298 GEOLOGY OF THE OOMSTOOK LODE. 



The galvanometer Gi was one of low resistance, consisting of a few 

 hundred turns of wire around an astatic needle on silk fiber. The instru- 

 ment was quite delicate, and with the aid of proper metliods of interpolation 

 would easily have enabled the measurement of increments as small as a few 

 ten-thousandths of a degree centigrade. Unfortunately, the silk was too thick, 

 and the zero point of the instrument, as a consequence, too variable; while, 

 on the other hand, the strong winds of the region and the frail foundation of 

 the house itself rendered this accuracy unattainable, and we were obliged 

 to content ourselves with measurements accurate to a few thousandths of a 

 degree. Readings were made with a mirror and scale. 



Thus far E has been considered constant. As this is not the case, its 

 variations were measured by the aid of a second galvanometer, G^ (Fig. 20), 

 made by Mr. Grrunow, and described elsewhere.^ This instrument was 

 placed at a distance from the boiler, in the cellar, where the atmospheric 

 condition was tolerably uniform, and for convenience provided with a com- 

 mutator of its own, D. It will easily be seen that by breaking the circuit 

 at B and C and closing K, E will be in a simple circuit, including G.., and 

 that its value may be measured in terms of R, which is also included. 



The value of the constants a and h in the equation on page 295 was 

 determined by putting the ends of the thei'mo-element c in adjoining jars, 

 containing water at different temperatures." Ten or more observations were 

 usually made, from which a and h were calculated by the method of least 

 squares. 



I cannot but consider tliis method of measuring differences of tempera- 

 ture as theoretically very perfect. First of all, discrepancies due to Peltier's 

 phenomena are avoided, while the constants a and h are used precisely in 

 the same way in which they were obtained. Moreover methods of interpo- 

 lation are particularly applicable ; even a method of multiplication might 

 be thus employed. There can be no doubt that under more favorable cir- 

 cumstances the minimum difference of temperature measurable with cer- 

 tainty would be much smaller than I have been compelled to consider it. 



' This volume, page 327. 



•Notliaving a reliable barometer, all temperatures arc expressed in terms of the interval be- 

 tween zero and 100° C. of the best instrument at hand, this interval being arbitrarily assumed as correct. 

 On this assumption the stems of the thermometers used were calibrated. 



