OF NATURAL PHILOSOPHY. 163 



to them by the similar radiation of others surround- 

 ing them. Clouds and surrounding objects therefore 

 act as opposing causes by replacing the whole or a 

 great part of the heat so radiated away, which can 

 escape effectually, without being replaced, only 

 through openings into infinite space. Thus, at 

 length, we arrive at the general proximate cause of 

 dew, in the cooling of the dewed surface by radia- 

 tion faster than its heat can be restored to it, 

 by communication with the ground, or by counter- 

 radiation ; so as to become colder than the air, and 

 thereby to cause a condensation of its moisture. 



(168.) We have purposely selected this theory 

 of dew, first developed by the late Dr. Wells, as one 

 of the most beautiful specimens we can call to mind 

 of inductive experimental enquiry lying within a 

 moderate compass. It is not possible in so brief a 

 space to do it justice; but we earnestly recommend 

 his work* (a short and very entertaining one) for 

 perusal to the student of natural philosophy, as a 

 model with which he will do well to become fa- 

 miliar. 



(169.) In the analysis above given, the formation 

 of dew is referred to two more general phenomena ; 

 the radiation of heat, and the condensation of in- 

 visible vapour by cold. The cause of the former 

 is a much higher enquiry, and may be said, indeed, 

 to be totally unknown ; that of the latter actually 

 forms a most important branch of physical enquiry. 

 In such a case, when we reason upwards till we reach 

 an ultimate fact, we regard a phenomenon as fully ex- 

 plained ; as we consider the branch of a tree to 



* Wells on Dew. 

 M 2 



