Page:On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground.pdf/3

238 absorption" . This view, which was founded on too wide a use of Newton's law of cooling, must be abandoned, as Langley himself in a later memoir showed that the full moon, which certainly does not possess any sensible heat-absorbing atmosphere, has a "mean effective temperature" of about 45° C.

The air retains heat (light or dark) in two different ways. On the one hand, the heat suffers a selective diffusion on its passage through the air; on the other hand, some of the atmospheric gases absorb considerable quantities of heat. These two actions are very different. The selective diffusion is extraordinarily great for the ultra-violet rays, and diminishes continuously with increasing wave-length of the light, so that it is insensible for the rays that form the chief part of the radiation from a body of the mean temperature of the earth Langley, 'Prof. Papers,' No. 15, p. 151. I have tried to calculate a formula for the value of the absorption due to the selective reflexion, as determined by Langley. Among the different formul examined, the following agrees best with the experimental results: –

I have determined the coefficients of this formula by aid of the method of least squares, and have found –

$$a$$ represents the strength of a ray of the wave-length $$\lambda$$ (expressed in $\mu$) after it has entered with the strength 1 and passed through the air-mass 1. The close agreement with experiment will be seen from the following table: –