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
fallacious, and therefore worse than useless, because some condition
necessary to secure accuracy of result was neglected, in obtaining 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;
[Footnote: 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 aqueous vapor within ten feet of the earth's
surface.--Fragments of Science, 3d edition, 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 Tyndall'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 material life are performed. They
give the rise and fall of the mercury at heights arbitrarily taken,
without reference to the relations of temperature to human interests, or
to any other scientific consideration than a somewhat less liability to
accidental disturbance.] and when we are told that the annual fall of
rain on the roof of the observatory at Paris is two inches less than on