110 Professor E. L. Callendar [March 10, 



table, in which the rate of increase is expressed by finding the power 

 n of the absolute temperature T to which the viscosity is most nearly 

 proportional. The most concordant results were obtained by the 

 method of transpiration, and gave an average of • 76 for the index n 

 in the case of air. The more condensible gases gave larger values 

 for the rate of increase, but the value for hydrogen appeared to be 

 smaller. 



It will be observed that the results are not very concordant, but 

 the experiments are much more difficult and liable to error than might 

 be supposed. The most accurate method was that employed by 

 Holman, but even in this case the margin of uncertainty is considerable. 

 It would evidently be impossible to employ the method of transpira- 

 tion to any advantage for the determination of temperature, unless a 

 far higher order of accuracy could be easily attained. After repeat- 

 ing the majority of the more promising methods in detail, including 

 the original method of Maxwell, the writer came to the conclusion 

 that they were entirely unsuitable for the purposes of thermometry, 

 and would have abandoned the attempt entirely if he had not fortu- 

 nately succeeded in finding a more perfect way. 



In studying the flow of electricity through conductors, which is 

 in many respects analogous to that of a fluid through a fine tube, 

 electricians have been compelled, from the intangible nature of the 

 fluid with which they work, to elaborate the most delicate and power- 

 ful methods of investigation. One of the most useful of these methods 

 is generally known as the Wheatstone-bridge method, and is used for 

 measuring the resistance of a conductor to the passage of an electric 

 current. The method is equally applicable and equally exact for 

 determining the resistance of a fine tube to the passage of a gas. 

 The writer was already very familiar with the application of this 

 method in all its refinement of detail to electrical resistance thermo- 

 metry. The suggestion for applying it to the closely analogous pro- 

 blem of transpiration was supplied by the researches of W. N. Shaw, 

 F.E.S., who had already applied it, in connection with certain experi- 

 ments on ventilation, to the effusion of air through large orifices at 

 ordinary temperatures. 



The apparatus used by Shaw (described in the Proc. Eoy. Soc, 

 vol. xlvii., 1890) consisted of boxes to represent rooms, with apertures 

 about half a square inch in area to represent ventilators. Two of 

 these apertures were made in the form of adjustable slits. The 

 circulation of air through two rooms in parallel was maintained by a 

 gas burner, and the slits were adjusted to make the pressure in the 

 two rooms the same, as indicated by the absence of flow in a connect- 

 ing tube, containing a pivoted needle and vane as a current detector. 

 The balance was shown to be independent of the air-current when 

 that was varied from one to four cubic feet per minute. The effusion 

 resistance of an aperture was also verified to be approximately 

 proportional to the square of the reciprocal of the area, with apertures 

 of similar shape. This method of investigation was admirably adapted 



