HARRIS'S HTGEOMETER. 



181 



its rise. On the contrary, if the evaporation exceeds the absorption, 

 cold will be produced, causing the fluid to fall. The general range of 

 the scales made are from zero to 40°. In Mr. Harris's stove, under 

 the general treatment of orchidaceous plants, temperature ranging from 

 78° to 95°, the hygrometer usually ranged from 15° to 30°." 



R*p 



^ 



Fig. XXX. 



Of this instrument it has been complained that its divisions are in a 

 great measure arbitrary and different from those of the thermometer to 

 which gardeners are accustomed. But this is unimportant, inasmuch 

 as men soon become acquainted with the value of the indications of any 

 instrument, and it gives an absolute, if not a comparative result, which 

 latter may be dispensed with. Mr. WaUes has expressed his opinion 

 that " the wet-bulb thermometer, which has been long known, though 

 recently improved in form, under the name of Mason's Hygrometer, is 

 the one best fitted for the hothouse, being ' simple, self-acting, econo- 

 mical, and certain,' and requiring the least attention to keep it in 

 working order. Miason's instrument is, however,' not indispensable, 

 as every gardener may readily convert any common thermometer into 



