ON THE HOURLY OBSERVATIONS OF THE THERMOMETER. 197 
In Plate IX. are projected the mean summer and winter curves, 
from the two columns of Table VIII. These are extremely regu¬ 
lar, more especially the former : each point of each curve is the 
mean of 365 observations. The mean annual curve is also shown 
between these as projected from the last column of Table VI. 
The summer curve descends regularly from midnight until 4 
a.m., when the greatest cold occurs; it then ascends with ex¬ 
traordinary rapidity and regularity until 1 p.m., after which it 
descends with great regularity, but with less rapidity, to its 
minimum; the total mean range being about 11 degrees and a 
half. The period of the ascent is nine hours, and the descent 
fifteen hours, which numbers are to each other as 3 : 5. The heat 
of the day in summer, therefore, increases at Plymouth faster 
than the cold of the night in the ratio of 5 : 3. It is interest¬ 
ing to observe the difference between this and the summer curve 
at Leith, which rises less rapidly, and somewhat less regularly, 
until 3 p.m., after which it finally descends with rapidity to the 
minimum; the total mean range being about 8*5°. The period 
of its ascent is eleven hours, and the descent thirteen hours; 
hence the heat of the day increases in summer at Leith more 
rapidly upon the whole than the cold of the night in the ratio 
of 13 :11, being less than the rate of increase at Plymouth by 
nearly one fourth. 
The winter curve for Plymouth descends regularly from mid¬ 
night until 6 a.m., at which time the cold is greatest: from 
this time it rises until 1 p.m., as before, the period of its ascent 
, being seven hours, and that of its descent seventeen hours : 
hence we may conclude, in neglecting the intermediate grada¬ 
tions as before, that upon the whole the heat of the day ad¬ 
vances in winter at Plymouth more rapidly than the cold of the 
night in the ratio of 17 : 7 ? or as 5 : 3 nearly, being about -J 
times more rapidly than in summer. In the winter curve for 
Leith there is a gentle rise of temperature after midnight, which 
is not apparent at Plymouth. On comparing those curves in 
Plate IX. with the analogous curves of Sir David Brewster for 
Leith as shown by the dotted lines, we find some interesting 
points of difference. The afternoon branches of the Leith curves, 
for example, especially those of summer, as also the mean curve, 
are more full and protrusive than the similar curves for Plymouth. 
In the individual curves for Leith which compose the winter 
group, and from which the dotted yellow and blue lines in 
Plate IX. are deduced, there is a rise of temperature after mid¬ 
night, and then a subsequent fall; this, however, does not seem 
ever to occur at Plymouth, nor does it occur in the summer curves 
for Leith. 
