1884.] 
in Land and at Sea . 
197 
the same daily period of velocity as is found in the interior 
of the continents. Of this a signal proof is given by the 
indications of the anemometer at St. Helena and Ascension. 
5. The diminution and inversion of the daily periodicity 
of the wind on high mountains does not depend on the 
absolute height of above the sea-level, but on the free ex- 
posure of the station on a summit of sufficient height above 
a varied region. Thus Koppen made observations on the 
Puy de Dome and on the Pic du Midi. On the former he 
observed a decrease and on the latter an increase of the wind 
at noon.. The explanation of this apparent anomaly is that 
the station of the Puy de Dome is situate on an open 
rounded summit ; that on the Pic du Midi lies 500 metres 
below the summit. 
6. Researches on the relation of the velocity of the wind 
to the gradient, which have been undertaken by Mr. C Ley 
for England, and by Herr Sprung for the German coasts, 
give the accordant result that at 8 a.m. one and the same 
gradient is accompanied by a stronger wind in summer than 
in winter ; stronger, also, with a north-easterly than with a 
south-westerly wind. But according to the researches of 
Hann and others the decrease of temperature upwards is on 
the average greatest in summer, and in time of north- 
easterly winds. As the magnitude of this decrease is gene- 
rally parallel to the strength of the perpendicular exchange 
of air, it follows that for equal gradients there ensues a 
stronger wind when this exchange of air is brisk than when 
it is feeble, as the theory demands. 
7. A particular importance attaches to the observation 
made independently by HH. Hjelstrom and Sprung, that 
cloudiness has a well-marked influence on the daily period 
of the wind. During east winds with a bright sky the daily 
variation is more distinct than during westerly winds ac- 
companied by cloudy weather. A clear sky which permits 
free radiation aCts in two directions : by day it increases the 
vertical exchange of air by augmenting the vertical decrease 
of temperature, and consequently it intensifies the velocity 
of the wind in the lower strata of the air. At night, by re- 
frigerating those strata it produces a firm equilibrium in the 
vertical direction, and thus withdraws the lower air from the 
influence of the upper, more rapidly moving, atmospheric 
strata. If the gradients are the same, there is therefore a 
stronger wind by day and a weaker by night in clear weather 
than under a cloudy sky. 
8. Perhaps the most interesting feature of the daily pe- 
riodicity of the wind is its relation to the daily variation in 
