ORDINARY MEETINGS. XXXI 



observations was made with the same cord and the same arrangements 

 as the first. This series had to be made somewhat hurriedly, as will be 

 seen by the dates of the individual experiments, but the experim'ents 

 were made with the same care as those of the first series. It will be 

 seen from the above table that (1) the treatment to which the cord had 

 been subjected in the first series had diminished its rigidity, the values 

 being smaller throughout than in the first series, and (2) that the initial 

 diminution of rigidity with increase of tension, its final increment with 

 tension, and the occurrence of a minimum point are as marked in the 

 second series as in the first. 



To find out if the occurrence of the minimum point was due to the 

 defective character of the attachments at the ends of the cord, Mr. 

 Macdonald made a number of observations with new modes of attach- 

 ment. These consisted of brass tubes in one end of which three 

 longitudinal cuts had been made, the ends of the three strips thus 

 formed being bent inwards and provided with teeth. The ends of 

 the cord having been passed into these tubes, the strips were firmly bound 

 to the cord by means of wire. Unfortunately Mr. Macdonald had Hot 

 (sufficient time to make more than rough observations with the new 

 arrangement. Such observations as he was able to make seemed to show 

 that the minimum point had disappeared. Whether its disappearance 

 was due to the more satisfactory attachments, the passing away of the 

 heating effect or the fatigue of the cord, Mr. Macdonald hopes to deter- 

 mine by further experiments. 



The above values of the rigidity agree fairly well with Mallock's 1 

 determinations, Mallock having found that the rigidity of what he calls 

 'soft grey" india-rubber, determined dynamically, ranged from 5.52 to 

 8.76, if expressed as in the above table, and that for "hard grey" rubber 

 it ranged from 10.77 to 13.94. Mr. Macdonald's rubber could not be 

 designated as either soft or hard ; and his values are intermediate 

 between Mallock's for the soft and the hard specimens. 



The volume of the cord underwent very little change during either 

 the first or the second series. In both series, however, there is an 

 unmistakeable though small increase of volume with stretching ; but 

 whether it was due to the stretching or to the series of oscillations to 

 which the cord had been subjected does not of course appear. 



1 Proc. R. S. Lond.. 46, 233, 1889. 



