xlii INTRODUCTION. 



Example : 



Giv^en / = io°4 C ; /, = 8f3 C. and B = 740 mm. , to find the vapor pressure 

 and dew-point. 



Table 43, with the argument /, = 8.°3 C, gives/, =8.15 mm. 



Table 44, with t — t^=2°i and B = 740 as arguments, gives 1.03 mm. as 

 the value of the last term of the expression for/. Hence we have the vapor 

 pressure, /= 8.15— 1.03 = 7. 12 mm. The value of the temperature in Table 

 40, corresponding to this vapor pressure, is the dew-point d = 6.°3 C. 



Table 45 . Re la live h u m idity — Tempera hi re Cen iigra de . 



Table 45 gives the relative humidity of the air in hundredths, having 

 given the air temperature t and the dew-point d in Centigrade degrees. 

 It is computed by the formula 



Relative humidity = ^, 



/ and F being the maximum pressures of aqueous vapor corresponding to 

 the temperatures d and / as given in Tables 36 and 43. 



The top argument is the dew-point d, extending by 5° intervals from 

 -15" to 30° C. 



The side argument is the depression of the dew-point t — d, given for 

 ever\' o^a C. from o.°o to io°o ; for every o.°5 from io°o to 20^0, and for every 

 1° from 2o.°o to 30^0. 



Example : 



Given the air temperature 21° C. and the dew-point 17° C, to determine 

 the relative humidity. 



With / — flf = 4° C. for the side argument, and d — 1"]° C. for the top 

 argument, the table gives 78 per cent as the relative humidity. 



REDUCTION OF SNOWFAI^L MEASUREMENT. 



• The determination of the water equivalent of snowfall has usually been 

 made by one of two methods : (a) by dividing the depth of snow by an 

 arbitrary factor ranging from 8 to 16 for snow of different degrees of com- 

 pactness ; (<^) by melting the snow and measuring the depth of the 

 resulting water. The first of these methods has always been recognized as 

 incapable of giving reHable results, and the second, although much more 

 accurate, is still open to objection. After extended experience in the 

 trial of both these methods, it has been found that the most accurate and 

 most convenient measurement is that of weighing the collected snow, 

 and then converting the weight into depth in inches. The method is 

 equally applicable whether the snow as it falls is caught in the gage, or a 

 section of the fallen snow is taken by collecting it in an inverted gage. 



