the absolute dryness, and the 

 weight in grains of a cubic foot 

 of air, tables have been con- 

 structed empirically from experi- 

 ments at Greenwich, combined 

 with Regnault's tables of Vapor 

 Tension, for the use of which we 

 are indebted to the courtesy of 

 MESSRS. QUEEN & Co., Phila- 

 delphia. 



If the air be very dry, the dif- 

 ference between the two. ther- 

 mometers will be great ; if moist, 

 less in proportion, and when fully 

 saturated, both will be alike. For 

 different purposes, different de- 

 grees of humidity are required, 

 and even in household use, that 

 hygrometrical condition of the 

 atmosphere most beneficial to one 

 person, may frequently be found altogether unsuit- 

 table for another. " Dry" bulb 70 and "wet" 

 bulb 62 to 64 indicate average healthful hygrom- 

 etrical conditions ; any other relative condition re- 

 quired may easily be found by experiment, and 

 then, dispensing with calculations, or reference to 

 tables, it is only necessary to see that the two ther- 

 mometers stand in the required relation to each 

 other. 



The price of Mason's Hygrometer ranges from 

 $2.00 to $17.50. 



MASON'S HYGROM- 

 ETER. 



122 



