.Mr. C. E. Stromeyer. [Apr. 19, 



it to represent 0'0000120 in., or 10 per cent. more. Both instruments 

 A and B were used, and in the Table each experiment is marked with 

 u distinguishing letter. In the earliest experiments (marked 

 spirit lamp was used for illuminating purposes; it was enclosed in 

 au asbestos-lined casing, but this soon got very hot, and must have 

 affected the readings. Later on a Bunsen burner was used, ami tlie 

 test piece and instrument screened from its radiant heat. These ex- 

 periments are marked Aj, but even now the heat made itself felt, an 

 the value l//t, last column, might in most of the experiments, :i- 

 as those marked B a , be reduced 5 per cent. In the case of those 

 marked B s , the test piece was placed in position and the lamp lit fi 

 .30 to 60 minutes before commencing the readings. 



In most of the early experiments (compare Columns 3 and 4) fi 

 ten, and even twenty bands were counted between each reading 

 the steelyard of the testing machine* This was not only 

 fatiguing to the eye, but it was subsequently impossible to dete 

 whether any interference bands had been wrongly counted. In t 

 later experiments, two, or at the utmost three, bands were coun 

 for each steelyard reading. Judging by the results, the centi 

 position of each band can be estimated to within 10 per cent., and i 

 many experiments the total number counted exceeded 20. Each 

 piece was strained to the maximum intended load before each expe 

 ment ; but, in spite of this, the first experiments were always slight! 

 unsatisfactory, and have generally been rejected. 



The author's original intention had been to use the instrument 

 both for measuring the longitudinal extension and the cross contrac- 

 tion, but as this instrument did not give reliable results as rega 

 extensions, other strain indicators had to be used. 



I. Professor Kennedy's Lever Gear (Ci). The short end of a HI 

 lever ended in a point, which was inserted into the centre puncl 

 mark at one end of a test piece. The fulcrum was connected to an 

 arm, which was fixed to the other end of the test piece,, and the long 

 arm of the lever acted as a pointer. The leverage was 100 to 1. 

 This instrument measured the elongation only on one side of the test 

 piece, and would not give reliable results. In many of the experi- 

 ments (those marked Cj) the instrument was first fixed on one side of 

 the test piece and then on the other. The same remarks apply to 

 the following gear, Dj and D,. 



II. Mr. Stromeyer's Rolling-pin Gear D,. Two flat plates with 

 projecting centre points at either end were attached to the test piece. 

 The rolling pin, which was placed between the two plates, and la-Id 

 there by helical springs, was a fine piece of hardened steel wire, to 

 which a large straw pointer was attached. In the first experiments 

 the leverage was about 300 to I ; in the later ones it was nearly 

 1,000 to 1. 



