14 CONCLTJSIOlSrS on AT^CIEITT CLIMATES. 



and every traveller, every lover of rural scenery, every agricnl- 

 turist, who will wisely use the gift of sight, may add valuable- 

 contributions to the common stock of knowledge on a subject 

 which, as I hope to convince my readers, though long neglected 

 and now inartificially presented, is not only a very important but 

 a very interesting field of inquiry. 



Measurement of Man's Influence. 



The exact measurement of the geographical and climatic 

 changes hitherto effected by man is impracticable, and we pos- 

 sess, in relation to them, the means of only qualitative, not quan- 

 titative, analysis. The fact of such revolutions is estabhshed 

 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 re- 

 mote 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 Gli- 



mates. 



The invention of measures of heat and of atmospheric moist- 

 m-e, pressure and precipitation, is extremely recent. Hence, an- 

 cient physicists have left us no thermometric or barometric rec- 

 ords, 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 whoUy confined to 

 excessive and exceptional instances of high or of low tempera- 

 tures, to extraordinary falls of rain and snow, and to unusual 

 floods or droughts. Our knowledge of the meteorological condi- 

 tion of the earth, at any period more than two centuries before 



science, though attended with some highly beneficial results, has impeded the 

 progress of Physical Geography by discouraging its pursuit as unworthy of 

 cultivation because incapable of precise results. 



