Rs a ‘gk oi i ae 
251 
tion with the surrounding hills, it is interesting to compare the 
relative temperature of the air and water at different periods of 
the year. This we are able to doin the present instance, the 
temperature of the river at the bottom of the Institution Gardens 
having been regularly taken each day, at the same hour as the 
other observations, since October, 1868. 
The following Table accordingly gives the mean temperature of 
the air and of the river for every month in the year, calculated 
from six years’ observations, 1869-1874; along with the mean 
number of days in each month on which, at 9 a.m., the dry-and 
wet-bulb thermometers were either coincident or within one 
degree of coincidence. 
Means. Jan. | Feb. | Mar. | April.| May./June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. 
Temperature } I 2 2. 6 62.1 6 0.6 8 
of Air. 4t-4 | 42.3 | 43-9 | 50-2 | 52.3 | 57.9 | 635 | 62.2 | 57.6 | 50.6 | 43-4 | 40. 
Ditto river. 42.t | 42.8 | 45.3 | 51.4 | 56.0 | 62.5 | 66.1 | 64.7 | 50.4 | 52.6 
Number of 
days of Coin- 
cidence of }| 12.0 | 7.3 | 4.6 
dry-and wet- 
bulbs. m 
It will be noted in the above Table, that in every month of the 
year at 9 a.m. the mean temperature of the river exceeds the 
mean temperature of the air, the excess being greatest in spring 
and summer, and least in autumn and winter. It exceeds the 
mean night temperature of the air to a much greater degree. 
These conditions of atmosphere quite account for the frequency of 
fogs in Bath,* which originate in the night when, from the differ- 
ence between the temperatures of the air and water being then 
greatest, the vapour ascending from the river is more copiously 
condensed than at other times. After sunrise these temperatures 
_ * Dr. Tunstall says—“ Fogs are of very rare occurrence, being seldom seen 
. in the valley of Bath.” Climate of Bath, p. 55. This statement is not at all 
in agreement, either with my own observations, or with those of the Institution 
registers. 
