50-TON BUCKTON MACHINE. 5 



at all times be floating that is to say, it must be acted 

 upon by no other forces than its own collective weight, the 

 pull on the specimen, and the pressure upon its main knife 

 edge. It will be noticed that at the right-hand end of the 

 beam in the illustration there is a pillar G, terminating at 

 its upper end in two stops H and S. The beam is free to- 

 oscillate between these limits, but cannot pass beyond 

 them. When the beam is horizontal and in equilibrium, 

 its end is about midway between these stops, and it should 

 remain as nearly as possible in this position throughout a 

 test. If the beam is -resting upon the bottom stop, some 

 of the weight of the beam and poise is being taken up by 

 it, and the position of the weight is no indication of 

 the load on the specimen. On the other hand, should the 

 end of the lever rest against the upper stop, the load on 

 the specimen will be greater than would appear to be the 

 case from the scale reading, because there is an additional 

 and unknown force acting upon the beam through the 

 stop, and at the full leverage of the machine. 



It is, therefore, extremely important that the beam be 

 always kept floating freely between the upper and lower 

 stops. To maintain this condition, use is made of the 

 hydraulic appliances. At F is a vertical hydraulic cylinder 

 bolted to the lower end of the standard of the machine. In 

 this cylinder works a piston fitted on the upper side with a 

 cup leather. A piston rod passes through the top of the 

 cylinder, and is made watertight by a second cup leather. 

 The upper end of this rod screws into the crosshead which 

 carries the lower holding shackle. In the earlier machines, 

 two cup leathers were fitted to the piston, and water under 

 pressure could be admitted either above or below the 

 piston above, when it was desired to increase the load on 

 the specimen, and below, when the piston had to be 

 brought back to its normal position after the completion of 

 a test. In the later designs the piston is brought back to 

 its normal position by means of a balance weight acting at 

 the end of a small lever ; this is shown in the figure. 



Suppose a bar, S, to be in the machine, and the load to be 

 moved along the beam. If the beam is perfectly horizontal 

 to begin with, the stretching of the specimen, Avhich 

 necessarily accompanies an increase of load, will cause it to 

 fall towards the lower stop, very slightlv at first, while the 

 specimen is still elastic, and more rapidly at a later stage,. 

 when permanent set begins to take place. It, therefore, 

 becomes necessary to bring the beam back to its horizontal 



