156 
MR. GLAISHER ON THE RADIATION OF HEAT, 
the grass plat to another, was lower than when it was placed on a similar piece of 
metal which had remained unmoved. 
A piece of metal, if so placed that part of it be in contact with grass and the other 
part not, that part of it in contact with the grass will be at a lower temperature than 
that which is not in contact. 
The thermometer whose bulb is in the focus of the mirror when the clouds are low 
remains stationary and frequently reads higher than any other thermometer however 
placed ; the reason seems to be, that the heat radiated from the cloud being received 
on the reflector and reflected to the bulb in the focus, exceeds that radiated from 
the thermometer. 
The remaining results in Table XLV. are from experiments on garden mould, 
gravel, saw-dust and paper ; also from those made by placing the bulb of a thermo- 
meter nine inches above wood, and protected from lateral wind by wood one foot 
high, at the distance of nine inches from the bulb all round ; and from those made by 
placing the bulb of a thermometer a quarter of an inch above water. Of these, paper 
exhibited the greatest cold; and from many unrecorded experiments, it always ex- 
hibited a great degree of cold. The next in order were saw-dust, garden mould, nine 
inches above wood, a quarter of an inch above water, and lastly gravel ; this last- 
mentioned substance apparently exhibited the lowest radiating power of any substance 
contained in this class ; but the circumstance of thermometers placed on garden 
mould and on gravel reading higher than when placed on other substances, was 
found to be in consequence of the observations having always been made on the 
ground, of which they formed a part, and the respective surfaces of which were 
readily supplied with heat from beneath, and thus prevented from exhibiting a great 
degree of cold from their situation and not from the nature of their substance. This 
was proved from the circumstance that small parcels of garden mould and gravel 
placed on the raised board, in which situation but little heat passed from the board 
to their surface, were found in a few nights to exhibit very low readings of thermo- 
meters placed on them. After having found this, I placed a thermometer with its bulb 
one inch below the surface of the garden mould, and at the same time another one 
inch below the surface of the ground under grass, and I found that the readings of the 
latter were usually some degrees higher than those of the former at times when the 
sky had been clear for some time, plainly indicating the cause of the higher readings 
on the surface of the mould to be the very ready passage of the heat from beneath 
to its surface ; and, therefore, as before observed, arising from its situation and not 
from a property inherent in itself. I made no experiments on the amount of heat 
thus conducted to the surface of the mould and of gravel, but it evidently must be 
equal, or very nearly so, to the amount radiated from those substances. I made 
some experiments on the quantity and on the rapidity of heat conducted upwards 
from one inch below the surface of the ground under grass ; these were made by 
placing thermometers with their bulbs one inch below the ground, on the surface of 
