HYG 



[448 ] 



HYM 



In making an observation it is first ne 

 cessary to note down the temperature o 

 the air ; next turn the instrument, si 

 that when the muslin-covered ball is heli 

 in the hand, the ether may escape into tin 

 blackened ball; and it should also be 

 held till the included thermometer rise 

 a few degrees above the temperature o 

 the air, when it should be replaced on the 

 support. Then drop, or gently pour, s 

 little ether on the muslin. The evapora 

 tion of this extremely volatile substanci 

 produces cold ; and attention must be in 

 stantly directed to the black glass bal 

 and included thermometer. The lattei 

 will be seen falling rapidly ; and at lengtl 

 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 

 wiil be thus deposited till the thermo 

 meter falls to 10, 20, or 30 below the 

 temperature of the air. But at whatever 

 temperature the dew forms, that tempe- 

 rature should be noted as the dew-point 

 and the difference between it and the 

 temperature of the air, at the time, is the 

 degree of dryness according to the indi- 

 cations 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 50, 

 as indicated by the included thermo- 

 meter, there are then said to be 15 of 

 dryness. Again, supposing the tempe- 

 rature 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, not- 

 withstanding, somewhat greater in the 

 latter case than in the former. If 1000 

 represent complete saturation, the quan- 

 tity 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 C , the moisture will be repre- 

 sented by 623 ; these numbers being as- 

 certained by tables prepared for the pur- 

 pose. The difference, however, in such a 

 case is so small, 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 con- 

 venient to reckon by the degrees of dry- 

 ness, bearing in mind that the dryness of 

 the air is indicated by the difference be- 



tween the temperature of the air and of 

 the 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 this the 

 air is parching. Gard. Chron. 



HYGRO'PHILA. (From hygros, moist, 

 and phileo, to love; referring to the 

 habitat of the plant. Nat. ord., Acanthads 

 [Acanthaceoe], Linn., 14-Didynomta 2- 

 Angiospermia. Allied to Ruellia.) 



Stove evergreens, from the East Indies. Cut- 

 tings of young shoots in sandy soil, in heat; peat 

 and loam. Summer temp., 60 to 80; winter, 

 45 to 55. 

 H. longifo'lia (long-leaved). Purple. June. 1821. 



ri'ngens (gaping). 1820. 



salicifo'lia (willow-leaved). Violet. June. 1822. 



HYLE SINUS PINIPERDA. A species of 

 beetle, which preys upon the pith of young 

 shoots of sickly or recently-felled Scotch 

 and spruce- firs. It is not very injurious 

 in this country. 



HYLOTONIA KOS.ZE. A saw-fly, which 

 injures rose-trees seriously, by puncturing 

 in rows their young shoots, and depositing 

 its eggs in the holes. The best remedy 

 is spreading a cloth beneath the trees in 

 the evening, and killing the insects 

 shaken down upon it. 



HYMENIE'A. Locust-tree. (From Hy- 

 men, the god of marriage ; referring to 

 he leaflets being joined. Nat. ord., Le- 

 guminous Plants [Fabacese]. Linn., 10- 

 Decandria \-Monogynia. Allied to Bau- 

 linia.) 



Fine, close-grained, hard wood ; and the resins 

 Anime and Copal are produced by these stove 

 evergreen trees. Cuttings of firm young shoots 

 n spring, inserted firmly in sand covered with a 

 jell-glass, in bottom-heat ; peat and rich loam. 

 Summer temp., 60 to 85; winter, 55 to 60. 

 H. Candollia'na (Decandolle's). 30. White. 

 Acalpulco. 1824. 



Cou'rbaril (Courbaril). 40. Yellow, purple. 



W. Ind. 1688. 



verruco'sa (warty-podded). 20. White. Ma- 



dagascar. 1808. 



HYMENANTHE'RA. (From hymen, a 



membrane, and anthera, an anther, or 



ollen-bag. Nat. ord., Violetworts [Vio- 



acese]. Linn., 5-Pentandria i-Jkfono- 



gynia. Allied to Alsodeia.) 



Greenhouse evergreen shrub. Cuttings of 

 oung shoots, getting a little firm, in spring, in 

 and, over peat, and well drained, under a bell- 

 lass; sandy peat, with a third of fibry loam. 

 Winter temp., 40 to 45. 



H, denta'ta (toothed-teawed). 6. Yellow. May, 

 N, Holland. 1824. 



