ig6 Daily Variations of the Strength of the Wind [April, 
the formation of cumuli demands a certain degree of atmo- 
spheric moisture, and depends also on perpendicular currents 
which do not necessarily extend down to the lowest stratum 
of the air. 
The correctness of these views is demonstrated by the faCt 
that a series of conclusions which necessarily follow are 
unmistakably confirmed by observation. These are accord- 
ingly here enumerated : — 
1. Wherever the daily variation of temperature and the 
retardation by friction are both very trifling, the mid-daily 
intensification of the wind — or, more stridtly, the nightly 
lull of the wind — is wanting, even at the surface of the 
earth. This occurs on the open sea. The very slight fric- 
tion at the surface of the water determines a very slight 
increase of the wind on ascending. Again, the perpendicular 
currents are substantially not connected with a daily period, 
as the daily fluctuation of temperature, even in the lowest 
stratum, does not exceed 0*7° C. 
2 . Where the intensified perpendicular circulation of the 
air in the warmest part of the day brings air from tranquil 
strata, there occurs an inversion of the period. The warmest 
hours of the day are then marked not by an increase, but by 
a decrease in the strength of the wind. This is the case on 
the summits of mountains which rise to a certain height 
over a varied landscape with a well-marked daily fluctuation 
of temperature, but which do not themselves greatly influence 
the surrounding atmosphere. 
3. The daily period of the wind displays characteristic 
relations to other phenomena, especially to vapour-tension. 
In all districts where the circumstances are favourable to a 
periodical powerful development of the perpendicular circu- 
lation at the hottest time of the day, and where accordingly 
the mid-daily intensification of the wind is distinct, the 
vapour-tension, as far as is known, has its minimum in the 
warm hours and its twofold maximum in the morning and 
evening. This decrease at the time of greatest heat can be 
explained only by the faCt that air, poorer in moisture be- 
cause warmer, arrives from the upper regions to the surface 
of the earth. This phenomenon occurs only at the bottom 
of the atmosphere in valleys or on plains. On the mountain 
tops the vapour-tension, like the period of the wind, is in- 
verted, the maximum occurring when the warm moist masses 
of air rise up from the low-lands. 
4. On those sea-coasts in the region of the trades which 
have no regular sea-breeze, the trade-wind blowing over the 
land blows more strongly by day than by night, and shows 
