54 PHYSIOGRAPHY [ch. hi. 



so common a phenomenon as the formation of dew was 

 thoroughly understood. Observations on the subject had 

 indeed been made at a much earlier date, but it remained 

 for Dr. W. C. Wells, an American settled in this country, to 

 undertake a systematic inquiry into the conditions under 

 which dew is deposited. After much patient investi- 

 gation he published, in 1814, his famous essay entitled 

 TJu ITieory of Dezi', and the simple explanation which he 

 set forth in this essay has been corroborated by subsequent 

 investigators. 



Atmospheric moisture is precipitated not only as rain and 

 dew — the particular forms which have been studied in this 

 chapter — but also occasionally as snow and hoar-frost ; 

 the formation of these will form the subject of the next 

 chapter. 



