used it is 

 necessary to avoid rapid changes, and especially rapid in- 

 creases of pressure. For this reason, when a weight has to be 

 added to the load carried by the rotating piston of the gauge 

 (Gi, G 2 , G 3 ), means must be provided for allowing the extra 

 weight to come on gradually. The plan adopted is to 

 suspend the weight from three triggers on a cage carried 

 upon a ball-bearing at the end of a fairly extensible spring.. 

 The suspended system is lowered by the slow unwinding of 

 the cord to which it is attached; and as the unwinding 

 proceeds, the extra load is placed gently in position and its 

 weight gradually transferred from the cage to the pile of 

 weights below it, the spring shortening meanwhile. As 

 soon as the cage has been relieved of the whole of the 

 weight, the three triggers withdraw automatically, leaving 

 the cage clear for lifting. The gradual removal of weight 

 is effected by the same mechanism ; but in this case the 

 triggers are differently set, so that they are adapted for 

 picking up instead of for releasing. 



To obtain an increment of pressure less than that due to a 

 weight unit, a light container is placed on the top of the 

 weights, and above it is suspended a tin funnel containing 

 the requisite quantity of shot. On opening the orifice of 

 the funnel the shot runs into the container at a rate which 

 depends (within limits) only on the area of the orifice and 

 not on the head of shot. 



Automatic compensation for leak. — It is sometimes necessary 

 to maintain a pressure sensibly constant for a considerable 

 time. To this end the head of the pump-screw is fitted with a 

 ratchet-clutch, Q,fig.l(the " free-wheel " clutch of a bicycle), 

 to one member of which is attached a horizontal lever L. 

 It happened to be most convenient to arrange this lever to 

 the right hand of the pump, which meant that the down-stroke 

 of the lever would be the operative one. As it was desired 



