I 



284 



Meteorology. 



[Sect. IX. 



depend 



jena on -v 



_ X 



ery different principles. That which we 

 recommend in preference to, and as having almost univer- 

 sally superseded in practice every other,* consists of two 

 thermometers, the bulb of the one being dry and of the 

 other wet ; being kept so by a covering of muslin, con- 

 nected by a wet roll of cotton (lamp wick) with a small 

 Clip of distilled or rain-water placed close beneath it, so 

 as to absorb and communicate water by capillary attrac- 

 tion. In frosts this arrann;eruent Is unavailing^, and water 



must be poured on the muslin envelope, and allowed to 

 freeze into a coat of ice, from which evaporation will still 

 go on and depress the temperature, as if still liquid. They 

 should be placed and observed in such locality as shall 



afford the best chance for procuring a fair indication of 



the moisture of the general atmosphere, and by no means 

 in any confined or ill-ventilated situation between decks, 

 where many persons habitually congregate, or which, from 

 any other cause, is usually or periodically damp The 

 whole instrument should be protected with a cap of wire- 

 work to defend it from injury : this, if it interfere with 



hould be removed a quarter of an hour 

 before the observation. In reading the thermometers begin 

 with the dry one, and use all the precautions recommended 

 with respect to the ^^ external '' thermometer. Enter the 

 simple readings, but at the head of each column place the 

 zero correction (with its proper sign) required for its ther- 

 raom^eter fa general rule for all thermometric entries), and 



the readings, s 



\ 



■ The hair hygrometer is delicate, exceedingly liable to derangement, 

 and, except prepared with extraordinary care, uncertain. DanielFs dew- 

 point hygrometer, excellent in theory, is very costly on account of its 

 great consumption of ether, and scarcely useable in hot climates, owing 

 to the difficulty of preserving that liquid. 



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