x INTRODUCTION. 



regards the requisites for the formation of dew, as laid down by Dr. 

 Wells. Dew, according to that gentleman, is caused by the following 

 circumstances : 



1st. A fall of temperature in the stratum of air in contact with the 

 soil. 



2d. The soil must be previously heated, and the vapours rise, 

 so as to be encountered by a descending current, which brings the 

 vapours back to the earth without the air being saturated. 



3d. After sunset, when the weather is calm and serene, the soil 

 radiates heat, and its temperature descends several degrees below 

 that of the contiguous stratum of air; and this causes the deposition 

 to take place. 



4th. The fall of temperature always precedes the formation of 

 dew. 



5th. Dew is deposited more copiously on bad than on good con- 

 ductors of heat. 



According to our observations : 



1st. Dew is not accompanied or preceded by any fall of temperature 

 near the surface. 



2d. The deposition even takes place before sunset, and very fre- 

 quently during a brisk breeze, the sky being clear, and stars visible. 



3d. The temperature never descends or changes, and in all cases 

 has been found two or three degrees above the contiguous air. 



4th. There is no fall of temperature in the stratum of air in contact 

 with the ocean. 



5th. All substances, whether good or bad conductors of heat, are 

 equally covered with dew or the deposition. 



It will thus be seen that the requisites for the formation of the 

 phenomenon of dew, according to Dr. Wells's theory, are totally 

 wanting in the production of the same phenomenon when it came 

 under our observation. 



It is not my wish to controvert or deny the truth of Dr. Wells's 

 beautiful theory, so long received as the true solution of dew, but it is 

 plainly deducible from the facts stated, that dew is otherwise formed, 

 or that which we have witnessed is a totally distinct phenomenon, as 

 far as its formation goes, or that there are two distinct and nearly 

 opposite modes in nature of producing the same result. The theory 

 of the formation of dew by Dr. Hutton appears to me to account for 

 the phenomenon as witnessed by us, and to establish that the same 

 causes produce it as go to the formation of the mists before referred 



