1884.] 
in Land and at Sea. 
193 
than that of the variables just named. The most striking 
characteristic of this periodicity is the intensification of the 
wind in the hottest time of the day, — a feature which, ac- 
cording to Hann, is recognised in all continental stations, 
whatever may be their climate or the prevailing character of 
their winds. 
This daily maximum of the strength occurs in Upsala, 
Cracow, Hamburg, and Dresden, at 1 p.m. ; at Vienna, 
Birmingham, Liverpool, Toronto, Calcutta, and Ascension, 
at 1.30 p.m. ; at Prague, Oxford, Batavia, and Melbourne, 
2 p.m. ; Petersburg, Halifax, Mauritius, 2.30 ; Bern, 3 ; 
and Rome, 3.30 p.m. At these places the entire daily 
variation in the strength of the wind takes place while the 
sun is above the horizon. During the night the wind is 
equally low, and even in the first hours after sunrise it varies 
little from the nocturnal value. The wind arises, especially 
on clear days, some hours after daybreak, and subsides again 
at night. In the intermediate time it increases towards 
noon ; and in plains, in the interior of tropical and sub- 
tropical continents, it reaches daily almost the intensity of 
a storm. This is especially the case in the region of the 
trade-winds and in the dry season. 
In the open sea, as has been recently shown by Mr. 
Buchan from the observations made on board the Challenger, 
the case is quite different. In the middle of the ocean there 
is scarcely any appreciable daily variation in the strength of 
the wind. It retains during the night the same relatively 
high figure which appears by day. According to two con- 
cordant observations, however, the trade-wind shows a slight 
periodicity, the maximum being at 8 a.m. and the minimum 
at 4 p.m. 
The increase of the speed of the wind in the warmer 
hours of the day, observed as a universal fact on land, is 
confined, however, to strata of air which are for considerable 
distances in direct contact with the earth’s surface. In the 
free, upper regions of the air, the wind is not stronger at 
noon than at morning and evening, but weaker. This fact 
was first demonstrated by Hellmann on Mount Washington 
and on the Righi. Koppen gives several other instances of 
a similar nature. 
As far back as 1840 Espy gave a satisfactory explanation 
for the main features of the daily periodicity of the wind : — 
“ The beginning of the formation of ascending columns of 
air in the morning will be accompanied by an increase in the 
strength of the wind, and its strength will grow with the 
increase of these columns, both phenomena keeping step 
