268 REPORT OF STATE BOARD OP HORTICULTURE. 



but the results have not been, as a rule, satisfactory, although the smoke 

 was equally dense. However, quite successful results were obtained by Mr. 

 Buck, Mr. La Rue, and others in the Vacaville and Sonoma sections by 

 burning damp stable manure in sacks scattered through the orchard. 



After considering the question, the writer was convinced that the protec- 

 tion of the northern wheat fields must be due to something besides the 

 checking of radiation by the cloud of smoke, for the heat which is radiated 

 from the earth to the cloud is absorbed by the cloud and not reflected. Con- 

 sequently, unless the air is almost perfectly calm to a considerable elevation, 

 the heat is carried away as the smoke drifts off with the wind before much 

 is radiated by the cloud of smoke back to the earth. 



Damj) smudge fuel preferable — It was observed that, as a rule, whenever 

 damp fuel was used, the efforts at protecting were more successful than at 

 other times. In the case of dry smudge material, the heat of the fire raises 

 the temperatui'e of the air about the burning fuel to hundreds of degrees 

 above the surrounding air. It is consequently greatly expanded, and its 

 density diminished so that it rapidly rises and the neighboring air flows in 

 to take its place. This also is, in turn, heated and escapes upward, carrying 

 the heat and smoke of the fire with it. On reaching an elevation consider- 

 ably above the trees, it is blown away more or less rapidly by the almost 

 constant circulation of air at such an elevation. 



When damp fuel is used, a considerable portion of the heat of the fire is 

 expended in evaporating the water in the fuel, and the consequent upward 

 draft of the fire is lessened by this amount. 



The amount of heat consumed in evaporating water is very considerable. 

 The evaporation of a quart of water would necessitate the expenditure of 

 as much energy as would be needed in raising the temperature of the air 

 25° throughout a space ten feet square and deep. 



It is evident from this fact that the upward draft, which was so marked in 

 the case of dry fuel, is greatly diminished when damp fuel is used ; conse- 

 quently, the smoke remains nearer the surface, where the trees interfere 

 with the movement of the wind and tend to retain the smoke, thus increasing 

 the protection. 



Further, Tyndall has proven that vapor itself, even while invisible, acts 

 as a barrier in retarding radiation, and if it condenses rapidly enough it 

 will cause the small particles of water thus left suspended in the air to form 

 a cloud or fog, which will obscure the sky and prevent radiation. 



However, by far- the principal cause of the protection obtained from the 

 wet smudge properly belongs to the second class of methods, namely : 



RAISING THE DEW-POINT. 



By adding moisture to the air and thus making sensible the latent heat of con- 

 densation at a temperature above the danger point — As mentioned above, when 

 damp fuel is used a considerable portion of the heat produced by the fire is 

 expended in evaporating the water in the fuel. The vapor thus formed is 

 invisible and has all the properties of gas, and quickly distributes itself 

 throughout the surrounding space almost as rapidly as air will expand to fill 



