60 REPORT—1840. 
earth leads to many interesting and important practical con- 
clusions, upon which we cannot now dwell. 
59. Whilst we admit the phenomenon of the decrease of 
temperature with height to be the normal one, and other cases 
exceptions, we must not omit to mention an important class of 
real exceptions, which deserve particular study. 
60, The superior radiating power of Earth to Air is the 
cause of the seemingly preternatural depression of the tempera- 
ture of the ground in clear evenings observed by Six, Wilson, 
and Pictet, and so well applied by Wells to the theory of 
dew. It is evident, that under the circumstances which favour 
the development of the cause (viz. Radiation), the effect must 
extend more or less above the surface; and, consequently, up 
to a certain point the temperature will zzcrease with height. 
This question has lately been treated of in an interesting paper 
by Prof. Marcet*, who has arrived at the following con- 
clusions :—1. It is a constant phenomenon about the time of 
sunset, except in the case of violent winds. 2. It attains a 
maximum immediately after sunset. 3. The increase of tem- 
perature with height extends to 100 or 110 feet at the most. 
4. It is most conspicuous when the ground is covered with | 
snow. 
61. This leads us directly to the important subject of 
E. Radiationt, 
whether solar or terrestrial; in its bearings, perhaps, the most 
important and interesting at present connected with Meteoro- 
logy. We speak now principally of instruments and primary 
results; in the next section, of conclusions to be drawn from 
them in connexion with great cosmical questions. 
62. The earth acts by absorbing radiant heat from the sun 
and (perhaps) other heavenly bodies ; and it radiates it again 
according to new laws towards space. Each of these effects, and 
the modifications which circumstances introduce into them, may 
be made the subject of separate experiment. 
63. Since a thermometer with a blackened ball absorbs more 
solar heat than a bright or transparent one, it was natural to 
suppose that the difference of indication of two such instru- 
ments might be considered as a measure (at least a relative 
indication of the force) of solar radiation. ‘The stationary dif- 
* Mémoires de la Société de Physique, &c. de Généve, tom. viii. (1838). 
+ See First Report, p. 222; Mahlmann, p. 64. I think it unnecessary to say 
anything of the progress of our knowledge of Primary Physical Laws of Radia- 
tion, because that is to be made the subject of a special report by Professor 
Powell. 
