3i6 



POPULAR SCIENCE MONTHLY. 



The amount of moisture present, or tlie humidity of the air, is 

 determined by a comparison of dry and wet bulb thermometers. 

 They are both ordinary thermometers; but the bulb of the latter is 

 covered with muslin that is wet. In the latest form of instru- 

 ment the thermometers are mounted on arms carried by a shaft that 

 is rotated by a crank which is geared to the shaft. The motion of the 

 shaft rotates the thermometers in vertical planes and causes the water 

 in the muslin to evaporate more or less rapidly according to the 

 amount of moisture in the air. This evaporation lowers the tem- 

 perature of the thermometer; and, from tables constructed after 

 long experiments, the degree of moisture can be determined by the 

 difference in temperature- between the two thermometers. 



The direction and speed of the wind, the hours of sunshine 

 and cloud, and the amount of rainfall are recorded by the " triple 

 register." The instrument has a cylinder on which is carried a 

 graduated sheet of paper for the record. The cylinder is sup- 

 ported by a horizontal shaft that is turned by clockwork. On the 

 shaft is secured a spiral wire that engages grooves in a bearing of the 

 shaft; so that, as the shaft turns, it and the cylinder are moved 

 along. This would cause a stationary pencil to trace a spiral on the 

 cylinder. The shaft carries two arms ])nranel to itself that pass 



Fig. 9. — The Triple Register. Rear view. 



through a yoke which is turned by the clock, this arrangement per- 

 mitting the shaft to recede from the clock under the action of the 

 spiral. The cylinder revolves four times in twenty-four hours, so 

 that each record passes across the paper four times. 



The wind vane is an arrow-shaped vane mounted on a vertical 

 shaft. The tail of the arrow is gradually broadened laterally 

 to give it greater steadiness. A sleeve is fastened on the shaft 



