198 
FIFTH REPORT — 1835 . 
On the Periods of Time at which the Annual Daily and other 
Curves cross the line of Mean Temperature . 
If we consider the march of temperature through the day, it 
is obvious that the temperature of certain periods is the same as 
that of the mean temperature of the whole twenty-four hours; 
and since the thermometer increases and decreases regularly 
once in each day, we may infer that the periods of mean tem¬ 
perature are generally two, one of them being in some point of 
the increase at morning, the other at some point of the decrease 
at evening. We may extend this consideration in a similar 
way to other periods greater than a day; such as a month, 
three months, six months, or a whole year. And hence, if a 
line of mean temperature be drawn for the respective curves in 
Plates VI., VIII., IX., &c., it is evident that each of these curves 
will cross that line twice in the course of their progress. We 
shall first, by references to the preceding tables and plates, endea¬ 
vour to determine the particular hours, the mean temperature of 
which, as deduced by observation through the whole year, is 
equal to the mean temperature of the whole twenty-four hours, 
as deduced from similar observations. 
The determination of the two times of the day at which the 
mean temperature happens, is an object of great interest in the 
science of meteorology; it enables an observer to arrive at the 
mean temperature of any given place by two observations, or 
even one, each day. These critical times, however*, may vary 
materially, both with the latitude and height above the sea : 
hence the necessity of ascertaining by extensive observation the 
laws of this variation. 
It appears from Table VII. that the mean temperature of 
Plymouth, for the whole year, is 52*90.; we may take this as 
the average mean temperature of twenty-four hours, as shown 
in the last column of Table VI. Now the critical times in the 
twenty-four hours at which this mean temperature happens, 
will, from Tables II., IV., and VI., be as follow: 
Table IX. 
Years. 
Times of Morning 
Mean Temperature. 
Times of Evening j 
Mean Temperature. 
h m 
h m 
1833. 
7 59 
7 3 
1834. 
8 13 
6 58 
1 Mean. 
8 6 
h 
t 
Mean bv Table VI. 
■ %f 
8 9 
Hr 
t 
I j ■ ■! . .. ■» 1 CTBBB—WSWW I I I III I I 'T T rim i I l l ' l ~ *~ P1 
* Edinburgh Phil. Trans, vol, x. p. 379. 
