IN PASSING THROUGH THE ATMOSPHERE. 
257 
stations. Not only did the continuity of the law which both series followed prove the 
exactness of the reduction of intensities obtained with one instrument into degrees of 
the other, but what I have called unexpected, was the fact that an equal ordinate or 
intensity should be indicated for a passage through an equal mass of air at both 
stations. For that mass of air, it is to be observed, was very differently composed in 
the two cases. On the Faulhorn, a very oblique transit through the rare air, superior 
to 8400 feet, was requisite to give the same mass as a less oblique transit through the 
whole atmosphere, in order to arrive at the lower station. It is very far from evident 
that an equal loss should take place in both cases : yet when the observations were 
piojected in the form of Curve XV. Plate XXII., without regard to the station at 
which they were made, they were found to range perfectly well together, so that one 
and the same interpolating curve passed naturally and easily through either series, or 
through both. 
83. I first sketched by the eye, and without respect to any theory, a curve which 
appealed to satisfy the observations of the 25 tli of September, which curve, it is to 
be observed, was to give the law of extinction of heat in the atmosphere, and by its 
arbitrary prolongation, to assign the solar intensity beyond the atmosphere, and the 
absorption due to a vertical transmission. 
Sect. VI . — Concerning the Law of Extinction. 
84. Many familiar facts connected with the extinction of heat and light in passing 
thiougb media, some of which have been adverted to in the earlier part of this paper, 
tender it very unlikely that the part of the solar rays which affects the actinometer 
should suffer a uniform relative loss in the successive strata of air. Perhaps no 
medium whatever merely extinguishes light without colouring it, and if it colours it, 
the light, being first deprived of those portions or rays for which the medium in 
question is comparatively opake, will be more and more freely transmitted through 
similar successive obstacles. We have seen (Art. 13.) how Lambert established the 
law of the progressively-increasing diathermancy of successive plates of glass, a re- 
sult confirmed by De la Roche and Melloni : and as we have found that, notwith- 
standing the rarity of the atmosphere, its resistance to the passage of even the solar 
rays is considerable, it is a most probable thing that a similar law should hold in that 
case. A very slight inspection indeed of Curve XV. shows that it rises much faster 
than in a simple geometrical progression. What law will best satisfy the obser va- 
tions . J and how far are we justified in pushing it beyond the limits of experience, as 
for instance, to the surface of the atmosphere ? 
85. The safest induction would appear to be by endeavouring to generalize the 
law of extinction of heat in various media. But here we are met by peculiar difficul- 
ties. A certain number of experiments have been made on the extinction of heat 
from terrestrial sources in passing through different thicknesses of media, such as 
glass, rock crystal, or water. I allude particularly to those made by M. Melloni, 
mdcccxlii. 2 L 
