APPLIED TO METEOROLOGICAL OBSERVATIONS. 
129 
Twice during the day the temperature of the air is at its mean value, and these 
times are as follows in the several months : — 
h m h m 
In January at 10 0 a.m. and again at 8 0 p.m. 
In February at 9 30 a.m. and again at 6 40 p.m. 
In March at 9 10 a.m. and again at 7 20 p.m. 
In April at 8 40 a.m. and again at 7 0 p.m. 
In May at 8 25 a.m. and again at 7 30 p.m. 
In June at 8 0 a.m. and again at 8 0 p.m. 
In July at 8 0 a.m. and again at 8 6 p.m. 
In August at 8 20 a.m. and again at 7 20 p.m. 
In September at 8 55 a.m. and again at 7 20 p.m. 
In October at 9 0 a.m. and again at 7 0 p.m. 
In November at 9 25 a.m. and again at 6 45 p.m. 
In December at 10 0 a.m. and again at 7 20 p.m. 
To determine the mean temperature of the air, it might therefore seem that it 
would be sufficient to take an observation at one of these two periods ; but it must be 
borne in mind that at these times the changes of temperature are rapid, and, con- 
sequently, if the observation be made a little too soon or a little too late, very con- 
siderable errors might be committed ; therefore, observations at these times, unless 
they are made very accurately with respect to time, are not worthy of implicit con- 
fidence. 
The better way is to take observations several times during the day, and at such 
times that the algebraical sum of the corrections is a minimum. 
The best plan, however, is to take observations at those hours which are the least 
liable to interruption by the avocations of the observers, and to apply to their mean 
results the necessary corrections. Of the hours which are equally convenient, those 
are preferable about which the least changes are taking place, as then a small error 
in the time of observation will entail little or no error in the readings. 
The mean temperature of the air has hitherto been considered by most observers 
to be that which is intermediate to the maximum and minimum of the day; and in 
most places in England the mean temperature has been deduced from these two 
elements, by taking a simple arithmetical mean between them. The true mean in 
the summer months is widely different from the half of the sum of the maximum 
and minimum readings. The mean temperatures, of nearly all places in England, 
have therefore been estimated too high. At Greenwich this empirical mean has 
been found always to exceed the true mean ; but the amount of the error is variable 
in the different seasons, but it is found to be the same in the same months of different 
years. The following table gives the correction for all the months of the year. 
MDCCCXLVIII. 
s 
