488 meteorol:>gy. — night cooling. 



plains which are sufficiently elevated to hare a mean temperature 

 of from 50° to 58° Fahr. are exposed to suffer from frost ; it fre- 

 auently happens that a crop of wheat, barley, maize, or potatoes, of 

 the richest appearance, is destroyed in a single night by the effect 

 of radiation. In Europe during the fine nights of April and May, 

 when the air is calm and the sky clear, buds, leaves, and young 

 shoots are frequently cut off, are frozen ; in a word, although a ther- 

 mometer in the air indicates several degrees above the point of con- 

 gelation. Market gardeners and others who are much exposed to 

 loss from this cause, ascribe the effect to the light of the mc in of the 

 months of April and May ; and they ground their opinion upon the 

 fact that when the sky is clouded, the destructive effects of frost are 

 not apparent, although the same temperature of the atmosphere be 

 indicated by the thermometer. 



In the lower ranges of the Cordilleras, farmers also ascribe the 

 same injurious consequences to the light of the moon, wuth this dif- 

 ference, that according to them the destructive influence continues 

 throughout the year ; and it is not unworthy of remark that, in the 

 neighborhoods of Paris and of London, the mean temperature of the 

 months of April and May (from 50° to 57°, or 58° F.) represents ex- 

 actly the invariable climate of those places among the Andes, where 

 the effects of frost upon vegetation are particularly to be apprehend- 

 ed. M. Arago has shown, that the cold ascribed to the light of the 

 moon is nothing but a consequence of the nocturnal radiation, at a 

 season when the thermometer in the air is frequently at from 40° to 

 43° F. and the sky is clear and calm. At this temperature a plant, 

 radiating into space, readily falls below the point of congelation, 

 and then the hopes of the gardener and farmer are destroyed. The 

 phenomenon takes place particularly in a bright night : and if the 

 moon happen to be up when it occurs, the influence is ascribed by 

 the vulgar to her light. Were the sky clouded, the principal con- 

 dition to radiation would be wanting ; the temperature of objects on 

 the surface of the ground would not fall below that of the surround- 

 ing medium, and plants would not freeze unless the air itself fell to 

 32° F. 



The observation of gardeners, therefore, as M. Arago remarks, 

 was not in itself false, it was only incomplete. If the freezing of 

 the soft and delicate parts of vegetables in circumstances when the 

 air is several degrees above the freezing point, be really due to the 

 escape of caloric into planetary space, it must happen that a screen 

 placed above a radiating body, so as to mask a portion of the heav- 

 ens, will either prevent or at least diminish the amount of the cooling. 

 And that this takes place in fact, appears from the beautiful experi- 

 ments of Dr. Wells. A thermometer, placed upon a plank of a 

 certain thickness, and raised about a yard above the ground, oc- 

 casionally indicates in clear and calm weather from 6° to T or 8° F. 

 ess than a second thermometer attached to the lower surface of 

 he plank. It is in this way that we explain the use of mats, of 

 ayers of straw, in a word, of all those slight coverings which gar- 

 deners are so careful to supply during the night to delicate plamts ai 



