38 Proceedings of the Roj/al Irish Acadcm/i. 



diagram which would ensue if any wheel were regarded as passing completely 

 off the girder. 



Passing round the frame and _ tightly stretched over four small pulleys set 

 on the back plate near the corners is an endless chain. A stud projecting 

 downwards from the locomotive is attached to the upper horizontal side of 

 this chain, so that the chain carries the locomotive with it as it moves. To 

 the right-hand (vertical) side of the chain is attached a handle, not shown 

 in the figure, by means of which the chain may be moved round the pulleys 

 at the corners of the frame. To the left-hand (vertical) side is attached 

 a slider on which is fixed a small pin over which threads pass, as shown in the 

 figure ; these threads passing through eyelets in the face-plate are kept tight 

 by weights behind. The eyelets are arranged in a vertical line 2| in. (repre- 

 senting 10 feet on the assumed scale) to the right of the pin, and the 

 intervals between them are proportional to the assumed loads on the wheels, 

 so that the line joining the eyelets forms a " load-line " or force-polygon for 

 the loaded girder. The distance between the extreme eyelets is equal to 

 the distance between the supports of the girder, which fixes the scale of force 

 at 4 tons to the inch, and the pin over which the threads pass is attached to 

 the chain at a point which is as far below the highest eyelet as the C. G. of 

 the locomotive is from the right-hand support of the girder. The lines 

 between the eyelets,, the pin, and the threads may be regarded as forming a 

 force- or load-polygon, a pole, and a set of polar vectors for the system of loads 

 on the girder. 



To the lower (horizontal) side of the chain is attached a slider which 

 moves along a groove in a brass plate attached to the back of the frame, and 

 which carries a horizontal pin (shown above the number 3 at the bottom of 

 fig. 10). This pin is set at the point on the chain which is vertically under 

 the middle of the span when the C. G. of the locomotive is over it. Arranged 

 in a horizontal row on a brass back-plate are six fixed pins, the extreme right 

 and left pin being vertically under the right and left supports of the girder, 

 the intervals between the pins being proportional to the wheel-loads, and the 

 height above the moving pin attached to the chain being equal to the distance 

 of the pin (pole) on the extreme left from the vertical load-line. 



The six pins in the horizontal row carry short brass tubular arbors at 

 right angles to which are soldered small sleeves ; through these sleeves pass 

 sliding rods attached to the moving pin on the bottom of the chain. To the 

 outer pair of these arbors, and outside the face-plate, are attached two long 

 aluminium hands or pointers, while the remaining four arbors carry short 

 crank-arms behind the face-plate, and so not shown in the figure. Vertically 

 above these four arbors and at heights which on the scale of bending- 



