HYGROMETRY. 



591 



Of lilt 



This Table affords the meant of knowing by inspec- 

 tj O n the relation between the degrees of Saussure's hy- 

 grometer, and the density of the vapour to which the 

 instrument is exposed ; and, consequently, it enables 

 us to determine the absolute quantity of moist it re con- 

 tained in a Riven portion of air, the temperature of 

 which is 50* Fahrenheit. All that u necessary for this 

 purpose is, to multiply .00246714, the weight in grains 

 of the moisture in a cubic inch of vapour at the tempe- 

 rature of 5O*, by the relative tension opposite the de- 

 grees of the hygrometer, and the product will be the 

 weight in grains of the moisture in a cubic inch of air. 

 Thus, let it be required to determine the quantity of 

 moisture in a cubic inch of air, when the barometer 



stands at 30 inches, the thermometer at 50*, and Saus- Hygrome- 

 stire's hygrometer at 77. "T- 



The tension corresponding to 77 being .5674, if '"""""Y" 1 '' 

 we multiply .00246714 by this number, we obtain 

 .00139985, as the weight in the fractional part of a 

 grain, of the moisture in a cubic inch of air, in the 

 supposed state of things. 



b4. As the formula? by which the preceding Table Extension 

 was constructed, are neither very easily deduced, nor of the Table 

 capable of being applied without some degree of diffi- to ot ' 1 ' r 

 culty, it is natural to enquire how far the results ob- te 

 tained for the temperature of 50 are applicable to other 

 temperatures. At first sight, one would be apt to con- 

 clude, that since, in all cases, the hygrometer stands at 

 100 when it is exposed to vapour at the maximum ten- 

 sion for the temperature, whatever the temperature may 

 be ; at inferior tensions it would, in like manner, al- 

 ways point to the same degree, when the tensions have 

 the same relation to the maximum tension of their re- 

 spective temperatures. This is certainly the most na- 

 tural conclusion ; and if it were well founded, it would 

 be easy to determine, with the assistance of the Table, 

 the absolute quantity of moisture in a given volume of 

 air tor any temperature, in the same manner as has al- 

 ready been done for the temperature of 50. There is 

 reason to suspect, however, that the affinity of the hair 

 for moisture is somewhat modified by temperature; 

 and that the hyperbola, which connects the tensions of 

 the vapour with the degrees of the hygrometer, under- 

 goes a corresponding change with regard to the rela- 

 tions of its co-ordinates. That something analogous to 

 this takes place might have been inferred from the ex- 

 periments of Saussure, described in j 74, by which it 

 appears, that the indications of the instrument have a 

 different relation, at different temperatures, to the rela- 

 tive densities of the vapour at these temperatures ; and 

 Biot mentions a fact which lewis to the same conclu- 

 sion. 



The results of the Table, therefore, can only be ex- 

 tended to other temperatures by approximation ; but 

 until additional experiments shall have determined the 

 necessary corrections, the tensions corresponding to the 

 degrees of the hygrometric scale for the temperature of 

 5O, may be applied to other temperatures, without lead- 

 ing, we may presume, to any great error. To ascer- 

 tain how far the results obtained by this extended use 

 of the Table coincided with the quantity of moisture in 

 the atmosphere, as determined by our formula in $ 63, 

 we made the following observations, 'which we have 

 arranged in a tabular form, in order that the coinci- 

 dence may be more distinctly perceived. 



The two last columns contain the weight in fraction- 

 al part* of a grain of the moisture in a cubic inch of the 

 ir at the time the observations were made ; the first hav- 

 ing been derived by our formula for a thermometer with 



a moistened bulb, and the last by Riot's table for Saus- 

 sure'a hygrometer. It is proper to remark, however, 

 in reference to these results,, that the degrees of Saus- 

 sure's hygrometer were obtained, not by actual inspec- 



