202 PROCEEDINGS OP THE AMERICAN ACADEMY. 



error involved is nearly auuulled. Hence, although it would be better 

 to compute 8 t, and express this as a percentage of the absolute tem- 

 perature T, the added labor did not seem justified by the small gain. 

 The exponential equation applied to the Barus data yields 



2;;e =: 0.7G91t^-^''^— 1730, or 

 e^ = 0.7691 T^-'^^, and yS = 1730 mv. 

 Range of data 350° to 1075° C. 



[N.B. This equation was deduced with the value 0°C. == 273° .7 abso- 

 lute, whereas in all subsequent tables 0°C. = 273°. absolute is em- 

 ployed as a more probable value. The numerical values of the 

 constants are therefore subject to a slight modification, but as for the 

 present purpose we are concerned only with 8, which would not be 

 sensibly changed, the recomputation is not worth while.] 



Columns six and seven, Table II., give 8 and its percentage value 

 for the exponential equation. 



llie Barus Equation, — • The excessive labor involved in the evalua- 

 tion of the constants P, Q^ P', and Q' of Barus's proposed equation 

 detracts so seriously from its usefulness that I have also allowed it to 

 deter me from computing them for the above tabulated values. The 

 comparison of tlie values of 8 for his " Klquation 3," and for an ap- 

 proximate exponential of my ow'n based on the same data, is, however, 

 decidedly in favor of the latter. 



The logarithmic equation applied to the Barus data yields 



2^6 = 2.665^1220^ 



or its equivalent, ^ 



log 2^6 = 1.220 log t + 0.4-2570. 



The deviations are given in the last two columns of Table II. 



Holborn and Wien Data. — This important comparison * of the 

 rhodo-platinum thermo-couple with the porcelain' bulb air thermometer 

 up to high temperatures was performed under the auspices of the 

 Reichsanstalt at Berlin, and appears to be on the whole the most im- 

 portant and reliable contribution to this subject in recent years. The 

 experimental work was evidently conducted with great care, and 

 although not showing the concordance of results, nor the multiplica- 

 tion of observations of Barus's work, yet in respect to stem-exposure 

 correction, to exposure of the thermal junction, and to direct measure- 

 ment of the coefficient of expansion of the bulb, it is probably more 



* Holborn and Wien. Zeit. f. Iiistk., XII. 257, 296 (1892). Also in full in 

 Wied. Ann., XLVII. 107 (1892). 



