462 METEOROLOGY. 



just correction for the water wliicli falls, especially when there are very 

 light showers, which cannot be measured by the pluviometer ; whence 

 the evaporation given by the Gasi)arin atmometer during the months 

 when there has been rain will be always inferior to the reality. To 

 have, therefore, the true ratio between the indications of the two atmo- 

 meters, and to correct those of the Gaspariu instrument, we proceed in 

 the following manner : From the sums of evaporation, given by the two 

 instruments in days when there was no rain, is deduced the ratio be- 

 tween the two evaporations for each month, and from the evaporation 

 observed with the atmometer of Vivenot, on the days of rain, is derived, 

 by means of the ratio, the corrections to be made in the indications of 

 that of Gasparin. 



In the following table will be found the monthly snras of this atmo- 

 meter thus corrected, and which ditfer from those given in the single 

 bulletins, because the aforesaid correction is then not applied. Hence, 

 to determine a formula representing the evaporation according to the 

 Gasparin atmometer, it is to be remembered that in this instrument the 

 action of the force of the wind must needs be greater than for that of 

 Vivenot, and that it is necessary, moreover, to take into account the di- 

 rect action of the solar rays, as we have not the temperature of the 

 water by that atmometer. We will sui^pose this action of the solar rays 

 proportional to the sine of the meridian altitude of the sun, for the effect 

 of the solar rays may be modified by the presence of clouds, diminishing 

 that action whenever they interpose between the sun and station. On 

 the other hand, the action of the daily temperature and the humidity 

 will, for this instrument, be the same as for that of Vivenot ; whence 

 the coefficients relative to these two elements will remain the same. If, 

 then, we calculate the part of the evaporation due to these two elements 

 and subtract it from the daily evaporation of the Gasparin atmometer, 

 the remainders of evaporation will be due to the direct action of the 

 solar rays, the direct action of the wind, and the serenity. With these 

 remainders will be established the equations of condition from which 

 are to be found the coefficients, «, v, and /, relative to the sine of the 

 meridian altitude of the sun, the serenity, and force of the wind. 



In the table which follows will be found united all the necessary ele- 

 ments, that is to say, the mean value of the sine of the sun's altitude 

 for every month, which we will denote by sine h, the volume of the 

 clouds, the force of the wind, daily evaporation of the Gasparin instru- 

 ment, the total for every month, by both the atmometers, and their 

 relation ; finally, the remainders of that of Gasparin, calculated as has 

 been above stated. 



