OE INTERNAL FEICTION OF AIE AND OTHER GASES. 



263 



same distance from the axis as the actual edge of the disk. Instead of A=^r* in 

 equation(9), we must therefore put A=^r*-{-2'7rr'u, and instead of b we must put b—j3. 

 The actual value of ^ r* for each surface in inches^lll2"8. 



The value of I in inches and grains was 175337. 



It was determined by comparing the times of oscillation of the axis and disks without 

 the little magnet, with the times of the brass ring (fig. 4) and of the tube and weights 

 (fig. 7). Four different suspension wires were used in these experiments. 



The following Table gives the numbers required for the calculation of each of the 

 five arrangements of the disks. 



If I is the Napierian logarithmic decrement per second, and L the observed decrement 

 of the common logarithm (to base 10) of the arc in time T, then 



L=lT\og,,e (20) 



If n is the coefficient of ^ in the periodic terms, and T the time of five complete vibrations, 



wT=10;r (21) 



Let 



K=A:Tlog,„^, (22) 



then K is the part of the observed logarithmic decrement due to the viscosity of the 

 wire, the yielding of the instrument, and the friction of the air on the axis, and is the 

 same for all experiments as long as the wire is unaltered. 



Let /x„ be the value of [ji, at temperature zero, |U, that at any other temperature 6, then 

 if [J, is proportional to the temperature from absolute zero, 



/^=(l+aa)//,„, (23) 



where a is the coefficient of expansion of air per degree. 

 Equation (9) may now be written in the form 



//,„Q(l+^)(l+a9)T+K=L, (24) 



where 1+a: is the series in equation (9),x being in most cases small, and maybe calcu- 

 lated from an approximate value of (*„. 



The values of Q are to be taken from the Table according to the arrangement of disks 

 in the experiment. 



In this way I have combined the results of forty experiments on dry air in order to 



