588 



HYGROMETRY. 



Hygrotne . 

 try. 



Hygrome- 



The indi- 

 cations of 

 hygrome- 

 ters relative, 

 not abso- 

 lute. Ne- 

 cessity of 

 attending to 

 tempera- 

 ture and 

 pressure. 



The difference between the point of deposition de- 

 termined by the formula, and the mean minimum tem- 

 perature for each month, of the particular year in which 

 these observations were made, scarcely ever exceeds 

 half a degree, except in April. The great difference in 

 that month seems to be owing, partly to the copious 

 discharge of moisture from the atmosphere, during the 

 two preceding months, the rain for February and 

 March amounting, by Mr Gordon's meteorological 

 journal, to 3.997 or nearly 4 inches ; and partly to the 

 unusual depression of temperature, for the season, dur- 

 ing the last of these months. 



71. It may be proper to remark, in reference to the 

 above results, that the indications of an hygrometer, of 

 whatever kind, even when they are numerically the 

 same, may imply very different portions of moisture in 

 the air. Thus the hygrometer in April and May stood 

 at 23, though the absolute quantities of moisture in the 

 atmosphere, during these two months, were extremely 

 different; a proof that the reports of hygrometrical ob- 

 servations only tend to mislead, when the temperature 

 and pressure of the air at the time they were made, are 

 omitted. Hence also we can perceive the reason why 

 no hygrometer can have its scale accurately graduated 

 by exposing it to a particular temperature, in order to 

 obtain the point of extreme dryness. 



grometry 

 to small 

 portions of 

 gaseous 

 fluids. 



Appiica- 72. If the researches of hygrometry were confined to 

 tion of hy- the atmosphere, the methods which we have already de- 

 uTomptrv scr ihed for detecting the quantity of moisture contain- 

 ed in a given portion of it, would be sufficient for all 

 the purposes of meteorology ; but these methods are 

 scarcely applicable to small quantities of aerial fluids, 

 the hygroscopic state of which it is often necessary to 

 determine, in order to conduct with accuracy their che- 

 mical analysis. This branch of the subject is of consi- 

 derable importance to the chemist ; and indeed, with- 

 out some knowledge of it, he cannot investigate with 

 success the properties of aerial fluids, which demand 

 much delicacy of research. 



73. Saussure was perhaps the first person who en- 

 *7saussure. deavoured to ascertain, in a systematic and philosophi- 

 cal manner, the relation between the degrees of the hy. 



Researches 



grometer, and the quantity of moisture in determinate 

 portions of air, at different temperatures ; but though 

 he prosecuted his inquiries with much ingenuity and 

 care, the methods he employed to obtain a solution of 

 the problem did not admit of the same degree of accu 

 racy as those which were afterwards employed by Gay 

 Lussac. The instrument, however, which he invented 

 for these researches, was admirably adapted for the pur- 

 pose, as the extreme tenuity of the substance used in 

 the construction of it, rendered it peculiarly fit for being 

 introduced into small portions of air, without affecting, 

 in any sensible degree, the hygroscopic state of the sur- 

 rounding medium, by the moisture it absorbed. 



74. Having inclosed his hygrometer in a vessel con- 

 taining air previously dried by caustic alkali, Saussure 

 introduced under the receiver, at successive intervals, 

 small quantities of water, by means of bits of moisten- 

 ed linen ; and after allowing these to remain a sufficient 

 length of time, he again withdrew them, and determi- 

 ned the loss they had sustained by evaporation, obser- 

 ving at the same time the progress of the hygrometer, 

 for each additional portion of moisture. He performed 

 this experiment at various temperatures, and found that 

 at the same temperature the index uniformly stood at 

 the same point, when the quantity of moisture evapo- 

 rated was the same. Among other results, he found 

 that a French cubic foot of air at 15. 16 Reaumur took 

 up, in the form of vapour, 11.069 grains French of 

 moisture, expanding at the same time -% of its original 

 volume ; and that the same quantity of air, at the tem- 

 perature 6. 18 Reaumur, was able to hold in the vapo- 

 rous state 5.65 grains French. If we reduce these re- 

 sults to English measures, the former would be .004339 1 > 

 and the latter .0022123 grains in a cubic inch, for the 

 respective temperatures 66. 11 and 45.91 Fahrenheit; 

 differing little from the results given by our formula in 

 39, as is easily ascertained by comparing them with 

 the quantities of moisture for the corresponding tempe- 

 ratures in the Table. In both cases, the hygrometer 

 stood at 98. The quantity of moisture answering to 

 other divisions on the scale of the instrument were pro- 

 portional to the numbers in the following Table, in 

 which complete saturation for each temperature is ex- 

 pressed by unity. 



