16 M'Keevor's Voyage to Hudson's Bay. 



be several degrees above the freezing point. The effect is, na 

 doubt, considerably hastened by the cold produced by evapo- 

 ration from the surface. This, however, exerts a very limited 

 influence ; for, after it has arrived at a certain tempe- 

 rature, the " shell of air" which comes in contact with it, 

 before making any ascensional effort, will discharge a quantity 

 of its surplus heat, and thus preserve the temperature of the 

 fluid within certain limits. Something similar to this may be 

 observed while passing through the fields of a cool evening. 

 We often observe dew, or hoar-frost, deposited on twigs, grass, 

 and other substances, though the air, even a few inches above 

 their surface, is several degrees above temperature. With re- 

 gard to the uses which this singular phenomenon may serve : 

 perhaps, in consequence of the heat extricated during the 

 process of congelation, it may thus prevent the temperature of 

 the numerous organized bodies, contained in those situations, 

 from being reduced to a degree which would be incompa- 

 tible with their healthy functions. It will thus render those 

 substances the same services as the deposition of dew and hoar- 

 frost does to the tender plants, the caloric, given out during 

 these processes, preserving them from the cool air which is so 

 prevalent during our summer evenings. 



It will in all probability be objected to the suggestions I 

 have here thrown out, that they are in direct opposition to the 

 observations of Mr. Lesslie, who asserts, that when the Can- 

 nister Reflector and Differential Thermometer were plunged 

 into water, that no radiation can be observed; and hence this 

 ingenious philosopher concludes, that no radiation will take 

 place, except when the radiating body is surrounded with an 

 elastic medium. I may remark, however, that the experiments 

 which he adduces in support of this opinion, are by no means 

 decisive of the point. Substances cool so rapidly, when 

 plunged into water, that there is scarcely time for the differen- 

 tial thermometer to be affected ; and, besides, the heat could 

 scarcely accumulate in the foral-ball, in such quantity as to 

 occasion a sensible rise. Moreover, I can see no reason what- 

 ever why radiant caloric should not pass through water as well 

 as air. They are both fluids ; they receive and transmit slow 

 communicating caloric in a precisely similar way, namely, by 

 a constant recession or migration of heated particles ; they 

 agree in many of their chemical relations, such as exterior 

 solvent power, &c. ; they also agree in possessing elasticity ; 

 though water is by no means susceptible of the same degree 

 of condensation as air ; still, however, that it possesses this 

 property, in a slight degree, is obvious, from the common 

 amusement in which boys indulge, of projecting a stone forci- 



