194 ESSAY 



small body, the bulb of a thermometer, possess- 

 ing the temperature of the atmosphere, is placed 

 before a larger cold body, rendered equal in 

 effect to one still larger, by means of a concave 

 metallic mirror. In this situation, the small 

 body radiates heat to the larger, without re- 

 ceiving an equivalent from it, and, in conse- 

 quence, becomes colder than the air through 

 which its heat is sent, notwithstanding that it 

 is continually gaining some heat, both from the 

 air which surrounds it, and from the walls and 

 contents of the apartment, in which the experi- 

 ment is made. Dew, therefore, would as readily 

 form upon the thermometer in this experiment, 

 as it would upon one suspended in the open air 

 at night, under a clear sky, provided that the 

 two instruments were equally colder than the 

 atmosphere, and that this was in both cases 

 equally near to being replete with moisture *. 



* The invention of this experiment having been ascribed 

 a few years ago to Mr. Pictet of Geneva, various English 

 writers have shown, that it occurs in several much older 

 foreign authors. But I have not seen any mention made of 

 its having been also long since known in this country. That 

 it was so appears from the following extract of a letter, 

 written by Mr. Oldenburgh to Mr. Boyle in 1665. " I met 

 the other day in the Astrological Discourse of Sir Christopher 

 Heydon, with an experiment, which he affirms to have tried 

 himself, importing, that cold accompanies reflected light, by 

 employing burning spherical concaves, or parabolical sections, 



