Af/ricultural Meteorology. 
329 
and summer seasons, wliile the two periods at which the average 
ascent of the temperature in spring and its descent in autumn 
respectively intersect the line of mean annual temperature would 
form the centres of the spring and autumn quarters. The ex- 
tremes generally follow, according to L. H., about a month after 
each solstice ; the means about the same interval after the equi- 
nox — that is, the average yearly mean coincides with the daily 
mean observed about the 24th of April and the 23rd of October, 
which, by his proposal, would become the middle of spring and 
the middle of autumn. 
The accompanying diagram explains better to the eye the 
foregoing description. The middle point of each quadrant con- 
tains respectively the maximum and minimum and the two means 
of the annual temperature. 
Midsummer, 23rd July. 
Temp. 65*^. 
Temp. 34° '0. 
It has been shown that the continental summers, though hotter 
than our own, are by no means exempt from mischievous alter- 
nations. Among those most destructive may be reckoned those 
sudden changes in the upper strata of air which give rise to the 
had-storms, to which all France and North Italy is more or less 
exposed. It has been remarked that some parishes are only 
visited thus after considerable intervals of time ; others every 
VOL. IX. ■ z 
