W Meteorological Observations, [Jan, 



tnore moisture to it until it receives the maximum it can 

 hold at its actual temperature, or, by reducing its tempe- 

 rature to that point which corresponds to the moisture il 

 already contains. The latter is called the point of satu- 

 ration, or the dew point. — Suppose, for example, that a 

 cubic mass of air 40 inches every way at the temperature of 

 17^ contains 200 grains of moisture ; in order to render it 

 absolutely damp, it would be necessary either to add 19.4 

 grains of humidity, or to reduce its temperature to 15^ 

 The following then, are the problems that immediatel|r 

 present themselves to us, in this branch of meteorology ; 

 1st, to ascertain the actual quantity of moisture in the air; 

 2d, the point of deposition or dew point ; and 3d, th<^ 

 dryness of the air that is to say, the additional quantity of 

 moisture that would be required to render it perfectly 

 humid. 



Various methods have been proposed for estimating the 

 dryness, and of consequence the opposite humidity of the 

 air. I shall only take notice of the two which are in most 

 general use, viz. those of Daniel and of Leslie*. — Mr. 

 Daniel employs the evaporation of ether to produce a cold 

 sufficient to cause the deposition of moisture from the air, 

 and thus to detect the dew point. His hygrometer, or dew 

 point instrument, constructed on this principle, acts tolera- 

 bly well in the colder parts of Europe ; but many serious 

 objections apply to it, which render it totally useless in hot 

 and dry climates. Under all circumstances it is difficult to 

 ascertain the exact point of deposition ; and even when 

 this is done, with all the accuracy the instrument admits of, 

 it will generally be found to be too high, owing to an origi- 

 nal defect in its construction. In the hot climate of India, 

 there is always a great waste of ether by evaporation, and 

 it is impossible to procure it pure, unless from Europe. 

 But the most serious objection remains to be stated; 

 it frequently happens that the air is so excessively dry, 

 that the very best ether is not capable of reducing the 



* The different organic substances, such as hair, whale bone, ivory, &c. that 

 have been used as hygroscopes, being incapable of affording those exact indica- 

 tions, which the advanced state of the science now demands, have, 1 believe, been 

 universally discarded, except by a few Bleteorologipt^; od the continent ol Eu- 

 Tope. 



