Dcdo and Hoar- Frost. i^S 



I shall conclude by repeating my own explanation in a few 

 words. In ordinary states of the atmosphere, the air is more 

 dense towards the surface of the earth, and, in consequence of 

 the refraction thereby produced, distant elevated objects appear 

 higher than they really are ; but, in the case under consider- 

 ation, a stratum of heated, and, consequently, rarefied air, rest- 

 ing on the surface of the ground, produces an effect the very 

 reverse : distant elevated objects are seen lower than they are, 

 by an eye placed near but below the upper surface of the rare- 

 fied stratum, while from this situation of the eye they will at 

 the same time be seen nearly in their true places. 



Observations on Dew and Hoar-Frosty principally with re- 

 ference to a paragraph in III. 562.— If we leave solution and 

 precipitation out of the question, and admit that water exists 

 in the solid, liquid, and aeriform states, in consequence of 

 being combined with a less or greater quantity of heat, we 

 shall remove some of the difficulties stated by J. M. Having 

 thus premised, we may expect, from analogy, that, if the heat 

 thus necessary to retain water in its aeriform state be abstracted, 

 it will reassume its liquid form, and, if more heat continue to 

 be withdrawn, it will take the solid state, and freeze. This, in 

 fact, we find to be the case. A greater or less quantity of 

 aqueous vapour is always mixed with the air of the atmosphere ; 

 if, therefore, a body at a temperature considerably lower than 

 that of the atmosphere be exposed to the air, it will abstract 

 heat from the air and vapour in contact with it, till at last the 

 latter, being deprived of that portion requisite to sustain it in 

 the gaseous state, will deposit itself on the body in the form 

 of dew. The degree just necessary to produce deposition is 

 termed the dew point. If the temperature is sufficiently low, 

 the dew will freeze and become hoar-frost. 



From this it follows that dew, speaking of it as a condens- 

 ation of aqueous vapour, may be formed at any temperature 

 under the boiling point ; but, in the more common accept- 

 ation of the word, dew will not be formed except when the 

 surface of the ground is at a considerably lower temperature 

 than the air immediately over it; and this will rarely be the 

 case except when the atmosphere is calm and clear, and the 

 sun is absent. For though all bodies on the earth's surface 

 are constantly throwing off their heat by radiation, yet, while 

 the sun, the great source of heat, is shining, they will re- 

 ceive more than they radiate; and when the sky is- overcast, 

 the heat radiated by the clouds will prevent their cooling 

 down much below the atmosphere : but a cloudless sky 

 radiates little or no heat towards the earth, consequently 

 bodies on its surface exposed to this clear sky nwist neces- 



G G 3 



