AT NIGHT, FROM THE EARTH, ETC. 
211 
The numbers in the column of mean excess exhibit the mean difference between 
the reading of a self-registering minimum thermometer placed in air, at the height of 
four feet, and protected as much as possible from the effects of radiation, and the 
readings of similar thermomefers placed as stated in the first column. The first re- 
sults are those relating to grass; that deduced from grass exposed to three-fourths of 
the sky, is about three-fourths of that deduced from grass exposed to the whole of 
the sky ; and that from long grass is larger than that from short grass. Of all the 
substances upon which experiments have been made with spirit thermometers, those 
on which the lowest readings have occurred were the filamentous, viz. wool and flax, 
and they were nearly alike. 
The next class of bodies consisted of metals ; of these lead exhibited the lowest 
readings in the mean ; but this must have arisen from the circumstance of its having 
been generally used on the clearest nights only, during which there was found to be 
a very small difference between its readings and that of the thermometer in the re- 
flector; in fact in all simultaneous observations it was found that the readings of 
the latter thermometer had no advantage over the readings of one placed on lead. 
The thermometers which were placed within a few inches of the top of grass ex- 
hibited higher readings than those of the last class, and those which were from one 
to three inches from the top of grass, generally read the same as that in the reflector. 
The reading which most nearly agreed with that in air was on garden mould, 
which was about a mean between that on long grass and that in the air ; these ob- 
servations were made on ground undisturbed ; the readings would have been still 
nearer those in the air, had the ground been frequently disturbed so as to have been 
loose, as it was found in this state to admit the heat to pass more readily from beneath 
to the surface. 
The numbers in the next column represent the relative radiating power of the 
several substances deduced by considering the mean result from long grass to be 
represented by 1000. 
The following columns of the table represent the mean results for each substance 
on nights wholly cloudy, but the clouds of different heights. This result is very im- 
portant, as it shows that the amount of radiation may be large on a wholly cloudy 
night, providing that the clouds be high. The differences between the results on a 
cloudy night when the clouds are high and when they are low, are very great ; the 
numbers in the Table exhibit the mean difference between the results deduced when 
the clouds have been high, and when moderately low. 
The last column shows that on a cloudy night, providing the kind of cloud be other 
than cirrostratus, the amount of radiation may nearly equal that on a cloudless night. 
At a very early stage in the investigation it was found that the variation in the 
height of the clouds had a very considerable effect on the reading of a thermometer 
placed on any substance fully exposed to the sky, during those nights that the sky 
was wholly covered with an apparent uniform cirrostratus cloud. 
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