THE HYGROMETER. 179 



pected manner air is rendered dry, notwitlistanding the operations of 

 syringing, steaming, &o. Daniell's hygrometer is constructed with 

 reference to these considerations. The figure represents two hollow glass 

 balls, containing ether, and communicating by the glass tabe which 

 rests on the support. The ball which forms the termination of the 

 longer leg is of black glass, in order that the formation of dew on its 

 surface may be the more perceptible ; it includes the bulb of a delicate 

 thermometer, dipping in the ether, its scale being inclosed in the tube 

 above the ball ; and whatever change takes place in the temperature of 

 the ether is indicated by this thermometer. The other ball is covered 

 with muslin. In making an observation, it is first necessary to note 

 down the temperature of the air, next to turn the instrument so that when 

 the muslin-covered ball is held in the hand the ether may escape into 

 the blackened ball ; and it should also be held tiU the included ther- 

 mometer rises a few degrees above the temperature of the air, when it 

 should be replaced on the support. Then drop, or gentiy pour, a little 

 ether on the muslin ; the evaporation of this extremely volatile sub- 

 stance produces cold, and attention must be instantiy directed to the 

 black glass ball and included thermometer ; the latter will be seen 

 falling rapidly, and at length a ring of dew will appear at the line 

 which runs across the black ball, — quickly if the air is very moist, 

 slowly if the air is dry. If the air is in a very dry state no moisture 

 will be thus deposited tDl the thermometer falls to perhaps 10°, 20°, or 30° 

 below the temperature of the air ; but at whatever temperature the dew 

 forms that temperature should be noted as the dew-point and the differ- 

 ence between it and the temperature of the air at the time is the degree of 

 dryness according to the indications of this hygrometer; thus, in a 

 moderately dry day, let it be supposed that the temperature of the air 

 is 65° in the shade, and that the muslin requires to be kept moist, 

 before dew is formed, till the blackened ball containing the ether has 

 its temperature reduced to 60°, as indicated by the included ther- 

 mometer, there are then said to be 15° of dryness. Again, supposing 

 the temperature is 85°, and the dew-point found, as before, to be 70°, 

 the degree of dryness is still expressed by 15°; but the quantity of 

 moisture diffused in the air is, notwithstanding, somewhat greater 

 in the latter case than in the former. If 1000 represent complete 

 saturation, the quantity of moisture when the temperature is 65° and 

 the dew-point 50°, will be 609 ; but when the temperature is 85°, and 

 the dew-point 70°, the moisture wiU be represented by 623 ; these 

 numbers being ascertained by tables prepared for the purpose. The 

 difference, however, in such a case, is so small that it is not worth 

 taking into account in a horticultural point of view. But as these 

 numbers can only be ascertained by calculation it is more convenient 

 to reckon by the degrees of dryness, bearing in mind that the dryness 

 of the air is indicated by the difference between the temperature of the 



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