HOGG. — FRTCTION AND FORCE DUE TO TRANSPIRATION. 



141 



these experiments. Tlie change in the temperature of the apparatus 

 from day to day was rarely as much as 0°.5 C, and was generally much 

 less than this during the course of one experiment. 



The proportionality already referred to would evidently be more 

 exact if the constant part of the resistance, i. e., that due to the im- 

 perfect elasticity of the fibre itself, could be determined and taken 

 from the total resistance. An effort has been made to determine what 

 is the resistance due to the air in the vessel alone. The method of 



L 



40 



;i: 



30 



z: 



g? 



■f 



?-- 



20 



T- 



1 



10 



O 10 20 30 40 50 



Figure 6. The curve shows the relation between pressure and logarithmic 

 decrement over the ranjje of pressure, p — 0.5o0 mm. io p — 0.00085 mm. The 

 unit of pressure is 0.01 mm. The unit of logarithmic decrement is 0.00333. 



procedure was to pump out the air to a certain low pressure, measure 

 the decrement, then pump to a still lower pressure, measure the dec- 

 rement again, and so on. If the exhaustion be carried far enough, a 

 limiting value for / should be approached. This did, indeed, happen, 

 but the limiting value seemed larger than one would have expected it 

 to be ; for it would seem that the friction in such a suspending fibre 

 should be exceedingly small. The following results show how the dec- 

 rement and the pressure diminished as the pumping proceeded. Pump 

 began at 10.30 a.m., when the pressure was about 0.001 mm. 



