232 
PROFESSOR- FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
19. The scale of the actinometer is an arbitrary one obtained by the direct com- 
parison of one instrument with another ; a method which, as we shall see, admits of 
great accuracy. Sir John Hf.rschel has, indeed, proposed to convert his degrees 
into “ actines,” each of which represents “ that intensity of solar radiation, which at a 
vertical incidence, supposing it wholly absorbed, would suffice to melt one-millionth 
part of a metre in thickness from a sheet of ice horizontally exposed to its action, per 
minute of mean solar time*”. It may be apprehended, however, that an arbitrary 
comparison will always be found more available in practice. The scale of Leslie’s 
photometer should also be considered as arbitrary ; the method of graduation by 
conversion into hygrometric degrees being wholly inaccurate. 
20. Leslie, from his experiments made on the sun’s force at different elevations, 
concluded that one-fourth of the solar rays are absorbed during a vertical transmis- 
sion through the atmosphere in clear weather at Edinburgh'!'. His experiments were 
made under the revolving dome of an observatory^. Thus the indirect heating and 
cooling influences were in some degree avoided. These influences are of a very 
material kind, and prove the impossibility of obtaining even an approximation to 
useful results without allowing for them. The photometer of Leslie indicates as 
much effect (according to Professor Kamtz) due to the light reflected from the at- 
mosphere as to the direct solar influence ; certainly a most startling result, but one 
entirely confirmed by my own observations. The part exclusively due to solar in- 
fluence being taken by M. Kamtz, he finds from Bouguer’s formula an extinction of 
no less than thirty per cent, of the solar rays in reaching the summit of the Faulhorn, 
by a vertical transit through the atmosphere, the pressure of which amounts there to 
only about twenty-one English inches §. 
21. M. Pouillet, of Paris, described some years ago an apparatus for measuring 
solar radiation, in which the errors of other statical contrivances were in a great 
measure avoided, by enclosing the thermometer in an envelope maintained at 0° cent., 
with the exception of a small hole which exactly admitted the direct rays from the 
solar disk. Since that time he has adopted Herschel’s dynamical method, which 
he has applied to a modification of the actinometer, which he terms a pyr heliometer ; 
reserving (rather unfortunately, I think) the term actinometer, which was already so 
fitly appropriated, to a separate apparatus for measuring nocturnal radiation. These 
instruments and their applications are described in an ingenious and interesting 
memoir read to the Academy of Sciences, 9th July 1838, printed in the Comptes 
Rendus, and privately circulated under the title of “ Memoire sur la Chaleur Solaire 
sur les Pouvoirs Rayonnants et Absorbants de 1’ Air Atmospherique, et sur la Tempera- 
ture de l’Espace.” 
22. The pyrheliometer is composed of a thin metallic chamber containing water, 
* Royal Society’s Instructions, p. 65. 
f Article ‘ Climate,’ Encyclopaedia Britannica. 
+ See my Supplementary Report on Meteorology in the British Association Report for 1S40, p. 63. 
§ Lchrbuch der Meteorologie, iii. 14. 
