1 
44 
ture exhibit a similar periodical change ? For the present, I 
will merely suggest that a clue to the correct explanation of 
this apparent anomaly may perhaps be found in the conclu- 
sion at which Professor Forbes arrived from a discussion of 
his observations made in Switzerland, in conjunction with 
Professor Kaemtz, on the intensity of solar radiation at 
different elevations in the atmosphere, namely, that the 
heating rays of the sun consist of two kinds, — one kind, of 
high intensity, which suffers little or no loss in passing 
through the atmosphere ; and the other, of much lower 
intensity, and therefore much more absorbable. Now, if 
we suppose that the relative quantities of the two kinds of 
rays undergo periodical changes, such that the maximum of 
the one corresponds to the minimum of the other, it will be 
evident that when the lower strata of the atmosphere receive 
less heat by absorption, owing to a diminished supply of the 
rays of low intensity, the ground will receive more from the 
simultaneous increase in the quantity of the rays of high 
intensity, and this being communicated to the lower atmo- 
sphere by conduction and convection, as well as by radiation 
upwards, will restore the equilibrium, and tend to produce 
uniformity in the mean annual temperatures. 
On a former occasion I urged the desirability of giving 
more attention than has hitherto been done to the oscil- 
lations of mean daily temperature, and have shown, in 
the present discussion, their importance in connection 
with the subject of solar radiation. I may now add 
that they appear to have an intimate connection with 
magnetic phenomena. Among the tables given by the 
Astronomer Royal in the volume of Greenwich observations 
for 1859, is one showing the monthly mean horizontal 
magnetic force at Greenwich for the nine years 1849-1857, 
corrected for secular variation. The line No. 2, in diagram 
E, is a projection of the numbers in this table, and the line 
No. 1, in the same diagram, shows the monthly mean daily 
