146 
MR. J. F. MILLER ON THE RELATION BETWEEN THE 
and with a standard instrument. But as perfect thermometers are very difficult to 
obtain, most of such instruments will differ from a standard at one point or other of 
the scale. They should therefore be carefully compared, and the errors allowed for, 
either at each reading, at the end of every month, or at some stated period. The 
goodness of the instruments, or the application of the corrections, is the more essen- 
tial, as any error in either or both the thermometers is considerably increased in the 
resulting temperature of the dew-point. The bulbs should be carried fully an inch 
below the metal or wood scale. The wet bulb should be covered with book-muslin, 
and the best conductor of the water from the cistern to the bulb is a piece of cotton 
lamp-wick, which must be changed as circumstances require, as in time it is apt to 
lose its capillary action. In winter, the water in the vessel will frequently be frozen, 
and the wet bulb quite dry : in this case the bulb must be moistened, when a cqatiog 
of ice will form on it, from which evaporation will take place : an hour or more will 
however sometimes elapse before the temperature of the moist bulb falls below that 
of the dry. 
It is well known that evaporation goes on freely from the surface of frozen 
water, even when the whole mass is converted into a solid block of ice. From the 
11th to the 16th of December 1846, during which a brisk breeze prevailed, the loss 
from my evaporation gauge was 0'450 inch, or *075 per diem, the average of the 
month being but ‘033 per diem. In twelve days of frost in February, 1847, the eva- 
poration from the ice was "552, or ’046 per day, the average daily quantity for the 
month being *030. During ten days of keen frost, between the 20th and 30th of 
January, 1848, the ice had parted with 0'324 inch, or '032 per diem, the average daily 
loss for the month being only '024. For five days of December 1848, the loss was 
'037 per diem, the daily average for the month being '032 ; and from the 1st to the 
8th of January, 1849, the depth evaporated was equivalent to '017 per diem, the 
average daily loss during the month being '029 inch. Hence it appears, that notwith- 
standing the large amount of heat required, not only to liquify but to vaporize frozen 
water, the vapour thrown off by ice in an invisible form exceeds in amount the average 
daily evaporation from an equal surface of water in the winter season. This appa- 
rently anomalous circumstance probably arises from the extreme dryness of the 
atmosphere, and its consequently increased capacity for vapour in severe frosts, 
whereas at other times during the winter the air is very moist near the sea, being gene- 
rally not more than 2° or 3° above the point of saturation. 
The subject of evaporation has been very little investigated or treated of since the 
publication of Mr. Howard’s ‘Climate of London ’ in the year 1818, and having paid 
considerable attention to this department of meteorology for several years past, I may 
perhaps be excused (though not directly connected with the issue of this paper) from 
briefly giving the results of my own investigations, obtained at nearly the opposite 
point of the kingdom. Evaporation is scarcely ever entirely suspended, either 
during the heaviest rains, or when the air is completely saturated with vapour; at 
