RELATIVE HUMIDITY. 



225 



resented by descending lines. The left-hand marginal scales of both 

 are alike, and show the pressure of vapor from 0.1 inch to 1.9 inches of 

 mercury, which aqueous vapor at the different temperatures would sup- 

 port. 



It will be seen that the intervals between the lines in the right-hand 

 drawing widen apart from below upward in a very regular manner. 

 These intervals would, if measured and applied to a diagram, form a 

 curve much like those in the first drawing. 



The Relative Humidity, is the percentage of saturation, — 100 being 

 complete saturation, or all that the air will hold in invisible form, and 

 being absolute dryness, a condition never obtained in our climate with- 

 out artificial means. This also depends upon the temperature, and, with 

 a given difference between wet and dry bulbs, is greatest at high tem- 

 peratures. Calculated for degrees Fahrenheit, from — 30° to 100°, the 

 percentage at intervals of ten degrees, and from differences between wet 

 and dry bulb of the psychrometer, from 0° to 20°, are as follows : 



In this table, as in the preceding, the numbers increase toward the 

 right in every line except the first, and in every column they increase 

 upward from below. The gaining rate of the former is greatest at the 

 beginning, and, when reduced to a diagram, the curves are convex above 

 instead of being concave, as in the horizontal numbers in the jireceding 

 table, showing the rate at which the relative humidity diminishes with 

 an increase of heat. But, taking at a given temperature, the difference 

 between wet and dry bulb thermometers, from 1° to 20°, the quantities 

 in the vertical columns diminish rapidly as we follow them down, 

 the smaller differences at more rapid rates, but all upon the same gene- 

 ral law.^ 



iReduction-tables, based upon the formula) of Keguault, aad prepared by Prolessor 

 Arnold Guyot, have been furnished by the Smithsonian Institution to meteorological 

 observers throughout the country. The thermometers commonly used for psychrome- 

 trical observati ns have the centigrade scale. Other tables based upon the same for- 

 multe, but adapted to the Fahrenheit scale and to English inches of mercurial column, 

 prepared by the late Prof. James H. Cofhu of Lafayette College, Easton, Pennsylvania, 

 have also been issued, and from these latter, the two tables here presented have been 

 condensed. The formulie of Reguault are scarcely applicable to places considerably 

 above sea-level, and new tables adapted to greater altitudes are much needed, although 

 it may be possible to apply a correction that will render them comxiarable with those 

 taken at no great elevation above sea-level. 



15 F 



