22 
CAPTAIN W. DB W. ABNEY ON THE TRANSMISSION 
IX,' and jx are the exponential coefficients as before, a and a are the constants in 
the formula, la® = I', 2 being the air thickness, and I and T the intensities before 
and after transmission. 
We are now in a position to determine the constant k in the formula 
r = 
at seadevel with the barometer at 30 in. and in a very clear sky, for as 
therefore 
= 
•453 
308 
•00146 ; 
in the case of fairly clear skies, 
•497 
“ -308 
•00161 
In Part I. of this paper the minimum value of k was found to be ’0013, and a 
mean value'about '0017, so that these observations are fairly accordant. 
K may be taken to be a measure of the number of particles the rays encounter, 
and thence it may be concluded that the number of particles at any thin layer of the 
atmosphere is all. The formula, therefore, for the scattering of a ray of any wave¬ 
length at any altitude becomes 
J/ _ 
where c is a constant, h the height of the barometer, and x the air thickness, those of 
the zenith being “ unity.” 
In comparing the comparative scattering of a ray at the same zenith distance, but 
at different altitudes, \ax are constants, and 
r = 
where m is a constant and h the variable. This formula and that of the law of error 
are identical. 
XXX V. — -Conclusions. 
In conclusion, it should be remarked that the loss of light as light from trans¬ 
mission through the atmosphere is, and must be, very different to that of the heating 
effect of the solar radiation. The latter is not principally dependent on scattering 
by small ])articles, but on the absorption of aqueous vapour, Avhich is a very different 
matter. Langley has shown that the heating effect diminishes much more rajjidly 
as tlie barometric pressure is diminished than is usually supposed, and this is not to 
