218 



MESSRS. R. THRELFALL AND J. A. POLLOCK 



J. ALLAN BROUN. Proceedings Roy. Soc. Edin., vol. 4, p. 411. 1861. 

 BABINET. Compt. Rend., vol. 56, p. 244. 1863. 

 SIEMENS, C. W. Phil. Trans., 1876, Part I., p. 07 I . 

 MASCART. Compt. Rend., vol. 95, p. 631. 1882. 

 POYNTING. British Association Reports, 1886, p. 141. 

 Lord KELVIN. British Association Reports, 1886. p. 534. 

 VERNON BOYS. British Association Reports, 1887, p. 41. 



VON STERNECK. Mitt, der K. u. K. Militar-geodiitischeu Gruppe des K. u. K. 

 Militiir-geographischen Institutes, vol. 5, 1885, and vol. 14, 1895. 



GENERAL DESCRIPTION OF THE INSTRUMENT AND PRELIMINARY REMARKS. 



A and B (fig. 1) are two metallic rods capable of adjustment by sliding along their 

 common axis, but having no freedom to move transversely ; in our earlier experiments 

 we made use of the head stock and back centre of a watchmaker's lathe. 



Fig. 1 



n-ju, 



"ITT 



n 







C is a coach spring attached to A, and carrying a point H, to which the end of the 

 quartz thread is fastened. 



B is a brass axle capable of turning about the axial line HI, which also represents 

 the quartz thread ; it is attached rigidly to an arm carrying a vernier G. The 

 vernier moves over a divided circular arc a sextant being in fact employed. 



The quartz thread has as nearly as possible a diameter of '0015 inch = '0038 

 centim., and is very uniform; it is soldered up to the points prepared for it at H 

 and I. 



The distance of A from B is regulated so that the thread is stretched tight by the 

 spring at C. 



The length HI is 30 '5 centims. 



D is a piece of gilded brass wire 5 '3 centims. long, and its mass is 0'018 gram ; it 

 is attached to the thread by soldering, the thread lying in a little kink in the wire. 



It is adjusted so that its centre of gravity lies a little to one side of the thread. 



