48 The Ottawa Naturalist. [May 



for determining the relative humidity of the atmosphere. Evi- 

 dently, the drier the atmosphere, the greater will be the evapor- 

 ation from the muslin, and the heat necessary for evaporation is 

 abstracted from the mercury bulb, with a consequent fall in 

 temperature. From the difference in temperature of the dry and 

 wet thermometers a measure of the relative humidity of the air 

 is obtained. 



The instruments used for recording the wind are ; the vane, 

 for direction ; the anemometer, for velocity and the anemograph^ 

 which registers both direction and velocity. Add to the instru- 

 ments named the hygrometer and rain guage and we have the 

 necessary ones for a meteorological station. 



When from any cause the air at any part of the atmosphere 

 is cooled below its dew point, a portion of the vapour suspended 

 in it becomes condensed and converted into minute drops of 

 water, forming what is called a fog, or a cloud, according as the 

 condensation takes place near the ground or in some higher 

 region. The numberless forms of clouds make it difficult to so 

 classify and name them as to secure easy recoghition and ensure 

 uniformity of record. The classification introduced at the begin- 

 ning of the century obtains to the present day. In it three 

 simple or primary and four compound forms are recognized. 

 The cirrus is the high fleecy cloud ; the cumulus of moderately 

 low elevation and its simpler form the shape of conical heaps 

 rising from a horizontal base ; the stratus is the lowest of all, 

 generally gray masses or sheets of clouds with ill defined out- 

 lines. The compound ones fall intermediate between the 

 primary ones described. The highest cirrus cloud recently 

 measured at Toronto was about seven miles high, while some 

 European observers claim to have obtained fully double that 

 height. Observations on the motions of upper clouds are of 

 great importance, since from these movements can be gleaned 

 the only possible information as to the prevailing direction of 



