630 



extensive in the town of Colmar, where a smoke department has developed 

 and has been well organized ever since 1884. Colmar lies on a plain and 

 the danger from frost is greater on plains than in higher regions, as was 

 shown, for example, in 1903 in frosts in Florence. Here Pas'serini^ found 

 fruit trees and asparagus greatly injured at an elevation of 40 m. above 

 sea level, but perfectly healthy 100 m. higher. In Colmar iron carts were 

 used, which contained possibly 16 litres of fluid tar. After the tar had 

 been ignited they were drawn back and forth over one field and then taken 

 to the next place (possibly 150 m. distant). When the temperature fell to 

 1 degree above zero, the smoke department was notified and, at a tempera- 

 ture of zero degrees, the signal for lighting was given by means of gunshots. 

 As a rule, this began in the night between two and three o'clock. The very 

 heavy expense to which the administration was put, because of the smoke 

 department, was paid by a tax on the harvested grapes. 



W't have cited this special case because we believe that only such an 

 organization can have such sweeping results. 



Frost Prediction. 



On account of the expensiveness of producing smudges for the protec- 

 tion of plants, threatened by late frosts, it is naturally of the greatest im- 

 portance to be able to judge in advance approximately whether night frost 

 will occur. 



On this account, it is advisable to make use of the frost curve con- 

 structed by Lang (Miinich) (Cf. Fig. 150). This is based on psycho- 

 metric observations. If, in spring, the temperature falls in the afternoon 

 and the sky becomes clear, with a cessation of wind, the probability of night 

 frost increases. For the use of the figure, two exactly corresponding 

 thermometers are necessary. The mercury bulb of one is so wrapped in 

 gauze that the under end of the gauze dips into water, thus keeping the 

 cover of the ball moist. This thermometer, because of the constant evap- 

 oration of water, will stand lower than the one beside it showing the 

 ordinary air temperature. From the difference between these temperatures 

 the relative humidity and the position of the dew-point can be reckoned, 

 i. e., the temperature at which the water, contained in the air as dew, mist, 

 or rain, will be precipitated. In order, however, that these precipitations 

 of water vapor may become effective as a protective mantel against frost 

 danger produced by radiation, the formation of dew and mist must take 

 place at a temperature above zero; therefore, the point of condensation 

 must lie above zero. If this is not the case, and the air is dr}^, a night frost 

 may be expected. 



The mechanical manipulation will, therefore, be as follows : the height 

 of the dry thermometer is read first of all, then the difference between this 

 and the one with the moist mercury bulb is reckoned. The height of the 



1 Passerini. N., Sui danni prodotti alle piante dal ghiacciato dei giorni 19-20 

 April, 1903. Bull. soc. botan. ital. 1903, p. 308. 



