AT NIGHT, PROM THE EARTH, ETC. 
159 
the surface of the ground immediately beneath it by 21 0, 3, and less than that one 
inch below the surface by 27°'5 ; and the reading of that on short grass was less than 
that on the ground immediately beneath it by 10 o, 3, and less than that one inch be- 
neath the surface of the ground by 17°T. The mean rate of decrease of temperature 
at one inch beneath the surface of the ground under long grass was 0 o, 2 per hour, and 
under short grass was 0 o- 3 per hour ; and on the surface of the ground under short 
grass was 0 o, 4 per hour. The difference between the readings at one inch beneath 
the surface under long grass and under short grass was 4 0, 9, the former being the 
higher by this amount ; the difference between the readings on the surface of the 
soil under long grass and under short grass was 5 0, 4, that under long grass being the 
higher of the two ; and the difference of readings on long grass and on short grass 
was 5°*4, that on long grass being the lower of the two ; and thus the reading on 
long grass was as much less than that on short grass as that under long exceeded 
that under short : this circumstance affords a sufficient reason for the temperature of 
short grass being warmer than that of long grass, the heat passing so much more 
freely from the earth to it than in the case of long grass. 
1 shall conclude this part of this section, with remarking that the various amounts 
of dew deposited, at the same time, on different bodies at night, were found to be, as 
near as could be determined, proportional to the amounts of the depression of their 
temperature below that of the dew-point. Hence, it is evident that all hygrometers 
formed of any of these substances, or of any animal or vegetable substance, when 
exposed to the clear sky at night, will be cooled by the radiation of their heat, and 
will cool the air in contact with them ; and thus indicate a greater degree of humidity 
than actually exists ; and particularly so, should their temperatures descend below 
that of the dew-point, and dew be actually deposited upon them. 
The following table contains the results of special simultaneous observations, 
made in some cases in consequence of the results as deduced from the ordinary ob- 
servations not agreeing with each other, and in others to determine the amount of 
the correction due to the placing of the different substances on the raised board. 
