41 



HY GEOMETRICAL TABLES. 



HYGROMETERS, or instruments used for determining the amount of aqueous vapor 

 present in the air, are of three classes. In the first, we find the hygrometers 

 based on the absorption of moisture by hygroscopic substances, the best of which is 

 Saussure's Hair-Hygrometer ; in the second class, the Psychrometer, or wet-bulb 

 thermometer, which gives the temperature of evaporation ; in the third, the various 

 instruments designed for ascertaining the temperature of the dew-point. From the 

 data furnished by each of these instruments, and a table of the elastic forces of vapor 

 at different temperatures, the humidity of the air can be deduced with more or less 

 accuracy. 



The use of the hygroscopic substances as hygrometers having been nearly given 

 up on account of the inaccuracy of the results, the variability of the instruments, and 

 the difficulty, if not impossibility, of making them comparable, the psychrometer and 

 the dew-point instruments represent the two methods now usually employed in 

 Meteorology. The following set, therefore, contains extensive tables, in French and 

 English measures, for deducing the hygrometrical condition of the atmosphere from 

 the indications of the Psychrometer and of the dew-point instruments, to which have 

 been added tables of the weight of vapor, in a given space, at different temperatures, 

 an element often needed in Meteorology. 



As, however, the results deduced from the same data furnished by the observations 

 may considerably differ, according to the values of the elastic force of vapor, arid the 

 formulae used in the computation, the tables have been arranged in two series. 



The first series contains Regnault's table of the elastic forces of vapor, with tables 

 of the three kinds above mentioned, together with a corresponding set in English 

 measures. Tables V. to X. have been computed for this volume. 



The second series gives the table of elastic forces of vapor deduced from Dalton's 

 experiments, and adopted in the Greenwich Observations, together with the various 

 tables based on it. 



B 5 



