HY G 



320 



H YG 



ether has its temperature reduced to is expected to nppear; because the dew 



50°, as indicated by the included ther- 

 mometer, there are then said to be lo^ 

 of dryness. 



" Again, supposing the temperature 

 is So^ , and the dew-point found, as be- 



is most easily seen where the line di- 

 vides the bright and black reflections on 

 the bulb; and inasmuch as the change 

 may not be noticed the very instant that 

 it occurs, it is well to make a second 



fore, to be 70°, the degree of dryness observation of the temperature at which 

 is still expressed by 15^ ; but the quan- the dew clears off, and then take the 



tity of moisture diffused in the air is, 

 notwithstanding, somewhat greater in 

 the latter case than in the former. 



" If 1000° represent complete satu- 

 ration, the quantity of moisture, when 

 the temperature is 65° and the dew- 



mean of two. If they are both taken 

 equally late, the errors will balance 

 each other ; because in one case the 

 mercury is falling, and in the other 

 rising." — Gai'd. Chron. 



Mr. J. W. Harris, writing on the same 



point 50^, will be 609°; but when the subject, says : — 

 temperature is S5° and the dew-point I " As I have for the last three months, 

 70°, the moisture will be represented used an instrument for the purpose of 

 by 623; these numbers being ascer- regulating the moisture of the air in my 



tained by tables prepared for the pur 

 pose 



Fig. 93. 



orchidaceous house which has perfectly 

 The difference, however, in such answered my purpose, I am induced to 



offer it to your notice. It consists of an 

 , old-fashioned instrument commonly sold 

 I in the opticians' shops as Leslie's Dif- 

 ferential Thermometer. It is arranged 

 I so that, when not in use, the fluid stands 

 I at zero in the stem ; over the bulb of 

 the opposite stem I place a piece of 

 muslin, which has been well soaked in 

 a strong solution of common salt in wa- 

 ter. The muslin having been cut into 

 a circular shape, is laid on the bulb 

 whilst wet; and the moisture will make 

 it adhere sufficiently. A shelf, or brack- 

 et, with sides, top, and back, is made 

 for it to stand in, to seclude it from the 

 sunshine — which is of course essential 

 — and also to prevent the damp wall 

 from having effect upon the muslin, so 

 that it may draw all its moisture from 

 the atmosphere alone. It will be found 

 a case is so small it is not worth taking convenient to have a thermometer hung 

 into account in a horticultural point of on the same stand, as in all hygrometric 

 view. But as these numbers can only observations the state of the thermome- 

 be ascertained by calculation it is more ter must be attended to. The rationale 

 convenient to reckon by the degree of of its action is simple. If the absorp- 

 dryness, bearing in mind that the dry- tion of moisture exceeds the evaporation 

 ness of the air is indicated by the differ- from the muslin, heat will be generated, 

 ence between the temperature of the which will expand the air in that bulb, 

 air and of the dew-point. Thus, if the and drive the fluid up the opposite stem, 

 ring of dew is formed as soon as ether indicating the degree by its rise. On 

 IS applied, and only 1° difference is the contrary, if the evaporation exceeds 

 observable, the air is nearly saturated; the absorption, cold will be produced, 

 if the difference is o^ to 10°, the dry- causing the fluid to fall. The general 

 ness is very moderate ; while 15° to 20° range of the scales made is from zero 

 ofdifference indicate excessive dryness, to 40^. I believe, in my stove, under 

 and beyond this the air is parching." — the general treatment of orchidaceous 



Gard. Chron. 



plants, temperature ranging from 78" 



"The instrument," says Mr. Ross, to 95^, the hygrometer has ranged from 

 "should be held so as to obtain a por- 15=" to 30^. Of course, if the instru- 

 tion of bright reflection where the dew | ment were found to require it, it would 



