60 J. F. Miller, Esq., on the 
subjected to a temperature at or below the freezing point of water at any 
period of the year; and on the open heaths and meadow lands of Eng- 
land, where the radiation is not diminished by fortuitous accessions of 
heat from surrounding objects, the surface must often fall far below the 
point of congelation, even in the hottest months. 
In January 1848, a naked thermometer on the grass, placed on wool, 
was at or below 32° on 29 nights; in February on 12 nights ; in March 
on 23 nights; in April on 20; in May on 6; in June on 2; in July, 
none ; in August, none ; in September on 3; in October on 7 ; in Novem- 
ber on 13; and in December on 19 nights. 
The greatest difference I have ever found between the minimum of a 
thermometer placed on raw wool and fully exposed to the sky, and that of 
a six’s thermometer at 4 feet above the ground, and protected from ra- 
diation, was 21°; and this great disparity has occurred on two occasions, 
viz., on the night between the 4th and 5th February 1847, and during 
the night between the 3d and 4th May 1848 ; and on one or two other oc- 
casions it has amounted to 20°. Mr Glaisher, F.R.S., of the Royal Ob- 
servatory, who has thrown more light on this interesting and important 
subject than any other person, in the remarks accompanying his exten- 
sive and elaborate tables printed in the Philosophical Transactions, states 
that the greatest difference he ever observed between the simultaneous 
readings of two thermometers so circumstanced, was 25°, and this extra- 
ordinary difference occurred once only in a period of several years. 
But the differences which obtain between the minimum readings of 
self-registering thermometers placed on substances exposed on the sur- 
face, and in air, are not the maximum differences, unless the two minima 
occur at the same time, which is seldom the case. 
I have generally found the lowest temperature on the ground to occur 
some hours before midnight, and rarely after it ; whereas the greatest de- 
gree of atmospheric cold takes place about the time of sunrise, as is well 
known to those whose occupations cause them to pass the greater part of 
the night in the open air. 
The difference between the two minima should be increased by the 
difference between the readings of the air-thermometer at those times, 
which, according to Mr Glaisher, may amount to 10°. Hence, it is pro- 
bable, that when a difference of 20° or 21° obtained between the minima 
at Whitehaven, an absolute difference of 30° may have occurred during 
some part of the preceding evening or night. The greatest differences 
recorded by Mr Glaisher (with S.R. thermometers) were, on raw wool 
20°-4, and with flax 21°-8, which are nearly identical with the extreme 
differences found at Whitehaven. 
From the large amount of heat absorbed by the earth during the sum- 
mer, it might be supposed that the effect of terrestrial radiation would be 
greatest in the autumn and early part of winter, and least in the spring 
months. But a careful examination of my observations does not counte- 
nance this opinion; on the contrary, the radiation is occasionally small 
in the winter, and large in the summer months; but such excess or de- 
ficiency is caused wholly by accidental atmospheric conditions, which 
favour or retard the process. On the whole, it appears that under equal 
and similar circumstances, the amount of radiant heat thrown off by the 
