10 OK THE VISCOSITY OR INTERNAL FRICTION 



or the friction is equal to what it would have been if there had been no 

 slipping, and if the interval between the planes had been increased by ft + ft". 

 By changing the interval between the planes, a may be made to vary while 

 + ff remains constant, and thus the value of ft + f? may be determined. In 

 the case of air, the amount of slipping is so small that it produces no appre- 

 ciable effect on the results of experiments. In the case of glass surfaces rubbing 

 on air, the probable value of ft, deduced from the experiments, was /J = '0027 

 inch. The distance between the moving surfaces cannot be measured so accu- 

 rately as to give this value of ft the character of an ascertained quantity. The 

 probability is rather in favour of the theory that there is no slipping between 

 air and glass, and that the value of ft given above results from accidental 

 discrepancy in the observations. I have therefore preferred to calculate the 

 value of /* on the supposition that there is no slipping between the air and 

 the glass in contact with it. 



The value of /t depends on the nature of the gas and on its physical 

 condition. By making experiments in gas of different densities, it is shewn 

 that p. remains constant, so that its value is the same for air at 0'5 inch and 

 at 30 niches pressure, provided the temperature remains the same. This will 

 be seen by examining Table IV., where the value of L, the logarithm of the 

 decrement of arc in ten single vibrations, is the same for the same tempei-ature, 

 though the density is sixty times greater in some cases than in others. In 

 fact the numbers in the column headed L' were calculated on the hypothesis 

 that the viscosity is independent of the density, and they agree very well with 

 the observed values. 



It will be seen, however, that the value of L rises and falls with the 

 temperature, as given in the second column of Table IV. These temperatures 

 range from 51* to 74 Fahr., and were the natural temperatures of the room 

 on different days in May 1865. The results agree with the hypothesis that 

 the viscosity is proportional to (461 + ^), the temperature measured from absolute 

 zero of the air-thermometer. In order to test this proportionality, the tempe- 

 rature was raised to 185* Fahr. by a current of steam sent round the space 

 between the glass receiver and the tin vessel. The temperature was kept up 

 for several hours, till the thermometer in the receiver became stationary, before 

 the disks were set in motion. The ratio of the upper temperature (185 F.) 

 to the lower (51*), measured from 461 F., was 



1-2605. 



