IN PASSING THROUGH THE ATMOSPHERE. 
243 
agree) was six inches and seven lines French, or above seven English inches, being 
nearly one-fourth of the whole weight of the atmosphere. It was to be expected 
then, that if the opacity of the atmosphere for the heating rays at all approached the 
estimate of Lambert, the difference would be very sensible indeed, especially when 
in consequence of the rays passing through the interposed stratum of 6800 feet with 
a considerable obliquity, the resistance to their passage was magnified. 
50. The month of September was rather changeable, and a fall of snow occurred 
about the middle of it, which affected so unfavourably the state of the atmosphere, 
that, though apparently cleat- weather followed, the results deducible from the ob- 
servations were of the most anomalous kind. All the observations which the weather 
enabled us simultaneously to make at the top of the Faulhorn and at Brientz, Grin- 
del wald, or other places in the neighbouring valleys, have been carefully and exactly 
reduced and computed ; but it was not till near the close of the month that the atmo- 
sphere appeared to settle into a pure and steady autumnal condition. The 24th, 25th 
and 26th were brilliant and, so far as I observed, perfectly cloudless days. The 24th 
I spent partly with M. Kamtz on the Faulhorn. In the course of the 25th the ob- 
servations were continued from the morning till sunset by M. Kamtz on the Faul- 
horn, and by myself at Brientz. To this series of comparative results the chief atten- 
tion will be drawn. Those on the Faulhorn were made entirely in the open air, 
those at Brientz, until one o’clock inclusive, were made in a room, and afterwards in 
the open air, a discrepancy not altogether favourable to the series. At every hour the 
state of the atmosphere was ascertained by the barometer, thermometer, and moist- 
ened bulb hygrometer at both stations, thus giving as accurate a knowledge as the 
circumstances permitted, of the state of the intercepted column. 
51. It is evident that these observations, immediately to be detailed, may be treated 
in three ways entirely distinct. The opacity of the atmosphere may be deduced from 
observations at different hours at the upper station only, by Bouguer’s formula 
founded on the logarithmic law of the decrement of solar intensity ; the observations 
at the lower station give independent data ; and finally, the direct determination of 
the loss of solar heat in passing from the upper to the lower station, by a comparison 
of the two instruments, yields a distinct result exclusively derived from the action of 
the lower strata of the atmosphere in absorbing solar heat. For this last purpose 
then a rigorous comparison of the arbitrary scale of the two instruments is of the 
highest importance. The following ample series of experiments will show the degree 
of accuracy attainable in such observations. 
Comparison of Actinometers. 
52. The comparison of the arbitrary scales of the two actinometers marked B. 2. and 
G. 7, was obtained by making alternating sets of observations on the solar intensity 
with each. It has already been stated that the method of using the instrument is to 
expose it to the sun for a certain short time (say one minute), then to interpose a 
2 i 2 
