
400 ALEXANDER BUCHAN ON THE 
In the northern hemisphere, during the winter months, the forenoon maxi- 
mum rises to a greater extent above the mean of the day than the afternoon 
minimum falls below it at 71 per cent. of all the stations ; whereas the after- 
noon minimum falls to a greater extent below the mean than the forenoon 
maximum rises above it at only 29 per cent. of the stations. On the other 
hand, during the summer months, the afternoon minimum falls below the mean 
of the 24 hours to a greater extent than the forenoon maximum rises above it 
at 64 per cent. of the stations. These results are confirmed by the observations 
made in the southern hemisphere, thus suggesting that the influence of the sun 
and the other causes on which the diurnal oscillations depend, tend during 
the winter months of both hemispheres generally to raise the morning 
maximum to a greater extent above the mean than to lower the afternoon 
minimum below it ; and during the summer months the same cause or causes 
tend to lower the afternoon minimum to a greater extent than to raise the 
forenoon maximum above it. 
Geographical Distribution.—At all places in Table I. for which hourly 
values have been obtained, it will be observed that the oscillation is 
stated in three ways, viz.: 1, The difference between the highest and lowest 
hourly means ; 2, the difference between 9 A.M. and 3 P.M. means; and 3, the 
difference between the means at 10 a.m. and 4 p.m. It will be seen that the 
amplitude of the oscillation indicated by the 9 4.M.-3 p.m. and the 10 A.m.—4 P.M. 
means differs but little from that indicated by the highest and lowest hourly 
means, in cases where three years’ observations and upwards are available. 
The amounts of the oscillations deduced from the means at 9 A.M.-3 P.M. 
and 10 a.m.-4 p.M., at places where observations at these hours only were made, 
were corrected by adding to them the mean of the small differences found to 
obtain at two, or in some cases three places, between their oscillation for the 
same hours and the absolute oscillation as shown by the hourly means. Care 
was taken to make the comparison in every case with stations similarly situated 
as regards latitude, proximity to the sea, and elevation. The results thus 
obtained have been used in constructing the charts, the actual oscillations as 
observed being given in Table I. Table II., on the other hand, does not give 
actually observed, but only corrected oscillations for 49 places. At these 
places observations were not available at 9-10 a.m. and 3-4 p.m., but only at 
8 A.M. and 2p.M.; 7 AM. and 2 p.m, &c. To the mean oscillation obtained 
from these hours’ observations, a correction was added by which an approxi- 
mation to the true oscillation was obtained. This correction was determined 
from the hourly means at two or three places similarly situated as regards 
latitude, proximity to the sea, and elevation, It is, however, to be remarked, 
that less weight has in all cases been given to the means of Table II. than is 
given to those of Table I. 
