UNCERTAINTY OF MODERN METEOROLOGY. 21 



Uncertainty of Modern Meteorology. 



"We are very imperfectly acquainted with tlie present mean 

 and extreme temperature, or tlie precipitation and the evapora- 

 tion of any extensive region, even in countries most densely peo- 

 pled 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 fallacious, 

 and therefore worse than useless, because some condition neces- 

 sary 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 ; * and when we ai*e told that the annual fall of rain on 

 the roof of the observatory at Paris is two inches less than on the 

 ground by the side of it, we may see that the height of the rain- 

 trees not now growing in the same localities, are found in abundance. Every 

 generation 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 atmos- 

 phere 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 cli- 

 mate. See pp. 363-4, 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 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 himg 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 ia 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 perf oi-med. 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. 



