JANUARY. 29 



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Radiation. — The production of frost may be traced to the radiation of 

 heat from the earth. By radiation is meant the power possessed by bodies 

 on the surface of the earth, of sending forth their heat into space. It ap- 

 pears to be the means provided by nature for parting with the superfluous 

 portion of tiie sun's heat. Not only the surface of the ground has this 

 power of radiation, but the leaf that flutters in the breeze, and the atmos- 

 phere itself, have more or less of the same power. This process goes on 

 equally by night and by day, in the breeze and in calm ; but it is rendered 

 sensible chiefly on a clear calm night. It seems to proceed much at the 

 same rate, whether the thermometer be at 80° or 32°. These cold-produc- 

 ing rays (if the term be allowable) are obstructed in their course by the in- 

 tervention of opake bodies. Hence when the sky is densely covered with 

 clouds, radiation ceases. 



Such is the power of unobstructed radiation, that were it not for the sun's 

 heat, accumulated during the day in the soil and given out to moderate the 

 temperature by night, the thermometer would probably sink to zero every 

 clear calm night. This natural reservoir of heat is, therefore, the great 

 moderator of frost. Very severe frosts occur when the ground is deeply 

 covered with snow, and the heat of the soil is thereby kept in, and its diffu- 

 sion into the atmosphere prevented. A thermometer placed by the writer 

 on the snow in his garden, on the night of January 2d 1854, sunk 2° below 

 zero : while another placed on a small patch of ground which was cleared 

 of snow, only two or three feet distant, did not sink below 16°. [Hence the 

 earth is warmer and the atmosphere colder, when snow lies on the ground, 

 than when it is bare.] 



It has often been observed that frost is most severe on clear, calm nights, 

 and that the degree of cold depends much on local circumstances. As a 

 general rule, it may be stated to be the effect of radiation. [When the sky 

 is overcast with cl^ds, this canopy of vapor operates in the same way as a 

 tin cover placed oto" any heated substance. The heat of the earth which 

 is radiated into the atmosphere is shut in by the clouds and prevented from 

 escaping into the upper air. While this canopy of clouds remains over our 

 heads we are, as it were, in a heated room, the heat being supplied by radi- 

 ation from the warm ground or floor. Hence on a cloudy evening there is 

 seldom any precipitation of dew, and the grass is often as dry as on a bright 

 summer morn. — Ed.] 



It may be observed that the air near the surface of the earth is colder 

 than at three feet above it ; and the cold air sinks down into valleys, obeying 

 slowly the same laws which regulate the motion of the heavier fluid water. 

 A thermometer placed in the bottom of an artificial valley or basin of nine 

 inches deep, and three or four feet wide, well protected by wool or any non- 

 conducting substance from the heat of the soil, will sink 25° or 50° below 

 the temperature of the surrounding air, from simple exposure to the clear 

 sky on a calm night. — Gardeners^ Chron. 1854. 



