52 PHYSICAL CONSERVATION AND RESTORATION. 
is still an immense extent of North American soil where the 
industry and the folly of man have as yet produced little 
appreciable change. Here, too, with the present increased 
facilities for scientific observation, the future effects, direct and 
contingent, of man’s labors, can be measured, and such precau- 
tions taken in those rural processes which we call improve- 
ments, as to mitigate evils, perhaps, in some degree, insepara- 
ble from every attempt to control the action of natural laws. 
In order to arrive at safe conclusions, we must first obtain 
a more exact knowledge of the topography, and of the pres- 
ent superficial and climatic condition of countries where the 
natural surface is as yet more or less unbroken. This can 
only be accomplished by accurate surveys, and by a great mul- 
tiplication of the points of meteorological registry,* already so 
numerous ; and as, moreover, considerable changes in the pro- 
portion of forest and of cultivated land, or of dry and wholly 
or partially submerged surface, will often take place within 
brief periods, it is highly desirable that the attention of 
observers, in whose neighborhood the clearing of the soil, or 
the drainage of lakes and swamps, or other great works of 
rural improvement, are going on or meditated, should be espe- 
cially drawn not only to revolutions in atmospheric tempera- 
* The general law of temperature is that it decreases as we ascend. But, 
in hilly regions, the law is reversed in cold, still weather, the cold air descend- 
ing, by reason of its greater gravity, into the valleys. If there be wind enough, 
however, to produce a disturbance and intermixture of higher and lower 
atmospheric strata, this exception to the general law does not take place. 
These facts have long been familiar to the common people of Switzerland and 
of New England, but their importance has not been sufficiently taken into 
account in the discussion of meteorological observations. The descent of the 
cold air and the rise of the warm affect the relative temperatures of hills and 
valleys to a much greater extent than has been usually supposed. A gentle- 
man well known to me kept a thermometrical record for nearly half a century, 
in a New England country town, at an elevation of at least 1,500 feet above 
the sea. During these years his thermometer never fell lower than 26°— 
Fahrenheit, while at the shire town of the county, situated in a basin one 
thousand feet lower, and only ten miles distant, as well as at other points in 
similar positions, the mercury froze several times in the same period. 
