Laplace. It consists in tlie determination of the length of the path and the mass of air which a ray of light must traverse in passing through the earth's atmosphere at every different angle of obliquity. The author determines the numerical value of these quantities for all angles of incidence from 0° to 90°.
The fourth section contains an account of the observations made by the author in conjunction with Professor Kamtz in 1832. These were conducted in 1832 at the top and bottom of the Faulhorn, a mountain of the canton of Berne in Switzerland. The lower station was Brientz, and the intercepted stratum of air had 6800 English feet of thickness, corresponding in its weight to about one-fourth of the entire atmosphere. Frequent observations were simultaneously made with the actinometer and other meteorological instruments at both stations, and the loss of solar heat in passing through the in- tervening mass of air was thus directly determined.
In the fifth section, the observations made from sunrise to sunset, on one peculiarly favourable day (the 25th September, 1832), are carefully analysed ; and from the absorption at various obliquities, the law of extinction in the atmosphere, within the limits of obser- vation, is attempted to be deduced.
The sixth and seventh sections include the results of similar, but less perfect observations in 1832 and in 1841.
From the facts and reasonings of this paper, the author deduces, on the whole, the following conclusions : —
1. The absorption of the solar rays by the strata of air to which we have immediate access is considerable in amount for even mo- derate thicknesses.
2. The diurnal curve of solar intensity has, even in its most nor- mal state, several inflections ; and its character depends materially on the elevation of the point of observation.
3. The approximations to the value of extra- atmospheric radia- tion, on the hypothesis of a geometrical diminution of intensity, are inaccurate.
4. The tendency to absorption through increasing thicknesses of air is a diminishing one ; and in point of fact, the absorption almost certainly reaches a limit beyond which no further loss will take place by an increased thickness of similar atmospheric ingredients. The residual heat, tested by the absorption into a blue liquor, may amount to between half and a third of that which reaches the surface of the earth after a vertical transmission through a clear atmosphere.
5. The law of absorption in a clear and dry atmosphere, equivalent to between one and four thicknesses of the mass of air traversed vertically, may be represented, within those limits, by an intensity diminishing in a geometrical progression, having for its limit the value already mentioned. Hence the amount of vertical transmission has always, hitherto, been greatly overrated ; or the value of extra-atmospheric solar radiation greatly underrated.
6. The value of extra-atmospheric solar radiation, on the hypothesis of the above law being generally true, is 73° of the actino-
