194 
MR. GLAISHER ON THE RADIATION OF HEAT, 
The result deduced from the observation 
taken in the year 
" 1844, on July 16, at 21 . 
1844, on August 13, at 21. 
1844, on August 14, at 21. 
1844, on October 7, 21. 
1844, on October 10, at 21. 
_1844, on October 18, at 21. 
In all twenty-five instances out of 331 cloudless nights; on one of these nights, 
viz. that preceding 1842, August 28, 21 u the result was a reading of 0 o- 6 higher than 
that of the thermometer in air, but it is very probable that the reading was in error. 
The general amount on cloudless nights is from 7° to 10° ; and frequently above 10°. 
During the four years’ observations there were thirty-three cases in which the reading 
of the thermometer in air exceeded that of the thermometer in the reflector by quan- 
tities between 10° and 11°; there were nineteen cases between 11° and 12°; sixteen 
between 12° and 13°; eight between 13° and 14°; and there were four instances in 
which the excess amounted to 14° and was less than 15°; these cases occurred during 
the nights preceding 1842, June 16, 21 h and October 29, 2 l h ; 1843, February 12, 21 h 
and March 6, 21 h . (The differences in 1843 March were unusually large.) There 
were two cases exceeding 15°, both in 1842 on June 6 and 7 ; and there is one above 
17° which occurred during the night preceding 1843 March 2, 21 h . This is the largest 
difference shown during the four years’ observations, between the thermometer in 
the air and that in the reflector. I now proceed to speak of the results derived from 
the observations of the other instruments. 
Those from cloudy nights with all the other thermometers were nearly the same as 
those with the thermometer with its bulb placed in the focus of the reflector, except- 
ing that their readings were much less frequently higher than those of the thermometer 
in air than were those of that instrument. 
On partially cloudy, and on cloudless nights, the reading of the thermometers on 
grass, those placed one inch above it, those on raw wool and flax, were always much 
lower than that in the reflector ; and frequently, even on grass, the reading was as 
much below that in the reflector as the latter was below that in the air ; on wool and 
flax it was nearly always so. 
The greatest difference between the readings of that in air and that on short grass 
was 17°'8 ; and on long grass was 18 0- 8 ; and they took place during the night pre- 
ceding 1844, October 3 d 2 1' 1 (see Table XCVI.). The reading of the thermometers 
which were placed on wool and flax were frequently from 15° to 18°, and occasionally 
from 19° to 21° less than that in air. The greatest difference with raw wool was 
20 o- 4 ; and with flax was 21°'8, as observed 1844, April 24 d at21 h , and on October 3 d 
at 21 h respectively. 
The readings of the thermometer placed on garden mould were always (excepting 
when the sky was quite cloudy) nearer to that in the air than any of the other 
