AT NIGHT, FROM THE EARTH, ETC. 
195 
readings mentioned in this section ; and this reading was nearer and nearer to that 
in the air in proportion to the looseness of the mould ; the heat from beneath pass- 
ing the more rapidly to the surface in proportion to its looseness. It was also 
observed that the temperature about the roots of plants was lower in exact amount 
to the excess of heat conducted to the surface. 
It must be borne in mind that the differences exhibited between the minimum 
readings of self-registering thermometers placed upon substances and fully exposed 
to the sky, and those of a similar thermometer placed in the air, and protected as 
much as possible from the sky, are not the maximum differences, unless the two 
minima occur at the same time ; this generally is not the case ; that of a thermometer 
placed on any substance and exposed to the sky usually occurs in the evening, or 
before midnight ; whilst that in the air occurs at about the time of the rising of the 
sun. The difference between these two minima should be increased by the difference 
between the readings of the thermometer in the air at those times ; this may amount 
to 10° ; and it is highly probable that during a portion of the evening or night, which 
precedes the reading of the instruments, at times when 20° are exhibited between the 
minima, a difference of 30° has existed. 
From the preceding remarks it is evident that the differences between the readings 
of the thermometer in air and the others, have varied with every variation of the 
amount of cloud ; and by comparing the numbers in one table with those in another, 
it will be found that this difference is about the same in amount, with the same 
quantity of cloud at all times of the year. 
This will be more clearly seen in the next table containing the monthly means of 
all the numbers contained in Tables LI. to XCVIIL of the results of the observations 
of the thermometer in the reflector. 
2 c 2 
