UNCERTAINTY OF DATA. 
15 
a long series of generations; here, improvidence, wastefulness, 
and wanton violence; there, foresight and wisely guided per¬ 
severing industry. So far as they are purely the calculated 
and desired results of those simple and familiar operations of 
agriculture and of social life which are as universal as civil¬ 
ization—the removal of the forests which covered the soil 
required for the cultivation of edible fruits, the drying of here 
and there a few acres too moist for profitable husbandry, by 
draining off the surface waters, the substitution of domesti¬ 
cated and nutritious for wild and unprofitable vegetable 
growths, the construction of roads and canals and artificial 
harbors—they belong to the sphere of rural, commercial, and 
political economy more properly than to geography, and 
hence are but incidentally embraced within the range of our 
present inquiries, which concern physical, not financial bal¬ 
ances. I propose to examine only the greater, more perma¬ 
nent, and more comprehensive mutations which man has pro¬ 
duced, and is producing, in earth, sea, and sky, sometimes, 
indeed, with conscious purpose, but for the most part, as 
unforseen though natural consequences of acts performed for 
narrower and more immediate ends. 
The exact measurement of the geographical changes hith¬ 
erto thus effected is, as I have hinted, impracticable, and we 
possess, in relation to them, the means of only qualitative, not 
quantitative analysis. The fact of such revolutions is estab¬ 
lished partly by historical evidence, partly by analogical 
deduction from effects produced in our own time by opera¬ 
tions similar in character 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 espe¬ 
cially upon climatic changes, existed. 
