38 Heat, Moisture, and Evaporation, 



of expansion and contraction, and certainly not so well adapted 

 for common observation. The one was first tried by Dr. 

 James Hutton, and consists in observing the temperature as 

 reduced by evaporation. Thus the bulb of a thermometer, 

 being covered with muslin, and moistened with water, it 

 causes the thermometer to sink a number of degrees corre- 

 sponding to the state of the air in respect to moisture. In 

 principle it is a simple and elegant method, in practice not so 

 precise as the method next to be described. 



When air is cooled below a certain temperature, it deposits 

 part of the moisture it contains in the form of dew. Now 

 when air deposits dew it is saturated with moisture, therefore, 

 air always contains that quantity of moisture which would sa- 

 turate the same quantity of air when its temperature is reduced 

 down to the dewing point or temperature at which it deposits 

 dew. But if a body cooled down to the temperature of the dew 

 point be presented to a mass of air, dew is deposited on its 

 surface ; hence it only requires the combination of a thermo- 

 meter, with a means of reducing the body containing it to such 

 a degree of cold that dew deposits, to have an accurate means 

 of determining the dew point ; and, consequently, the quantity 

 of moisture in the air. 



A most ingenious instrument has been contrived on this 

 principle by Mr. Daniell, and it has lately been slightly im- 

 proved by Mr. Jones of Charing Cross. These instruments 

 are, however, rather too delicate and troublesome for ordinary 

 use, and something more simple seems to be desirable. The 

 state of an artificial atmosphere, in regard to moisture may, 

 perhaps, be most satisfactorily obtained by measuring the real 

 quantity of evaporation, it being the excess or the deficiency 

 of this quantity that affects the health of plants; and the 

 evaporation in a given time is nearly proportional to the dif- 

 ference between the quantity that would saturate the air and 

 the quantity it actually contains. 



In order that the quantity evaporated in a given time may 

 be sufficiently measurable, a proper surface should be exposed to 

 the air in a cylindrical vessel, having vertical sides {fig> 6.) ; and 



1 ! i i i i I i r i i I i i i i.l i i iTTi i i, i I i i i i 1 i.i i i I i i i iJl i i i-i I i i i i 



I io so eo 3S 



a tube of smaller diameter might be added, into which the water 

 may be changed to measure the quantity evaporated. If the 

 diameter of the cylindrical vessel be 1*58 inches, and that of 

 the tube half an inch, then each tenth of an inch in the cylin- 

 der will occupy an inch in the tube ; and as the mean evapo- 



