Elastic Constants of Quartz Threads. 105 



metres of the various scales with several millimetres taken 

 from the standard metre, with several taken from a callipers 

 by the Darling, Brown, and Sharpe Co., and with several 

 from a large callipers by Elliot Bros. The result was to 

 show that the Zeiss scales were as nearly right as I found 

 possible to estimate, and the Powell and Lealand wrong by 

 about 3 per cent. In order to check the method some very 

 thick threads, up to '3 millimetre diameter, were measured 

 under the microscope, and callipered directly both with 

 American and English micrometer- callipers. The result was 

 entirely satisfactory, though the experiment involved using a 

 lower power on the microscope. Some thin threads were also 

 measured, and though less suited to the callipers allowed the 

 high power to be used on the microscope and gave, as nearly 

 as could be read, identical results. At first there was a want 

 of consistency because care had not been taken always to use 

 the middle of the field. The advantage of using the glycerine 

 mounting was that it was much easier to focus on the edge 

 of the thread with certainty than was the case in air. It is 

 perhaps fair to say that the engraving of the Powell and 

 Lealand scale was very superior to that of the Zeiss, but it 

 was not so convenient, as only one tenth of a millimetre was 

 divided into hundredths, and, as has been said, was wrong by 

 3 per cent. 1 have to thank Professor Haswell and Dr. 

 Wright for the use of these scales. To get the best results I 

 found it necessary to use a variety of shades and screens so as 

 to obtain uniform illumination. In the thread measurements 

 the divisions and parts of a division occupied by the image of 

 the thread on the step-scale were estimated first, and then the 

 micrometer having been substituted for the threads the value 

 of a step-scale division was got by taking the number of 

 divisions occupied between the two nearest coincidences, 

 covering the part of the scale previously occupied by the 

 image of the thread. This is more accurate than using pencil 

 marks. I have dwelt on this measurement with more detail 

 than is perhaps necessary, because as the accuracy attained 

 here really conditions the final accuracy of the results, I have 

 wished to give an idea of what sort of accuracy I have suc- 

 ceeded in reaching. Most, if not all, of the threads were too 

 coarse for the optical method of measurement. 



Determination of the Coefficient of Torsion and of its 

 Temjjerature- Coefficient. 



In these experiments an arrangement was made permitting 

 the torsion to be investigated by the method of vibration at 



