Daniell's Meteorological Essays. 221 



indeed, that the course and velocity of the currents may be 

 considered, without any chance of disturbing our main argu- 

 ment, as unaltered. It will also be remarked, that while the 

 great aerial ocean is divided into two distinct strata, flowing in 

 opposite directions from south to north, the aqueous part, which 

 is nearly confined to the lower current, presses in a contrary 

 direction. The adjustment of these particulars remaining as 

 now supposed, the compensating winds flow on in the courses 

 which have been described, and the balance remains undis- 

 turbed." 



Although the general currents are so little affected by the 

 mere presence of aqueous vapour, the variations of tempera- 

 ture that are produced by its evaporation and condensation 

 will tend to produce changes in their direction ; these will be of 

 great importance, but modified by local circumstances. The 

 surface of the earth is not uniformly composed of one sub- 

 stance, but is partly covered by land, and partly by water. The 

 evaporation from surfaces of these two different natures will 

 be very different, and the changes of temperature arising from 

 this source must vary with every modification of local circum- 

 stances. The quantity of moisture, then, that the air of any par- 

 ticular place contains, will have an influence upon its climate ; 

 nor can any set of meteorological observations be complete, 

 when this is not employed as an element. The increase of 

 weight acquired by deliquescent salts, the tension of cords, the 

 shortening of whalebone, hair, and of some vegetable sub- 

 stances, have all been applied to this purpose, but with little 

 success. Aware of the great importance of this subject, Mr. 

 Daniell has planned and constructed a hygrometer that must, 

 when its merits shall be fairly appreciated, supersede all others. 

 Hygrometers, constructed of the substances we have already 

 mentioned, indicate merely the presence of different portions 

 of vapour, without affording any means of determining its abso- 

 lute quantity. Leslie has indeed proposed a modification of his 

 differential thermometer as a hygrometer; and observations 

 with this maybe applied to the tables of vapour existing in 

 space at a given temperature, as deduced from the experiments 

 of Dalton ; but his instrument, although beautiful and ingeni- 

 ous in the highest degree, is liable to objections from which Mr. 

 Daniell's is free. It requires abstruse calculations, and deli- 

 cate corrections, on the nature of which philosophers are by no 

 means agreed; and it has the disadvantage of having an arbi- 

 trary seale, instead of adopting one of those sanctioned by 

 usage in the common thermometer. 



I fa vessel containing a cold liquid be exposed to air of a tem- 

 perature considerably higher, it will be covered with a film of 



condensed 



