HYCr 



I 500 ] 



HYG 



for shewing the temperature; and by ; 

 taking two similar ones, covering their j 

 bulbs with a fold of muslin or silk, j 

 keeping one dry and the other wet, 

 with a thread of lios-silk acting as a J 

 syphon from a vessel of water, the | 

 greater the difference of temperature 

 indicated by the moist and dry ther- 

 mometer; the greater the deficiency of 

 atmospheric moisture. The nearer the 

 temperature of the moist and dry bulb, 

 the nearer is the air to being saturated 

 with moisture. To obtain more perfect 

 details Daniel's Hygrometer is the best 

 instrument. It is represented in the 

 following figure. It consists of two 

 hollow glass 

 balls containing 

 ether, and com- 

 municating by 

 the glass tube 

 which rests on 

 the support. The 

 ball which f onus 

 the termination 

 of the longer leg 

 is of black glass, 

 in order that 

 the formation of 

 dew on its sur- 

 face may be the 

 more percepti- 

 ble. It includes the bulb of a deli- 

 cate 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 ther- 

 mometer. The other ball is covered 

 with muslin. In making an obser- 

 vation it is first necessary to note 

 down the temperature of the air ; next 

 turn the instrument, so that when the 

 muslin-covered ball is heldiii the hand, 

 the ether may escape into the blackened 

 ball ; and it should also be held till the 

 included thermometer rises a few de- 

 grees above the temperature of the air, 

 when it should be replaced on the 

 support. Then drop, or gently pour, a 

 little ether on the muslin. The evapo- 

 ration of this extremely volatile sub- 

 stance produces cold; and attention 

 must be instantly 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 

 very dry, no moisture will be thus de- 

 posited till the thermometer falls to 

 10, 20, or 30 below the temperature 

 of the air. But at whatever tempe- 

 rature the dew forms, that temperature 

 should be noted as the dew-point; and 

 the difference between it and the tem- 

 perature 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 sup- 

 posed that the temperature of the air 

 is 05 in the shade, and that the muslin 

 requires to be kept moist, before dew 

 is formed, till the blackened ball con- 

 taining the ether has its temperature 

 reduced to 50, as indicated by the 

 included thermometer, there are then 

 said to be 15 of dryness. Again, sup- 

 posing 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 009; 

 but when the temperature is 85 and 

 the dew-point 70, the moisture will be 

 represented by (i'23 ; these numbers 

 being ascertained by tables prepared 

 for the purpose. The difference, how- 

 ever, in such a case is so small it is not 

 worth taking into account in a horti- 

 cultural point of view. But as these 

 numbers can only be ascertained by 

 calculation, it is more convenient to 

 reckon by the degree of dryness, bear- 

 ing in mind that the dryness of the air 

 is indicated by the difference between 

 the temperature of the air and of tlui 

 dew-point. Thus, if the ring of dew is 

 formed as soon as ether is applied, and 

 only 1 difference is observable, the air 

 is nearly saturated ; if the difference is 

 5 to 10 the dryness is very moderate ; 

 while 15 to 20 of difference indicate 

 excessive dryness, and beyond tin's the 

 air is parching. Gard. Chron. 



(From Itygros, moist) 



