METEOROLOGY. NIGHT FROSTS. 489 



certain seasons of the year. Before men were aware thai bodies on 

 the surface of the ground became colder than the air which sur- 

 rounds them during a clear night, the rationale of this practice was not 

 apparent ; for it was altogether impossible to conceive that coverings 

 so slight could protect vegetables from a low temperature of the air. 

 The means indicated, as simple as they are elTeciual in protecting 

 plants in the garden, are rarely applicable in farniing, where the 

 surface to be preserved is always very extensive. Nevertheless, in 

 severe winters, the frost by penetrating the ground would frequently 

 destroy the fields sown in autumn, were it not that in high latitudes 

 the snow which covers the surface becomes a powerful obstacle to 

 excessive cooling, by acting at one and the same time as a covering, 

 and as a screen preventing radiation. As a covering, because snow 

 is one of the worst of conductors, one of those substances which lor a 

 given thickness opposes the passage of heat most effectually ; it is, 

 therefore, an obstacle almost insurmountable to the earth beneath it 

 getting into equilibrium in point of temperature with the atmosphere. 

 As a screen, because in shelteririg the ground it prevents it from 

 undergoing the cooling which it would not fail to experience in clear 

 nights by radiation into the open firmament. It is familiarly known 

 in many parts of Europe, that the accidental want of the usual cov- 

 ering of snow will cause the loss of the autumn-sown crops of grain 

 It is on the surface of the snow that the great depression of temper- 

 ature takes place ; and the substance being a very bad conductor, 

 the earth cools in a much less degree. In the month of February, 

 1841, I made some experiments, which show that the snow which 

 covers the ground acts in the manner of a screen. I had first a 

 thermometer upc^n the snow, the bulb of the instrument being cover- 

 ed by from 0.078 to 0.1 17 of an inch of snow in powder ; second, a 

 thermometer, the bulb of which was situated completely under the 

 layer of snow in contact with the ground ; third, a thermometer 

 in the open air, at about 37 or 38 feet above the surface, on the north 

 of a building. The layer of snow was about four inches in thickness, 

 and had covered a field sown with wheat for a month. The sun 

 shone brightly upon the field on those days when my experiments 

 were made. 



Feb. 11. Five o'clock in the evening; the sun has been hidden 

 by the mountains for half an hour ; the sky is unclouded, the air very 

 calm : thermometer under the snow, 32"' F. ; thermometer upon tha 

 snow, 29° F. ; thermometer in the air, 36.3' F. 



Feb. 12. The night very fine, no clouds, the air calm. At seve.i 

 o'clock in the morning, the sun is not yet upon the field : thermom 

 eter under the snow, 26.2' F. ; thermometer upon the snow, 10" F. , 

 thermometer in the air, 26.3" F. 



At half-past five in the evening, the sun behind the mountains : 

 thermometer under the snow, 32° F, ; thermometer upon the snow, 

 29" F, ; thermometer in the air, 37.5° F. 



Feb. 13. At seven in the morning ; the sky gray, the air slightly 

 in motion : thermometer under the snow, 28° F. ; thermometei 

 upon the snow, 17° F. ; thermometer in the air, 25° F. 



