AT NIGHT, FROM THE EARTH, ETC. 
15,3 
If attention be paid to the placing of thermometers, so that their bulbs be in 
the shade, protected from rain and from the effects of radiation from walls (bv 
placing their bulbs at least six inches from them), but in other respects freely 
exposed to the air, and correcting their readings by the preceding formulae, it will 
ensure the obtaining the true temperature of the air from them at night. To avoid 
the effects of reflected heat during the day, the thermometers should be placed 
at a greater distance than six inches from walls, &c., and their readings corrected 
as before by the preceding formulae. No simple rules have hitherto been given for 
placing a thermometer so that from its readings the true temperature of the air could 
be deduced. 
The readings of thermometers placed on grass being found so variable, the unfit- 
ness of it to furnish the means of comparing the degrees of cold at night on the surface 
of the earth was evident. A much greater uniformity was observed in the results of 
experiments made with other substances, which were bad conductors of heat, and 
whose condition was always the same. 
Of all the substances experimented upon, those on which the readings of the ther- 
mometers have been the lowest were hare-skin and rabbit-skin* ; and upon the fila- 
mentous, as raw wool, flax, unwrought cotton wool, and raw silk, the readings upon 
all of which were more steadily less, on clear and calm nights, and exhibited a greater 
degree of cold, than those on grass. Among bodies of this class, raw wool exhibited 
a lower reading at all times than any of the rest ; the circumstance of hare-skin and 
rabbit-skin exhibiting a lower mean in the table is accidental, experiments on these 
substances having been made only during the finest nights, and on which nights raw 
wool exhibited lower readings than they did. The next in order is flax, both fine and 
coarse, and in its mean it exhibits a radiating power very nearly equal to that of 
raw wool ; but the last-mentioned substance was much more sensible on the approach 
of a cloud than any other substance: on those nights which were clear for many 
hours, raw wool obtained its lowest readings long before flax ; and the readings 
on wool, on the approach of a cloud, increased much sooner than those on flax ; and 
in consequence of the latter circumstance, the mean values deduced from raw wool 
and flax were nearly the same. In intervals of clear sky, between cloudy states of 
it, the difference between the readings of two thermometers, the one in air and the 
other on raw wool, was the greatest of any of the differences. The rapidity of the 
decrease of the readings on raw wool from a cloudy to a clear sky was about a degree 
per minute, so that a change in the readings of 15° has taken place in a quarter of an 
hour. The greatest difference between the temperature of a body at night on the 
surface of the earth, and that of air a few feet above the earth, was 28°‘5 ; this extra- 
ordinary difference occurred in 1844, April 8 d , at 8 h (See Table XLIV.). The times 
* Thus explaining the fact frequently noticed by sportsmen, that the snow upon which hares have been 
lying is never in the slightest degree melted. 
MDCCCXLVII. X 
