126 
MR. GLAISHER ON THE RADIATION OF HEAT, 
Each set of observations may be looked upon as a series of experiments made 
for the purpose of ascertaining the different tendencies of various bodies to become 
cold upon exposure to a cloudless sky at night. To many persons it must be a new 
fact, that a perfectly dry body, placed in contact on all sides with other bodies of 
the same temperature with itself when the sky is covered by clouds, shall on the sky 
becoming less cloudy or cloudless, become much colder than those bodies, to so great 
an amount as is exhibited in the following tables, and that it shall remain so for many 
hours; yet these circumstances were exhibited in every series of observations*. 
The formation of dew was found to depend solely on the temperature of the bodies 
upon which it was deposited, and it never appeared upon them till their temperatures 
had descended below that of the dew-point in their locality, as found by observations 
of a dry and wet bulb thermometer placed in their vicinity. 
The amount of water thus deposited was the greatest upon the substances whose 
temperatures were the lowest : among these bodies glass was found to radiate heat 
freely, and it very readily became wet with dew. In consequence of this property, the 
tube of a naked thermometer, which was lying on a substance entirely free from 
moisture, was frequently found covered by dew, and therefore it seemed probable that 
the temperature exhibited by the instrument was not that of the body in question. 
On such instances occurring an attempt was made to correct the error by enclosing 
the thermometer stem in a tube made of gilt paper ; the bulb alone, resting on the 
substance, remained exposed to the sky. The differences between the readings of a 
thermometer thus enclosed, and when naked, were found to be sensible, but small in 
amount ; it was observed that when the thermometer was wholly naked, the stem was at 
times wet when the bulb itself was dry ; and at all times much less moisture appeared 
on the bulb than on the stem, unless the disposition of the substance in question to 
become cold was the same, or greater than that of glass. The error arising from 
this cause was chiefly confined to the consequent contraction of the mercury in the 
stem, and not in the bulb, and which was considered to be avoided by the use of 
gilt paper: the error in all cases must have been small. It was found that the dif- 
ferences between the temperature of the air and of bodies on the earth, at night, in 
equally calm and clear weather, w r as the same at every period of the year, but it was 
found that the amount of dew deposited during such times was much greater in 
summer than in winter. This is easily accounted for, from the now well-known re- 
lation existing between temperature and moisture. At all seasons of the year, at 
night, the depression of the temperature of the dew-point below that of the air is 
small, or the air is in a state of saturation nearly, and therefore in summer a certain 
diminution of temperature would cause much more vapour to be changed into water, 
than an equal diminution in winter would do. 
Radiation of heat from the earth to the heavens must exist at all times both day 
and night, and in all states of the sky. Generally, when the sun is above the 
* The whole of these observations are placed in the Archives of the Royal Society. 
