112 



Professor H. L. Callendar 



[March 10, 



actually used for temperature measurement has sixteon taps and a 

 mirror rheoscope, and is a thousand times more sensitive. 



In order to apply the method to the measurement of extreme 

 temperatures, it is not sufficient to he able to measure resistance. It 

 is also necessary to determine the law of the variation of viscosity 

 with temperature. Here, again, recourse must be had to the method 

 of extrapolation. Fortunately, in the present instance, the tempera- 

 ture can be measured through a very wide range, aud the range of 

 extrapolation, being limited by the melting point of platinum, is not 



OUTLET 



RATIO 



COILS 



Fig. 4. — Diagram of Transpiration Balance. 



very great in comparison. It should be possible, therefore, by suffi- 

 ciently varying the conditions of the experiments, and by comparing 

 the behaviour of different gases throughout the whole range of tem- 

 perature, to arrive at a very fair degree of certainty with regard to 

 the essential nature of the phenomenon. Owing to want of leisure 

 for the work, the author's experiments have not as yet extended over a 

 sufficient range of temperature, except in the case of air, to warrant 

 the publication of any general conclusions with regard to the law of 

 variation of viscosity, or of any results at high temperatures obtained 

 by the method of extrapolation. It may be stated, however, that the 

 formula above quoted, according to which the viscosity varies as some 



