The Botany of Iceland - 01.12.1930, Page 184

174
H. M0LHOLM HANSEN
monthly mean temperatures. The table confirms the rule previously
laid down that the amplitude decreases with the distance from the
surface, and further that it is greatest for the sunny open soil, less
tor the wooded soil. This applies in still greater degree to the mean
temperature of the warmest month and to the annual mean tempera-
ture. The sequence is as follows: grassfield—» birchwood^- firwood.
From a knowledge of the daily and annual variations in tempe-
rature it is possible to determine the heat-conducting power of a
rock, this quantity being the less the slower the maximum and
minimum spread in the soil, or the quicker the amplitude decreases
downwards.
After Hann (1926, p. 800) we give the following values for a
series of natural rocks (minute, cm.).
Sandstone.............................. 1.39
Clay with an admixture of sand......... 0.82
Sandy soil............................. 0.52
Finnish granite........................ 1.14
Heath................................ 0.32
Boggy soil............................. 0.13
Loose snow............................. 0.16
Compact snow........................... 0.24
Ice.................................... 0.68
Frozen earth........................... 0.56
Earth not frozen....................... 0.32
However, Ihe heat-conducting power is not only different for
the different rocks, it also varies in the same rock according to the
air and water-content. The more solid and moist a rock is, the
better is its heat-conducting power.