228 DOVE ON THE NON-PERIODIC VARIATIONS IN THE 
high as 14°, we immediately see how little worth can be laid 
upon climatological conclusions, drawn from the observations of 
one or even of a few years. 
In the next tables the mean variability is represented, regard- 
ing as mean variability the differences, taken without reference to 
sign, of each month of several years from the general mean of 
the same month in the same period of years. For instance, let ¢ 
be the temperature of January in Berlin, determined by the ob- 
servations from 1807 to 1824, 7., ¢,,....., the temperature of the 
month in the single years in that period; and if we suppose 
d, = t,—t, 
d,=t, —t, 
ooo ST Orem 69 
dig = tg — t, 
d, +d, +.....d 
then the mean variability d = 18 where dj, doy 
18 
d,, are all positive values. 
[Here follow four tables, showing the mean variability in the 
years 1807 to 1824 for twenty places, 1820 to 1830 for twenty- 
three places, 1828 to 1834 for thirty-six places, and 1797 to 
1804 for fourteen places. ] 
These tables confirm generally the conclusions which have 
been drawn from the preceding ones. The mean variability is 
greatest in January, decreases until April, increases in our lati- 
tudes during summer, and is at a minimum in September. These 
relations are not so evident in southern Italy and England as in 
those places which have summer rains; a considerable variation 
of temperature between single years is caused by the late appear- 
ance of the rainy period, or by its total absence. The extent of 
variability in May is but small, and this explains why vegetation 
generally commences in this month. In winter the earth loses 
more heat during the night than it receives during the day, and 
therefore a clear sky generally causes a diminution of tempera- 
ture. In summer the exact reverse is observed; the clear days 
are the warmest. This agrees exactly with the thermic “ wind- 
roses ” of the different seasons, and shows the dependence of the 
temperature on the winds which prevail during each of them. 
In winter the highest temperature appears with winds which 
produce cloudiness; in summer those winds are the warmest 
which, according to the principle of rotation, blow after the 
clearing up of the weather. 
