AT NIGHT, FROM THE EARTH, ETC. 
155 
sand. The lowest readings occurred on lamp-black ; the next in order were chalk, 
whiting, charcoal, black-lead and sand; a quantity of the last-mentioned substance 
was also placed on the ground to the depth of three inches, and to an extent of nine 
square feet ; the difference between the mean readings of thermometers placed on 
sand in these two positions is probably accidental, arising from the observations in 
the former case having been taken on very fine nights only. 
Building materials formed a fourth class ; these consisted of glass, stone, brick, 
pantile, wood and slate. The lowest readings occurred on glass, and therefore this 
substance exhibited the greatest radiating power ; the next in order were wood, slate, 
pantile, brick and stone successively ; the three last-named substances were nearly 
equal. 
Metals formed a fifth class; these consisted of tinfoil, lead, zinc, copper, iron, 
white tin, blackened tin, and the thermometer placed in the focus of a polished 
metallic reflector ; of these lead and zinc of different thicknesses were experimented 
upon, and the same results were obtained (see Table XLVIL), from which circum- 
stance it is inferred that the thickness of a metal has no effect on its radiating power. 
The metals were all placed on grass, and from the circumstance of the ready passage 
of heat from one part of a metal to another, much heat must have passed from the 
earth to their upper surface when so placed. The lowest readings of this class were 
those of the thermometer in the focus of the reflector ; the next in order, and very 
nearly equal to it in amount, was copper, then blackened tin, lead, zinc, iron and 
white tin. Two of these metals, viz. lead and zinc, were placed at different distances 
from the earth, the plane of each sheet being parallel to the horizon in all cases, and 
in every individual experiment the same result was obtained as that exhibited in the 
means, viz. that a metal when raised above the ground, though only by one inch, 
was always warmer than one on the grass, though in the latter case, from its great 
conducting power for heat, some must have been received from the earth, whilst in 
the former case, heat only could have been derived from the air flowing past both its 
under as well as its upper surface. 
The temperature of metals as exhibited in these results, contrary to what was 
expected, is much below that of the surrounding air ; this is particularly the case 
with copper ; this metal radiates heat so freely, that with respect to its amount, 
it would be placed in the second class of substances. Many simultaneous expe- 
riments were made with copper when placed on one of the angles of the box, within 
which was placed the thermometer in the parabolic reflector, with the readings of 
the latter thermometer ; thus situated, the thermometer in the box was protected 
from lateral wind, whilst that on the copper was subject to the passing air ; and thus 
situated the readings on the copper were lower than those in the reflector, except at 
times when the air was passing quickly. 
Whilst speaking of metals, I may remark here that I never found the reading of a 
thermometer placed on a metal which had been moved successively from one part of 
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