OF METEOBOLOGICAL INSTRUMENTS. 



19 



FIG. 50. 



53. Mason's Hygrometer, exact as No. 50, but mounted on brass pedestal (fig. 53) 



220 



55. MASON'S HYGEOMETEB, as fig. 50, but of extra size, with expanded graduations, 



the tubes being fifteen inches long, and divided to 0.2 . 3 10 



56. Mason's Hygrometer, with 18-inch, tubes, divided to 5ths for delicate investiga- 



tions . 550 



57. POETABLE POCKET WET AND DET BULB HYGEOMETEB. A most convenient form 

 of instrument for travellers, especially designed by L. CASELLA as a com- 

 panion instrument to his Alpine, or pocket maximum and minimum re- 

 gistering thermometers, Nos. 46, 47, and pocket aneroid barometer, No. 118, 

 and Livingstone's rain gauge, No. 62 '. . . 220 



The wet and dry bulb hygrometer or psychrometer may be fairly said to supplant the use 

 of all others as an easy and practical means of indicating the humidity of the air. It consists 

 of two thermometers placed parallel, about four inches apart, with their graduations as nearly 

 as possible identical. The bulb of one is covered with thin muslin, from which trail a few 

 threads of lamp cotton ; these, being first wetted, are passed into a small attached vessel of 

 water, two or three inches distant, and the bulb thus kept continually moist, causes this 

 thermometer to indicate a lower temperature in proportion to the rate of evaporation, whilst 

 the dry bulb thermometer shows the temperature of the air. From the readings of the dry and 

 wet thermometers, the dew-point is obtained by means of the accompanying table. During 

 frost, however, when the capillary action of the cotton is stopped, the bulb should be wetted 



C 2 



