62 REPORT—1840. 
point of view they do not even afford the slightest approxima- 
tion to the truth. 
66. It does not, however, follow that the indications of the 
photometer are of no value, or that observations with it should 
be discontinued. There are some constant peculiarities in its 
action, so remarkable, as to suggest very interesting investiga- 
tions. The effect of the reflected light of the sky is always 
exceedingly intense; so much so, as to give rise to the most 
paradoxical effects with regard to the intensity of solar radia- 
tion, if neglected. Thus I have found the whole effect of the 
sun and sky in a bright April day in this country, when many 
white clouds were present, not very inferior to that of the most 
piercing sunshine of the most sultry day of the south of Europe, 
unaccompanied by a single cloud. What would be the indica- 
tions of the actinometer in these circumstances I am unable to 
state. M. Kamtz found, on the summit of the Faulhorn, that 
the direct solar effect on Leslie’s photometer was equalled, 
and often exceeded, by that of the diffuse atmospheric in- 
fluence*. 
67. Sir John Herschel has lately proposed to render his scale 
an absolute one, denoting by an actine ‘‘ the intensity of solar 
radiation, which, wholly absorbed at a vertical incidence, would 
suffice to melt a sheet of ice one-millionth of a metre in thick- 
ness in one minutet.”’ With an actinometer, which marked 
29°5 as the maximum effect which he had observed in Europe, 
Sir John Herschel found the solar radiation at the Cape of Good 
Hope to attain 48°°75 of the same scale, the intensities being 
in the exact proportion of those numbers f. 
68. M. Pouillet, of Paris, described, some years ago, an ap- 
paratus for measuring solar radiation, in which the errors of other 
statical contrivances were in a good measure avoided, by en- 
closing the thermometer in an envelope maintained at 0° c., with 
the exception of a small hole, which exactly admitted the direct 
rays from the solar disc§. Since that time, however, he has 
adopted Herschel’s dynamical method, which he has applied to 
a modification of the actinometer, which he terms a pyrhelio- 
meter; reserving (rather unfortunately I think) the term acti- 
nometer, which was already so fitly appropriated, to a separate 
apparatus for measuring nocturnal radiation. These instru- 
ments and their applications are described in an ingenious and 
interesting memoir read to the Academy of Sciences 9th July, 
* Lehrbuch, iii. 14. 
+ Poggendorff, xli. 559. Royal Society’s Report, p. 67. 
+ Comptes Rendus (Paris), iii. 506. 
§ Elémens de Physique, 1832, tom. ii. p. 703, fig. 356. 
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