IN PASSING THROUGH THE ATMOSPHERE. 
253 
70. Let us now examine the separate data which we have thus massed together. 
71. It is evident from the equations last written, that 
log — 
V 
= m 
x 
ought to be constant upon the hypothesis which we have provisionally adopted (that 
of uniform opacity and of uniformity of meteorological conditions). If, however, we 
divide the numbers in column 3 of Table F by those in column 4 (as is done in co- 
lumn 5), we shall find wide differences for the value of m. These may arise, — 1st, 
from changes in the constant of opacity m, which may naturally arise from meteo- 
rological variations ; 2ndly, from an error in the logarithmic hypothesis, which is 
founded on the physical supposition of a loss continually proportional to the intensity ; 
3rdly, from errors of observation. We shall consider these causes in succession. 
72. I. The most important meteorological element is undoubtedly the dampness of 
the air ; for we know that the formation of the slightest visible vapour instantly di- 
minishes the solar intensity. We can hardly doubt that this action must depend 
upon the relative dampness of the atmosphere, that is, upon the portion of moisture 
existing, compared to what could exist without deposition in an equal space, and not 
upon the absolute elasticity of the vapour : for it is plain that vapour of given elas- 
ticity would make a dense visible cloud at one temperature, and might yet be com- 
patible with intense relative dryness at another. I have therefore taken particular 
pains in the reduction of the hygrometric observations, and the course of the pro- 
gress of dampness at Brientz and the Faulhorn in curves V. and VI. is particularly 
worthy of observation. At the former, the lower station, the dampness is greatest in 
the morning and evening, and has a minimum between 1 and 2 p.m. At the upper 
station, on the contrary, the dampness increases almost continually from morning till 
night. These facts are perfectly normal*, and are readily explained by the continual 
rise of the imperfectly condensed moisture which occupies the valleys of the Alps 
every fine night in summer, and is gradually exhaled into the upper atmosphere by 
the action of currents and the increasing warmth of the inferior strata, — a phenome- 
non from which arises (amongst other effects) the very frequent formation, about 
noon in the finest weather, of clouds at a height of from 8 to 15000 feet, which again 
give way during the advance of evening as the vapour descends. 
73. The curve of mean dampness at both stations (VII.) exhibits a morning and 
afternoon maximum about 9 a.m. and 3 p.m., preceding somewhat the epochs of 
maximum loss of solar radiation already referred to. This is an important analogy, 
and an inspection of Curve VII. together with Curve XIV., which represents the loss 
of solar radiation in terms of the radiation at the upper station, will show a certain 
general, though not a precise analogy. 
74. It cannot, however, be affirmed that these experiments are at all sufficient to 
show the kind of dependence which the Opacity has upon the Dampness. The values 
* See Dove’s Repertorium, iv. 264. 
