Page:On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground.pdf/17

		Morning. $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad } }$						Noon. $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad } }$						Evening. $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad } }$
		${\text{D}}$.	${\text{W}}$.	${\text{I}}$.	${\text{I}}_{1}$.			${\text{D}}$.	${\text{W}}$.	${\text{I}}$.	${\text{I}}_{1}$.			${\text{D}}$.	${\text{W}}$.	${\text{I}}$.	${\text{I}}_{1}$.
Lone Pine.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	29.3	0.610	1.424	1.554	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	23.6	0.460	1.692	1.715	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	26.6	0.510	1.417	1.351	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
		21.1	0.840	1.458	1.583			26.9	0.590	1.699	1.721			23.2	0.740	1.428	1.359
Mountain Camp.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	23.5	0.088	1.790		$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	22.5	0.182	1.904	1.873	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	24.5	0.205	1.701	1.641	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
		23.5	0.153	1.749				24.5	0.245	1.890	1.917			22.5	0.320	1.601	1.527

At a very low humidity (Mountain Camp) it is evident that the absorbing power of the aqueous vapour has an influence, for the figures for greater humidity are (with an insignificant exception) inferior to those for less humidity. But for the observations from Lone Pine the contrary seems to be true. It is not permissible to assume that the radiation can be strengthened by its passage through aqueous vapour, but the observed effect must be caused by some secondary circumstance. Probably the air is in general more pure if there is more water-vapour in it than if there is less. The selective diffusion diminishes in consequence of this greater purity, and this secondary effect more than counterbalances the insignificant absorption that the radiation suffers from the increase of the water-vapour. It is noteworthy that Elster and Geitel have proved that invisible actinic rays of very high refrangibility traverse the air much more easily if it is humid than if it is dry. Langley's figures demonstrate meanwhile that the influence of aqueous vapour on the radiation from the sun is insensible as soon as it has exceeded a value of about $0.4$.

Probably the same reasoning will hold good for carbonic acid, for the absorption spectrum of both gases is of the same general character. Moreover, the absorption by carbonic acid occurs at considerably greater wave-lengths, and consequently for much less important parts of the sun's spectrum than the absorption by water-vapour^[1]. It is, therefore, justifiable to assume that the radiation from the sun suffers no appreciable diminution if ${\text{K}}$ and ${\text{W}}$ increase from a rather insignificant value (${\text{K}}=1$, ${\text{W}}=0.4$) to higher ones.

Before we proceed further we need to examine another question, Let the carbonic acid in the air be, for instance, the same as now (${\text{K}}=1$ for vertical rays), and the quantity of water-vapour be 10 grammes per cubic metre (${\text{W}}=1$ for

1. Cf. above, pages 246–248, and Langley's curve for the solar spectrum, Ann. Ch. et d. Phys. sér. 6, t. xvii, pp. 323 and 326 (1889); 'Prof, Papers,' No. 15, plate 12.