418 Ninth Annual Report of the 



Hygrometers. — The shelter also contains a hair hygrometer 

 (Richard's), whose registering mechanism is similar to that of 

 the thermograph ; but in this case the pea arm is moved by the 

 expansion and contraction of a bundle of hairs which are 

 rendered very sensitive to the action of moisture by a special 

 process of the makers. The index or datum of this hygrometer 

 is liable to change somewhat during prolonged wet or dry 

 periods; but gives very satisfactory results when adjusted from 

 time to time to agree with the values of humidity indicated by 

 the whirled psychrometer. (See '' Instructions to voluntary ob- 

 servers " for full information regarding the latter instrument.) 



A Richard dry and wet bulb recording hygrometer has also 

 been used at this station, but thus far has not proved as satis- 

 factory as the instrument just described. In principle it con- 

 sists of two Richard thermographs, one of which has its alcohol 

 tube moistened constantly by muslin dipping into a reservoir, 

 while the other tube is kept dry. The instrument is so con- 

 structed, however, that the records of both dry and wet bulbs are 

 traced upon the same cylinder. 



Evaporometer. — An evaporometer (Richard's) is located beneath 

 the shelter at a height of 2 feet from the ground ; direct sunlight 

 and driving rain or snow being excluded by louvred sides. The 

 evaporometer consists, essentially, of a pair of scales, one pan of 

 which carries the water, whose evaporation is to be measured, 

 and a pen attached by suitable mechanism to the scale arm, 

 which records on a rotating cylinder the rise or fall of the scale 

 beam as the quantity of water is diminished or increased. 



Actinomeier. — An actinometer or sunshine recorder (Richard's) 

 is mounted on a shelf out of a south window of the college. The 

 intensity of solar heat is measured by the difference in tem- 

 perature between two glass bulbs 4 inches in diameter, placed 

 side by side, one of which is coated with lamp-black while the 

 other is silvered. The two have the same temperature (nearly) 

 when the sky is overcast, but the black bulb absorbs the heat of 

 solar rays more rapidly than the bright under a clear sky. Each 



