268 
PROFESSOR FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
Curves XXL and XXII. Plate XXV. show the relative march of the actinometer 
at Grindelwald and the Faulhorn on the 23rd of September, during - the latter part 
of which clouds appeared at the upper station which were not visible, or at least not 
observed, at the lower ; and the effect on the inflection of the diurnal curve is the 
same as that noticed Art. 79, p. 255. 
106. Although these observations were never made except when the sun appeared 
to shine through a clear blue sky, the rate of extinction is enormously greater than 
in the formerly described more favourable circumstances. By selecting the observa- 
tions directly comparable, and reducing them as in Art. 62, I have found an 
absorption equal to three-fourths of the incident heat, the mean ratio to saturation 
being *6/ 1 7- But it must be confessed that no evident relation to the hygrometric 
condition of the air appears in the individual observations. 
Section VIII. — Observations in 1841. 
10/. These were made under very far from favourable circumstances at one station 
only, namely, on the lower glacier of the Aar, at an elevation of about 7000 feet above 
the sea. Although the sky was to appearance generally clear and of a deep blue du- 
ring the continuance of these observations, the occasional formation of slight clouds, 
and the feeble degree of dryness, considering the elevation, explains the compara- 
tively great opacity of the atmosphere which we deduce from these observations. 
The instrument was a different one and partly on a different construction from those 
formerly used, and they have not been compared, consequently the actinometric 
degrees are not convertible into one another. It may be doubted whether the sur- 
face of a glacier is not a very bad position for such observations, owing to the stra- 
tum of moist air, which in summer must generally rest upon it during the heat of the 
day, and the glare from the adjoining mountains is an evil not wholly to be avoided. 
I must state, however, that numerous comparative experiments which I made on this 
occasion with the actinometer and with Leslie's photometer, convinced me of the 
remarkable constancy and truth of the indications of the former. My immediate 
object was to verify a conjecture which I had published* respecting an anomaly in 
solar radiation described by Dr. Richardson in the arctic regions, as measured by 
the statical thermometric effect. The anomaly was that the maximum occurred 
in April or May, instead of in June or July, as elsewhere ; and the explanation I gave 
was that the disappearance of the snow from the earth’s surface in the month of May 
diminished the solar effect more than the sun’s greater elevation increased it. This 
was confirmed by finding the enormous indications given by Leslie’s photometer on 
the glacier of the Aar, at a time when the sun’s rays were not peculiarly intense, 
which I ascribed to the glittering reflection from the surface of the glacier, and from 
the amphitheatre of snowy mountains. When the instrument was placed on a rock, 
or merely on a piece of black wax-cloth laid upon the snow, it sunk in a very remark- 
able manner. The actinometer, on the other hand, when supported on a small box, 
so as just to avoid contact with the snow, gave appreciably the same result in both 
situations. 
1 08. The following are the most available meteorological observations made in 1841 : 
* Jameson’s Edinburgh New Philosophical Journal for 1841. 
