398 



MR. J. J. MANLEY: OBSERVATIONS ON THE 



which so often take place in the R.P. during a short period immediately following the 

 release of the beam. 



That some change which is equivalent to the lengthening of one arm or the 

 shortening of the other actually occurs appears to be established beyond doubt, not 

 only by the experiments instituted by myself, but also by independent experiments 

 conducted by several observers researching in this laboratory and using other 

 balances.' 



(3) Of the Possibility of the Lateral Displacement of a Knife-edge. 



So far as the writer is aware, there now remain only two possible ways of 

 accounting for the unexpected and anomalous behaviour exhibited by the beam 

 immediately after its release, viz. : (1) the lateral displacement, or its equivalent, of 

 a knife-edge, and (2) the changes in the temperature of the beam during loading. 

 In this section we confine ourselves to a consideration of the first-named possibility. 



On loading a balance the resultant strains set up within it will necessarily be 

 shared by the beam as a whole ; they would naturally tend to bring about some 

 slight movements among the blocks and somewhat numerous screws used for securing 

 the knife-edges. The stresses would probably exert the greatest effect when the 

 inclination of the beam during its oscillations is a maximum, for at that moment the 

 bases of the three agate prisms would lie upon planes the inclination of which would 

 also be a maximum, and the side-thrust exerted by the horizontal planes which are in 

 contact witli the knife-edges would then be as large as possible. 



Now we should expect any effects produced in this way to show themselves most 

 markedly at first, and then to die away as the knife-edges accommodated themselves 

 to the new conditions imposed by the load. Generally it would be exceedingly 

 difficult, if not impossible, to obtain any direct evidence other than that which may 

 be gathered from the data for the R.P., which would confirm or disprove the truth of 

 this supposition. But upon two occasions, when experimenting with a triangular 

 beam to obtain data for an R.P. curve, we were so fortunate as to actually observe a 

 series of minute joltings of the pointer. We believe that these were caused by 

 sudden yieldings of some one or more of the knife-edge screws. The changes in the 

 R.P. resulting from these joltings are indicated in curve a, Fig. III. 



104 



103. 



IOZ< 



101 



ion I y * Comptete Vttratio/i5. 



02 4 6 6 10 / 14 16 18 20 22 24 26 28 



Fig HI 



