52 Proceedings of the Royal Physical Society. 
directly governs the available solar radiation, and affects its 
amount no less surely than the degree of latitude. This is 
the state of the atmosphere, in respect to its transparency for 
the solar rays. The term transparency, as here and subse- 
quently used, must be taken as including not only transpar- 
ency to light, but to radiant heat (diathermancy), and to all 
other forms of radiant energy. The radiation values obtained 
above by calculation from latitude are, in point of fact, 
ideal values, for they take no account of the exceedingly 
powerful and variable factor—atmospheric absorption. The 
atmosphere determines for each place and season and day 
what proportion of the ideal radiation shall actually reach and 
affect the earth. Are there, then, any means of accurately 
representing this important element of climate? If some 
adequate expression could be devised for the state of the 
atmosphere in respect to its transparency for the solar ray, at_ 
different places and different times, it is obvious that it would. 
only be necessary to combine this expression with zenith 
solar seconds in order to obtain the true value of the actual 
radiation received. Meteorological science has made an 
important advance in this direction. Of late years a mass 
of information has been accumulated at different stations, by 
instruments designed to record the duration of bright sunshine. 
In point of fact, Campbell’s, which, with shght modifications, 
is the form most commonly employed, being essentially a 
large burning-glass, records the solar heat; and Jordan’s 
photographic recorder is affected by the chemical energy of 
the solar ray. Assuming, however, that these instruments 
may be fairly comparable, we have now a most interesting 
view of the duration of radiation of a certain intensity at the 
different localities. 
Table IX. shows the sunshine records for Strathpeffer for 
each month from December 1889, compared with the corre- 
sponding records for Braemar, Royal Observatory, Meteorolo- 
gical Office Westminster, and, as a typical English winter 
health resort, Falmouth. The two last columns exhibit the 
percentage of possible duration for Strathpeffer and London 
respectively. It will be observed that the brightest month 
of this short series is May (184 hours), and the dullest 
ee 
