50 Proceedings of the Royal Physical Society. 
diagram. In the table four equidistant periods, for brevity’s 
sake, have been taken—the two equinoxes and the winter and 
summer solstices. At the winter solstice, while the sun is 
LENGTH of DAY 
30.000 
LONDON 
“5 
20.000 ti 
Li i 
in 
Hi 
iy 
Wi | 
a 
above the horizon at Greenwich for 7 hrs. 36 mins., the 
ageregate radiation, supposing it to shine throughout the day 
without diminution from atmospheric causes, is exactly equal 
