844 
PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
Note 3, Art. 7. 
It will be observed that tbis dependence of tbe pbenomena on a relation between the size of the surfaces 
and the mean path of a molecule is essentially different from what has been a common, but as is herein 
shown, erroneous supposition, that the phenomena essentially depend on distance separating the opposite 
surfaces. The one supposition makes the action of the radiometer depend on the size of the vanes, but 
leaves it independent of the size of the envelope, while the other makes the action depend on the size of 
the envelope, but leaves it so far independent of the size of the vanes. 
Note 4, Arts. 41 and 104. 
The assumption that the coefficients A, to, and X, 2 , also to' and X g , equation (99), are the same for 
stucco as for meerschaum is equivalent to assuming that the only respect in which the interstices of these 
plates differ is that of coarseness. There is no a priori ground for making this assumption. The fact that the 
logarithmic homologues for stucco fit those for meerschaum through such a considerable range of densities 
proves the approximate truth of the assumption ; but it is possible, since c s and c m are arbitrary dimensions, 
that the curves for transpiration under pressure depending on A, to, X 1 , and X 2 may approximately fit for 
one value of —, and the curves for thermal transpiration depending on A x , to, X 1 , X 2 , to', and X 3 may 
Cm 
approximately fit for another value of —. If this were so log. ( — ); the shift necessary to bring the curves 
Cm \Cm/ 
into coincidence would not be the same for transpiration under pressure as for thermal transpiration, and 
as has been pointed out (Art. 41), this is to a certain extent the case, this ratio having the values 6 - 5 and 5'6— 
a difference which was sufficiently decided to call for notice, but which is not so large but that, as pointed 
out (Art. 41), it may possibly be due to the plates being hot in the one case and cold in the other. In 
any case the smallness of the difference is an additional proof that the interstices do not greatly differ as 
passages in any respect except that of size. 
Note 5, Art. 119. 
(a.) 
—g =: a { M (y? -f U 3 + M> 2 ) }. 
Whence at the surface when is sufficiently small we have by equation (18) 
neglecting ^' c ■ j 
H_3 P_ 
Cj 2 2 -/TT 
H 9 p' 
And when -1 is large, we have by equation (128) 
H lip' s , 
~o- -f=- -{“c — cc ). 
c \/ IT o 1 
Therefore substituting for A—_— in equation (140) 
