1^35.] Meteorological Observations. 



51 



temperature sufficiently law to reach the dew point. This 

 I have repeatedly found to be the case in Egypt^ and the 

 same thing has been remarked in India. 



Sir John Leslie's method consists in determining the 

 difference of temperature between a dry thermometer, and 

 one having its ball moistened with pure water ; and by 

 means of the law of atmospheric solution discovered by 

 %im, and the formula he has founded thereupon, all the 

 parts of the problem I have proposed above, may be easily 

 and exactly determined.* He has constructed a hygrometer 

 upon this principle, by applying it to his differential ther- 

 ■mometer ; but this has nothing to recommend it, in pre- 

 ference to the arrangement of the two thermometers, which 

 possess, the great advantages of being easily procurable, 

 cheapness, and portability. By this method nothing more is 

 l-equired than two delicate thermometers, with centigrade 

 pr millesimal scales, and one of them having its ball covered 

 with muslin or silk, which being wetted with distilled water^ 

 the depression below the other is noted. The form of the 

 thermometer I have recommended for the cethrioscope and 

 photometer, will be found most convenient for the hygrome- 

 ter, for when placed upon a table, the covered ball, when 

 once wetted, may be kept constantly moist by means of 

 some loose cotton threads, passing over it from a small bot- 

 tle filled with water, f 



Leslie's table of the quantities of moisture in the 

 state of vapour, contained in atmospheric air at different 

 temperatures, is calculated for the centigrade scale. To ren- 

 der it more useful, I have enterpolated it for every tenth part 



* Vide Leslie on boat and moisture, and the article meteorology in the sup- 

 plement to the Encyclopaedia Britannica. 



The accuracy of Leslie's formula has been called in question:; and I have no 

 doubt it requires revision. I feel confident, however, from;, some experiments 

 I have made in reference to this subject, but which it would be out of place to 

 detail here, that it affords results, which are seldom far from the truth, and are 

 always capable of comparison. Moreover, the great facility of its applicatioa 

 gives it a decided advantage over other methods that have been proposed. 



I do not pretend to pronounce any opinion upon the theoretical views of Leslie 

 regarding the nature of heat, or the manner in which water is maintained in the 

 state of vapour in the air. 



. t All these thermometers might have the same screw under the ball so that in 

 the event of one of them being broken, its place might be easily supplied. Thus 

 ahjgrometer might be converted into a photometer by removing the muslin or 

 silk, and sere vinjj on a small glass cover over iU bail previously biackeiiedj aud 

 "vice versa. 



