FOEilATION OF CLOUDS. 157 



traversed by eddiea thrown off by it, raia may be a consequence of 

 disturbances of the hygrometric equilibrium beyond what was com- 

 patible with the retention of the moisture by the air. A current of 

 cold air so brought into a body of warm air may cool this to a tempera- 

 ture at which it is saturated with a much less quantity than it con- 

 tains, and the excess wiU be precipitated as rain ; or a current of 

 warm air brought into a body of cold air may be so cooled down that 

 its surplus moisture is deposited in the form of rain. 



Thus are clouds produced ; and a change of wind occasioning a 

 slight rise of temperature may cause the cloud to melt away into 

 transparent air, by the re-absorption of the vapour of which the cloud 

 was formed. The phenomena I have seen at the Cape of Good Hope 

 a hundred times. They are there of constant occurrence. 



A correspondent from Sea Point writes : — " On Thursday evening, 

 during the hard south-east gale, some very remarkable and beautiful 

 cloud effects were witnessed. Dark masses of cloud came driving 

 across the Lion's Back at a furious rate, obscuring the brilliant moon- 

 light. But in a few seconds after passing the zenith, their speed 

 became checked, and, at the same time, they began rapidly to 

 dissolve. Some of the cloud masses hung for a few seconds 

 apparently motionless between the opposing currents of wind. 

 Others again yielded to the counter current, and rushed back 

 furiously from the north, meeting the driving masses from the 

 south, and, almost in the moment of apparent meeting, dissolving 

 utterly away. It was like the unavailable charge of a gallant 

 body of light cavalry against a masked battery of artillery, — swept 

 away before the cruel storm. Ttie sudden alternations of clear blue 

 sky, with black thunderous-looking masses of light fleecy clouds, and 

 the changing iridescent hues of the vapours as they drifted across 

 the moon, were very striking, and produced a scenic effect which 

 those who witnessed it will not soon forget.'' 



By the clouds may be traced the aerial currents, Writing on this 

 subject, in connection with the distribution of rain, it is remarked by 

 Cezanne that, " From the elevated summit of a mountain above 

 which shines the sun in a cloudless sky, one may see under his feet 

 a tide of clouds rising up the slopes on one side of the range. It 

 impels its mobile billows into all the anfractuosities of the gorges, 

 lashes and surmounts the buttresses which stop it, in every respect 

 as does the ocean covered with foam the black head of the reef of 

 rock. By all the rocks, by all the depressions of the chasm, the 

 cloudy wave presses on, and pours itself over on the slope on the 



