236 METEOKOLOGICAL OBSERVATIONS. 



any one month was 10"59 inches, which fell during the month 

 of December, 1876. Within the Truro Union there are 85,260 

 acres of land, which means that, provided the rain was evenly 

 distributed, there fell over this district during that month 

 91,184,214 tons of water. The highest rainfall for any one day 

 was 3-0 inches, or 25,815,222 tons, on October 4th, 1880. The 

 mean annual rainfall for Truro is 40*5 inches. 



Clouds are perhaps the most beautiful of all aerial 

 phenomena, their color and form ever changing in great variety. 

 As already mentioned, invisible aqueous vapour is diffused 

 throughout the air. If from any cause, such as the sudden 

 variation in the direction of the wind, the atmosphere is cooled 

 below its capacity for holding this vapour, aqueous deposition 

 occurs, with the result that we have clouds formed. There are 

 various kinds of clouds, the chief or primary forms being : 



Cirrus, white delicate feathery clouds, existing only at 

 great heights. Supposed to consist of ice particles. 



JVimbus, a dark uniform grey cloud without shape, from 

 which rain falls. 



Cumulus, a thick white cloud of which the upper part is 

 dome-shaped, the base being usually horizontal ; often occurs in 

 groups. 



Stratus, occurs in horizontal sheets. 



The proportion of sky covered with cloud is estimated, the 

 scale adopted being to 10; represents a cloudless sky, and 

 10 a sky completely covered. 



Snow, Sleet, and Hail. If the temperature of a cloud 

 should fall at any time to 32° F or lower, the tiny water particles 

 are frozen into crystals of ice, and instead of rain we get snow. 

 Sleet is a mixture of melting snow and rain. Hail consists of 

 small rounded masses of ice which fall in showers. These are 

 collected in the rain guage, melted, and measured as rain. 



Ozone, discovered by Sclionbein in 1840, is a concentrated 

 form of oxygen. It occurs in small traces in the atmosphere, 

 and is sometimes made the subject of observation. The usual 

 method consists in exposing to the atmosphere strips of paper 

 previously dipped in an emulsion of starch, to which a small 



