162 DISCOURSE ON THE STUDY 



the free passage of heat from the skin into the air, 

 so as to allow their outer surfaces to be very cold 

 while they remain warm within. 



(166.) Lastly, among the negative instances, 

 ( 150.) it is observed, that dew is never copiously 

 deposited in situations much screened from the open 

 sky, and not at all in a cloudy night; but if the 

 clouds withdraw, even for a few minutes, and leave 

 a clear opening, a deposition of dew presently begins, 

 and goes on increasing. Here, then, a cause is dis- 

 tinctly pointed out by its antecedence to the effect 

 in question ( 145.). A clear view of the cloudless 

 sky, then, is an essential condition, or, which comes 

 to the same thing, clouds or surrounding objects act 

 as opposing causes. This is so much the case, that 

 dew formed in clear intervals will often even eva- 

 porate again when the sky becomes thickly overcast 

 (Rule 4. 150.). 



(167.) When we now come to assemble these partial 

 inductions so as to raise from them a general con- 

 clusion, we consider, 1st, That all the conclusions we 

 have come to have a reference to that first general 

 fact the cooling of the exposed surface of the body 

 dewed below the temperature of the air. Those 

 surfaces which part with their heat outwards most 

 readily, and have it supplied from within most 

 slowly, will, of course, become coldest if there be an 

 opportunity for their heat to escape, and not be 

 restored to them from without. Now, a clear sky af- 

 fords such an opportunity. It is a law well known 

 to those who are conversant with the nature of heat, 

 that heat is constantly escaping from all bodies in 

 ;uys, or by radiation, but is as constantly restored 



