790 
Proceedings of Royal Society of Edinburgh. [sess. 
the diurnal variation of the force of the wind is exceedingly small, 
the difference between the hour of least and greatest velocity being 
less than a mile per hour ; and as the hours of occurrence of these 
very small maxima and minima vary with the different oceans, 
they cannot be regarded as true maxima and minima. 
Quite different is it with the winds observed by the “ Challenger” 
near land, the force of the wind there giving a curve as pronouncedly 
marked as the diurnal curves of temperature, pressure, or humidity. 
The minimum occurs from 2 to 4 a.m. and the maximum from 
noon to 4 p.m., the highest velocity being at 2 p.m. The curves 
from each of the five great oceans give one and the same result, viz., 
a curve closely congruent with that of the diurnal temperature. 
The differences between the hour of least and greatest velocity are 
these : — Southern Ocean, 6 J miles ; South Pacific, 4 \ miles ; South 
Atlantic, 3J miles ; and North Atlantic and North Pacific, 3 miles per 
hour. Another point of considerable importance is that in no case 
does the maximum velocity, attained near land about or shortly 
after noon, reach the velocity of the wind on the open sea. 
The diurnal variation in the amount of cloud is very small. 
There are, however, indicated two maxima, one about sunrise and 
the other early in the afternoon • and two minima, one at noon and 
the other from sunset to midnight, the differences not exceeding 6 
per cent, of the whole sky. The observation of the diurnal occur- 
rence of rain on the open sea is inversely as the temperature, 684 
days’ observations giviug 96 cases in the seven hours, from 9 a.m. to 
4 p.m., but 135 in the two hours from midnight to 2 a.m., these 
being respectively the times of minimum and maximum occurrence. 
Of the forty-five thunderstorms recorded, twenty-six occurred 
over the open sea and nineteen near land. Of those over the open 
sea twenty-two occurred during the ten hours from 10 p.m. to 
8 a.m., but during the remaining fourteen hours of the day only 
four were recorded. Hence, the important conclusion is arrived at, 
that over the open sea thunderstorms are essentially phenomena of 
the night, and occur mostly during the morning minimum of 
temperature and pressure, squalls reaching the daily maximum at 
the same time ; the phases of the curve of distribution during the 
twenty-four hours being thus the reverse of what obtains over the 
land surfaces of the globe. On the other hand, the maximum in the 
