THE SOUTHERN PLANTER. 



501 



which immedia'elj surrounds the grass is 

 cooled by contact to the same degree. It 

 then-sinks down and another portion of air 

 comes in contact with the blade of grass, 

 .and is in its turn cooled to the same extent, , 

 and so on until all the air between the blades 

 is two degrees lower than that of the air 

 farther up. The radiation, however, con- 

 tinues, and a stratum of air from the mass 

 already cooled two degrees more, which 

 sinks down as before, and so on until the air 

 between the blades is cooled to 4° below its 

 siornial condition ; and iia this way the pro- 

 cess may be continued until the temperature 

 descends to 8° or 10° below that of the stra- 

 tum of air a few feet above. In this way 

 we can readily explain the small amount of 

 d.ew deposited on the tops of trees, since the 

 air, as soon as it is cooled, sinks down to- 

 ward the ground, and its place is continu- 

 ously supplied by new portions of the atmos- 

 phere« To the same cause, we may attri- 

 bute copious deposition of dew on wool and 

 other fibrous materials which, though they 

 do not radiate heat more freely into space, 

 they entangle and retain the air between 

 their fibres, and thus allow the cooling pro- 

 cess we have described to go on. It would 

 appear that spider-webs radiate heat freely 

 into space, since they are generally covered 

 with a large amount of dew ; their insulated 

 position prevents them from renewing their 

 heat, but, according to the above principle, 

 a much larger amount of deposition ought 

 to be produced by the same material, were 

 it loosely gathered up into a fibrous mass. 

 The fact of the screening influence of the 

 clouds teaches us that a thin cloth, or even 

 a slight gauze, supported horizootally over 

 tender plants, is sufficient to neutralize the 

 radiation, and to prevent injury from frost 

 during the clear nights of spring or autumn. 

 The same effect is produced by artificial 

 clouds of smoke. 



Since radiation from the surface of the 

 earth is most intense on clear nights, when 

 the moon is visible, many of the effects 

 which are due to this cause, have been re- 

 ferred to lunar influence; for example, a 

 piece of fresh meat exposed to the moon- 

 light, is said to become tainted in a few 

 hours ; this may arise from the deposition 

 of moisture on the surface of the meat due 

 to the cooling from radiation. The moon 

 itself, however, acts as a cloud and radiates 

 back to the earth a portion of the heat 

 which it received from the earth, as well 



as a portion of that which it received from 

 the sun ; and hence Sir John Herschel has 

 referred to this cause., with apparent prob- 

 ability, the origin of an assertion of the 

 sailors, that ''the moon eats up the clouds." 

 He supposes that they may be dissipated by 

 the radiant heat from that body, which, 

 being of low intensity and but feebly pene- 

 trating the lower stratum of the atmosphere, 

 may serve to dissipate the clouds. Though 

 a WTong explanation is generally given by 

 the popular observer of natural phenomena, 

 and though effects and causes are frequently 

 made to change places in his explanations, 

 yet it is true, as Biot has properly said, that 

 the scientist who devotes himself assidu- 

 ously to investigate the subject of popu- 

 lar errors, will find in them a sufficient 

 amount of truth to fully repay him for his 

 labour. 



Formation of Fogs.-— The difference be- 

 tween a fog and a cloud relates principally 

 to the conditions under w^hich the}' are 

 severally formed. A fog has been aptly 

 called a cloud resting on the earth, and a 

 cloud a fog suspended in the atmosphere. 

 The circumstances under which a fog is 

 usually produced are the following : Either 

 the surface of the earth or water is warmer 

 than the air, or it is cooler. If the tempera- 

 ture, of a river or of a damp portion of 

 ground is higher than that of the atmos- 

 phere which rests upon it, the warmer 

 surface will give off vapour of an elastic 

 force due to its temperature. Should the 

 superincumbent air be extremely dry, the 

 vapour .will diffuse itself up through it in 

 an invisible form, without condensation, 

 and no fog will be formed until, by the 

 continuation of the process the air becomes 

 completely saturated; and then if an ex- 

 cess of heat remain in the evaporating sur- 

 face, the fog will be produced, and will in- 

 crease io density and height so long as a 

 difference of temperatEire continues. If, 

 however, a wind be blowing at the time, so 

 that successive portions of unsaturated air 

 are brought over the place, no fog will be 

 produced. A still atmosphere, therefore, is 

 a necessary condition to the accumulation 

 of fog. 



The foregoing is the usual method in 

 which fog is produced, for it is well known 

 that in cold weather the surfaces of lakes 

 , ami rivers are much warmer than the strata 

 of air which rest upon them. 



