8i! VAPORIZATION. 



the greatest importance to deduce from them the dew point, or 

 the tension of the vapour in the air, by an easy rule. Could 

 this inference be made with certainty, the wet-bulb hygrometer 

 is so commodious that it would supersede all others. I shall 

 place below the formula of Dr. August, which after constant 

 application for the last ten years, has received the general 

 sanction of the philosophers of Berlin. It was employed by 

 Humboldt and G. Rose in their recent expedition to Siberia, and 

 (as I was assured by the latter) with excellent effect.* 



In evaporating by means of hot air, as in drying goods in the 

 ordinary bleachers 5 stove, which is heated by flues from a fire 

 carried along the floor, it should be kept in mind that a cer- 

 tain time must elapse before air is saturated with humidity. 

 Mr. Daniell has observed that a few cubic inches of dry air con- 



* Dr. August's formula for deducing the tension of vapour in the air from the 

 temperature indicated by a wet and dry thermometer : 



Let x = the tension of vapour in the atmosphere, expressed in Parisian lines, 

 to be found. 



e i tension of vapour at the temperature indicated by the wet thermo- 

 meter, in Parisian lines, taken from a table. 



t = the temperature of the dry thermometer, by Reaumur's scale. 

 t' = temperature of wet thermometer, by the same scale. 

 b = the height of the barometer in Parisian lines, the normal height being 

 336 lines. 

 Then, for temperatures above zero Reaumur, 



x e , _ 4 (**') 0.0011 (336 b}.(t t'} 

 For temperatures below zero Reaumur, 



x= e' |0 *') - 001 (336 *).(/*') 



This formula is very simple in its application, as will be seen by a particular 

 example. 



Professor Erman made the following observation, May 20, 1.827, 2 A.M. 

 Dry thermometer, 19. 1 Reaumur. 

 Wet thermometer, ll.l 

 Difference of temperature, 8 degrees. 



The tension of vapour at ll.l is 5.56 Parisian lines ; from which subtract f of 

 the difference of temperature, which in this case is the number 3.00. The subtrac- 

 tion gives 2.56 Parisian lines. But the barometer stood 2 lines higher than 336 ; 

 there is therefore 0.0022 X 8 = 0.02, to subtract from 2.56 ; which gives 2.54 

 Parisian lines as the tension of the vapour in the air at the time of observation. 

 The above formula are deduced from the expressions 



0.558 (tt')b 

 * = *- 572 -*' ' 



where 512 is the latent heat of vapour at Reaumur. (Ueber die Fortschritte 

 der Hygrometric, von Dr. E. F. August, Berlin 1830.) 



