176 

 TABLE XIII. 



FACTORS FOR COMPUTING THE FORCE OF VAPOR, FROM THE READINGS OF THE 

 PSYCHROMETER, BV APJOHN's FORMULA. 



Dr. Apjohn's formula for deducing the force of vapor, and the temperature of the 

 dew-point, from the readings of the Psychrometer as given in the Proceedings of the 

 Royal Irish Academy for 1840, is 



f // f / _1 y A 



J ~ -I 88 -^ 30' 



when the readings of the wet-bulb thermometer are above 32° Fahr., in which formula 



j" = the force of vapor at the temperature of the dew-point in degrees of Fahr., 

 f' = the force of vapor at the temperature of evaporation given by the wet-bulb 



thermometer, 

 d = the difference between the readings of the dry and wet thermometers, 

 h 1= the height of the barometer in English inches at the time of the observation. 

 When the readings of the wet-bulb thermometer are below 32° Fahr., and the bulb 

 is covered with ice, the formula becomes 



f/l fl ^ V A 



J ~J 96 ^ 30' 



The factors in the following table, which is taken from the Greenwich Observa- 

 tions for 1843, represent gs X g^ and ^ x g^-, computed for all differences between 

 the wet and dry bulb thermometers, or values of d, from 0° to 21°. 



Use of the Table. 



To find out the force of vapor in the air, and the temperature of the dew-point, by 

 means of these factors, let the factor corresponding to d, or the difference between 

 the wet and dr}'' thermometer in the first column, be multiplied mto the observed 

 neight of the barometer, and subtract the result from the force of vapor, in Table 

 XL, due to the temperature of evaporation- indicated by the wet-bulb thermomf-ter ; 

 the rest is the force of vapor in the air at the time of the observation; and the tem- 

 perature of the dew-point is that which i« due to it in Table XI. 



Example. 

 The observation gives. 



Dry-bulb thermometer = 79° Fahr., or the temperature of the air. 

 Wet-bulb " := 69° " or temperature of evaporation. 



Difference "lO^ 



Height of barometer 29.7 English inches. 



In the Table, 2d part, is found, — factor for a difference of 10° = 0.00379 X 29.7, 

 or height of barometer = 0.113, which, subtracted from the force of vapor due to 

 69°, in Table XI., = 0.704 — 0.113, gives force of vapor in the air = 0.591 inches, 

 and temperature of the dew-point 62°. 5. 



When the temperature of the wet bulb is beloiv 32° Fahrenheit, the factors in the 

 firi5t part of the Table must be used. 



B 140 



