212 
MR. GLAISHER ON THE RADIATION OF HEAT, 
At the Royal Observatory at Greenwich, the reflexion of the lights of London on the 
clouds is well seen ; at times this appears as a narrow well-defined band of light, at the 
elevation of several degrees, and at other times as a broad diffused band; the lower 
limit of which is sometimes below the horizon. It was soon found that the difference 
between the reading of the thermometer in air and that on grass was greater, the 
greater the height of this band of light. On very many nights, and several times in 
the course of the same night, the height of the upper and lower edges of this band of 
reflected light was measured above the horizon, and from these observations that of 
the centre was determined ; at the same times the readings of the thermometers in 
air and on grass were taken. By these means it was found that when the centre of 
the reflected light was above the horizon of Greenwich by — 
4°, the excess of reading of the thermometer in air above that on grass was 1 0, 6 
6°, the excess of reading of the thermometer in air above that on grass was 2 0, 5 
8°, the excess of reading of the thermometer in air above that on grass was 3 0, 9 
And whenever the centre of the band was in height less than 4°, the lower limit was 
generally below the horizon, and the height of the centre could not be determined ; 
but at these times the differences between the readings of the two thermometers w r as 
seldom so much as a degree, and it was generally less, and frequently there was no 
difference. 
The Cathedral of St. Paul is very nearly the centre of London, and it may be con- 
sidered to be immediately under the centre of the reflected light ; its distance from 
the Magnetic House is more than 25,700 feet; at this distance one degree subtends 
about 450 feet ; the height of the ground where the observations were taken is about 
100 feet higher than the ground at London. 
From the above data, it appears that when the reflexion of the London lights was 
4°, 6°, and 8° high, the distances of the clouds from the earth was 1900, 2800, and 
3700 feet respectively. 
A similar investigation was made by measuring the distance of the band from the 
cross on the cathedral, whose height above the ground is about 400 feet, and results 
were obtained differing from the preceding by about fifty feet ; a much smaller 
number of observations, however, were used in deducing the latter results than were 
used in deducing the former. 
As a difference of 0 O- 7 took place in the reading of a thermometer on grass for 
every variation of a degree in the height of the reflected lights, it follows that a dif- 
ference of 1° in the readings of the thermometer on the grass indicated a difference 
of 630 feet in the height of the cloud. 
