194 Daily Variations of the Strength of the Wind [April, 
with the rising temperature. This acceleration of the wind 
is in part determined by the influx of air on the earth’s sur- 
face from all sides towards the centre of the rising columns, 
which thus produces inconstant breezes ; in part also by the 
descent of air in the regions surrounding the ascending 
columns. Such descending air retains the velocity which it 
possessed in the upper regions, and which is well known to 
be greater than that of air in immediate contact with the 
earth’s surface.” 
Without any knowledge of Espy’s explanation, Koppen, 
as far back as 1879, whilst discussing Hann’s observations, 
propounded the hypothesis that the daily periodicity of the 
wind depends on two conditions — 1st, the varying intensity 
of the perpendicular exchange of air in the lowest 1000 to 
4000 metres of the atmosphere, with a motion which on the 
average increases from below upwards, on account of dimi- 
nishing friction ; 2nd, the preference of the warmer hours of 
the day, in consequence of the appearance of steep gra- 
dients by difference of pressure at small distances. This 
explanation is practically at one with that of Espy, — a fadt 
so much the more interesting as the latter savant had at his 
command in 1840 but a scanty array of fadts for his infer- 
ences, whilst now a formidable series of arguments can be 
produced in favour of this hypothesis. 
The latter of the two causes mentioned consists in the 
especial occurrence of thunderstorms, with their accom- 
panying depressions, local whirlwinds, &c. The nature and 
the local movement of the gradients may be seen strikingly 
illustrated in Koppen’s memoir on the thunderstorm of 
August 9th, 1881. As these gradients may have very dif- 
ferent directions, and may be propagated very differently, all 
the winds may undergo in the course of time such temporary 
accelerations which, by their occurrence in the hottest part 
of the day, contribute essentially to the known form of the 
wind on land. 
But this cause accounts for merely a subordinate portion 
of the phenomena of the daily period of the strength of the 
wind. This is shown by the fadt that the mid-daily accele- 
ration of the wind is often very distinctly characterised in 
cases where the foregoing explanation is manifestly inad- 
missible. This is especially the case in great aerial currents 
which retain an approximately constant direction. As ex- 
amples we may take the trade-wind in the dry season in 
the interior of Africa and South America, and the persistent 
east winds accompanied by dry clear weather in the tem- 
perate zone. 
