MR. W. N. SHAW ON' HYOROMKTRir METHODS. 



119 



TABLE II. Air agitated by a Cardboard Fun. 



Some experiments to determine the effect of radiation upon a wet bulb are then 

 detailed. A wet-bulb thermometer was observed alternately in the sun's rays and in 

 shadow, in two positions in the same room a short distance apart. The air surrounding 

 the bulb was kept in motion for each of the positions ; the bulb in the sun gave, a 

 mean temperature of 20 '66 C., and in the shade 20 '14 C., the temperature of the air 

 being 25 '25 C. From this observation a calculation is made that the effect of radia- 

 tion upon a wet bulb under ordinary circumstances would not exceed '2 C. 



BELLI therefore considered that a satisfactory observation with a wet bulb that 

 could be used as a basis of calculation could be obtained, provided that the bulb is 

 small, not greater than 7 mm. in diameter, and that the air in its neighbourhood is 

 kept in a state of brisk agitation. 



These conditions being satisfied, sets of tables might be formed, each table corre- 

 sponding to a particular pressure, and showing, for that pressure, the pressure of 

 uqueous vapour for a given temperature of the air, and a given difference between the 

 wet and dry bulb. 



BELLI then proceeds to calculate a formula for the psychrometer, which comes out 

 for the pressure 760 mm. : 



e = E' - -592 (T - T'). 



This agrees remarkably well with AUGUST'S formula, e = E' 0'0007832(T T') P, 

 for the same value of the air pressure.* 



Some comparisons of results obtained by this formula are given. They are not very 

 consistent as they stand., but the tables used in their reduction have been considerably 



* See below, p. 124. 



