CONCLUSIONS ON ANCIENT CLIMATES. 15 
partly by historical evidence, partly by analogical deduction from 
effects produced, in our own time, by operations similar in char- 
acter to those which must have taken place in more or less 
remote ages of human action. Both sources of information 
are alike defective in precision; the latter, for general reasons 
too obvious to require specification; the former, because the 
facts to which it bears testimony occurred before the habit or 
the means of rigorously scientific observation upon any branch 
of physical research, and especially upon climatic changes, 
existed. 
Uncertainty of our Historical Conclusions on Ancient 
Climates. 
The invention of measures of heat and of atmospheric mois- 
ture, pressure, and precipitation, is extremely recent. Henee, 
ancient physicists have left us no thermometric or barometric 
records, no tables of the fall, evaporation, and flow of waters, 
and even no accurate maps of coast lines and the course of rivers. 
Their notices of these phenomena are almost wholly confined to 
excessive and exceptional instances of high or of low tempera- 
tures, extraordinary falls of rain and snow, and unusual floods 
or droughts. Our knowledge of the meteorological condition 
of the earth, at any period more than two centuries before our 
own time, is derived from these imperfect details, from the 
vague statements of ancient historians and geographers in re- 
gard to the volume of rivers and the relative extent of forest 
and cultivated land, from the indications furnished by the history 
of the agriculture and rural economy of past generations, and 
from other almost purely casual sources of information.* 
* The subject of climatic change, with and without reference to human 
action as a cause, has been much Ciscussed by Moreau de Jonnés, Dureau de la 
Malle, Arago, Humboldt, Fuster, Gasparin, Becquerel, Schleiden, and many 
other writers in Europe, and by Noah Webster, Forry, Drake, and others in 
America. Fraas has endeavored to show, by the history of vegetation in 
Greece, not merely that clearing and cultivation have affected climate, but 
that change of climate has essentially modified the character of vegetable life. 
See his Klima und Pflanzenwelt in der Zeit. 
