FUTURE QUANTITATIVE RESULTS. 395 



are limited to the ordinary range of atmospheric pressiire 

 and temperature, but we require additional ones for all 

 pressures and all temperatures, and for all combinations 

 of these. A probable result of the formation of such 

 tables will be a profound alteration of our views of na- 

 ture, because every single substance has more or less 

 different properties at every different pressure and tem- 

 perature. 1 



6 Now with respect to accurate measurement, theory 

 was left far behind by practice, and I need not to be 

 reminded how very much more accurate were the measure- 

 ments of resistance in the practical telegraphy of Dr. 

 Werner Siemens and his brother than in any laboratory 

 of theoretical science. When in the laboratory of theo- 

 retical science, it had not been discovered that the con- 

 ductivity of different specimens of copper differed at all, 

 in practical telegraphy workshops they were found to 

 differ by from thirty to forty per cent. When differences 

 amounting to so much were overlooked, when their very 

 existence was not known to scientific electricians, the great 

 founders of accurate measurements in telegraphy were 

 establishing the standards of resistance accurate to one- 

 tenth per cent. Dr. Werner Siemens and his brother 

 were among the first to give accurate standards of resist- 

 ance, and the very first to give an accurate system of 

 units founded upon those standards.' 2 



Accuracy of measurement is often obtained by means 

 of repetition, as in the pendulum ; also by means of coin- 

 cidence, as in the vernier. Descriptions of the numerous 

 ways in which measurements have led to discoveries, and 



1 See Chapter IV. p. 34. 



2 Address by Sir W. Thomson. On Electrical Measurement. Con- 

 ferences, Special Loan Collection, London, 1876, p. 247. 



