22 UNCERTAINTY OF MODERN METEOROLOGY. 
Uncertainty of Modern Meteorology. 
We are very imperfectly acquainted with the present mean 
and extreme temperature, or the precipitation and the evapora- 
tion of any extensive region, even in countries most densely 
peopled and best supplied with instruments and observers. 
The progress of science is constantly detecting errors of method 
in older observations, and many laboriously constructed tables 
of meteorological phenomena are now thrown aside as falla- 
cious, and therefore worse than useless, because some condition 
necessary to secure accuracy of result was neglected, in obtain- 
ing and recording the data on which they were founded. 
To take a familiar instance: it is but recently that attention 
has been drawn to the great influence of slight differences in 
station upon the results of observations of temperature and 
precipitation. Two thermometers hung but a few hundred 
yards from each other differ not unfrequently five, sometimes 
even ten degrees in their readings;* and when we are told 
Analogous changes occur slowly and almost imperceptibly even in sponta- 
neous vegetation. In the peat mosses of Denmark, Scotch firs and other trees 
not now growing in the same localities, are found in abundance. Every gen- 
eration of trees leaves the soil in a different state from that in which it found 
it; every tree that springs up in a group of trees of another species than its 
own, grows under different influences of light and shade and atmosphere from 
its predecessors. Hence the succession of crops, which occurs in all natural 
forests, seems to be due rather to changes of condition than of climate. See 
chapter iil., post. 
* Tyndall, in a lecture on Radiation, expresses the opinion that from ten to 
fifteen per cent. of the heat radiated from the earth is absorbed by aquecus 
vapor within ten feet of the earth’s surface.—Hvragments of Science, 34 edi- 
tion, London, 1871, p. 203. 
Thermometers at most meteorological stations, when not suspended at 
points regulated by the mere personal convenience of the observer, are hung 
from 20 to 40 feet above the ground. In such positions they are less exposed 
to disturbance from the action of surrounding bodies than at a lower level, 
and their indications are consequently more uniform; but according to Tyn- 
dall’s views they do not mark the temperature of the atmospheric stratum in 
which nearly all the vegetables useful to man, except forest trees, bud and 
blossom and ripen, and in which a vast majority of the ordinary operations of 
