184 OTHER HYGROMETERS. 



between that and the indication of the wet-bulb thermometer, the 

 dew-point can be ascertained by the accompanying table thus : — 



Supposing the temperature of the air, as indicated by the dry 



thermometer, is 70° 



Whilst the wet-bulb thermometer is 64° 



Degrees of dryness by this instrument 6° 



If we look in the left hand column, headed " temperature," we shall 

 find 70° ; opposite this, and under 6° in the top column, we find 56°, 

 the dew-point, or temperature at which the dew is deposited, according 

 to DanieU's hygrometer, and 70°— 56°= 14° the degree of dryness by 

 Daniell's instrument. By practice, or rather experience, a gardener 

 would form as true a notion of the condition of his plants, with regard 

 to moisture, by the indications of one instrument as he would by the 

 other. He would learn that by Mason's 3° was a moderate state of 

 dryness, but that 12° was excessive, just as easily as he would by 

 observing 7° of dryness was moderate, according to Daniell's, but 

 that 28° was parching (3 and 7, 12 and 28 are the corresponding 

 degrees on the two instruments). So far these instruments are 

 on an equality as regards their results ; whilst Mason's has the 

 advantage of not requiring any experiment to be made, nor an ex- 

 pensive substance like ether to be applied. 



Other modifications may be adopted, such as Rutherford's thermo- 

 meter; or by using Six's, or one constructed after Dr. Traill's method, 

 the maximum and miTiimiim of moisture can be registered by one 

 bulb. Finally there is Simmons's Hygrometer, or more properly 

 hygroscope, which has been much used, and of which a full account by 

 Mr. BelviUe wiU be found in the Gardener's Chronicle for 1847, p. 815. 

 The fault of this is that like all wood hygroscopes it is apt to get out of 

 order, and to lose its hygrometrieal property with time. 



By means of these and similar contrivances, we are at all 

 times able to ascertain exactly the quantity of water that exists 

 in an elastic state ia the air. When the hygrometer was first 

 brought into use, what was called a damp atmosphere was 

 frequently seen to indicate a degree of moisture faUing short of 

 •500, saturation being represented by 1000; and it was found 

 that "120 was not uncommon — a state of thiags suf&cient to 

 impair the vitality of the most vigorous vegetation. 



In this country, the changes of moisture are said to extend 

 from I'OOO, or saturation, to '389, or even so low as '120, under 

 a south wall for a short space of time ; " a state of dryness 



