48 

 II. 



PSYCHROMETRICAL TABLES. 



GIVING IMMEDIATELY THE FORCE OF AQUEOUS VAPOR AND THE RELATIVE HUMIDITY 

 FROM THE INDICATIONS OF THE PSYCHROMETER. 



CALCULATED BY M. T. HAECHENS. 



IN his Etudes sur VHygrometrie,* M. V. Regnault discusses the theoretical bases of 

 the formula of the Psychrometer, given by M. August, which was, 



x f Q.S63C-'') ^ 



in which h represents the height of the barometer ; t the temperature of the air given 

 by the dry-bulb thermometer ; t 1 the temperature of the wet-bulb thermometer ; f the 

 force of aqueous vapor in the saturated air at a temperature equal to t 1 ; x the elastic 

 force of aqueous vapor which exists in the air at the time of the observation. 



After having modified some of the numerical values, which form the coefficients, 

 M. Regnault adopted this formula, 



X 



But comparative experiments, made by himself, showed that by substituting the 

 coefficient 0.480 for that of 0.429, the calculated results, and those obtained by direct 

 observation, agree perfectly in the fractions of saturation, which are greater than 0.40. 

 This formula thus modified, or 



x =f - 4S0 ^_-/> h, 



has been used for calculating the following tables. In that part of the tables which 

 supposes the wet-bulb to be covered with a film of ice, or below the freezing point, 

 the value 610 t', which represents the latent heat of aqueous vapor, has been 

 changed into this : 6 10 + 79 t 1 = 689 t'. 



The only hypothesis made, is that of a mean barometric pressure /&, equal to 755 

 millimetres. If we take into account the causes of errors inherent to the psychrom- 

 eter, and to the tables of the force of vapor, by means of which the absolute force of 

 vapor is calculated, as well as to the differences of these tensions, taken at tempera- 

 tures differing only by one tenth of a degree, it will be obvious that the correction due 

 to the variations of barometric pressure can almost always be neglected. Neverthe- 

 less, a separate table has been calculated, giving the correction to be applied to the 

 numbers in the Psychrometrical Tables for the heights of the barometer between 650 

 and 800 millimetres. It will be found at the end of the tables. 



The disposition of the tables is the following : 



The temperatures are noted in centigrade degrees ; the elastic force of vapor in the 

 air, or its pressure on the barometer, is expressed in millimetres of mercury ; the rel- 



* Etudes sur FHygrometrie, par M. V. Regnault. Annales de C/iimie et de Physique^ 3 me Serie, Tom 

 XV., 1845. 



B 12 



