AT NIGHT, FROM THE EARTH, ETC. 
149 
the soil, over that conducted to the surface of the former, and not from the greater 
quantity of heat radiated into space from the long, over that radiated from the short ; 
such being the case, it was to be expected that the readings of a thermometer would 
vary with every variation of the length of grass upon which it was placed, and such 
was found to be the case. 
In fact, the readings of thermometers placed on grass were found to differ with 
every variation of length, fineness and closeness of its blades. My experiments 
have been made on that differing only in the length of its blades, and the differences 
arising from this cause were found to vary with every variation of the excess of the 
temperature of the air over that of long grass. The following are the mean results 
of experiments in this respect : — 
between 3° and 7° 
■between 7° and 10° 
, between 10° and 19° 
grass was J 
These differences were found to correspond to a mean temperature of the air of 44°, 
and that of long grass of 36°, or generally to all temperatures above 30° ; but when the 
reading of long grass declined below 30°, that of short grass was found not to decline 
nearly so rapidly: investigatingthe temperatures at readings below 30°, the following 
are the mean results : — 
When the excess of air- 
temperature at the height of 
4 feet above that of loner ( 
X 
the reading of the ther- "1 
mometer on short grass was 1 Vo.o 1*^ ** 
higher than that on long grass rom 
by 
O 0, 8 from 125 comparisons. 
comparisons. 
1°'9 from 86 comparisons. 
That when the reading on long grass 
was between 
'25 and 30") C2' 0 from 10 
20 and 25 2 4 from 31 
15 and 20 .. , , . , , , 3 - 3 from 19 , 
10 and 15 r exceeded that 011 sllort S rass b 7 - 4 - 8 from 14 > 
5 and 10 | 6'4 from 9 | 
0 and 5^ f_9'4 from 2 J 
From another investigation, it appeared that these differences were connected with the 
difference between the readings of the thermometer in air and that on long grass as 
follows : — 
CS 
a. 
a 
o 
O 
When the read- 1 at and above 30° f the excess of the'l one-eighth part, fof the excess of the reading 
ing of the thermo- ! at 20° J reading of the thermo- I one-fourth part, J of the thermometer in air at 4 
meter on long grass f at 10° 1 meter on short grass f one-half, | feet high, above that on long 
was J at 0° Iwas J the whole, Igrass. 
Hence it appears that at the reading of 0° of the thermometer on long grass, the reading 
of a thermometer on short grass is as much higher than 0°, as the reading of the ther- 
mometer in air at the height of 4 feet is higher than 0° ; or that the readings on short 
grass and that in air are alike, and therefore that the heat conducted from the earth 
equals in amount the heat lost by radiation ; and that for readings on long grass in- 
creasing from 0° in an arithmetical progression with a common difference of 10°, 
the difference between the readings on long and on short grass becomes less in geo- 
metrical progression with a common ratio of one-half ; till attaining a reading of 30° 
the difference equals the fourth term of the series, and this difference continues 
very nearly constant till 60°, above which temperature I have had but few expe- 
riments. If we call the excess of air-temperature above that of long grass tempera- 
ture by e, then the excess of the readings on short grass above those on long grass at 
