AQUEOUS ATMOSPHERIC PHENOMENA. 



469 



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A Thunder Storm. 



till the resistance opposed by the air becomes equal to their weight, when they continue 

 to fall with an uniform velocity. The velocity bears a certain ratio to the diameter of the 

 drops ; those of a thunder shower, which are large, pouring down faster than those of an 

 ordinary rain. The celerity of a small drop, ^th of an inch in diameter, he estimates at 

 11^ feet per second, upon acquiring its uniform velocity ; that of a larger one, ^th of an 

 inch, at 331 f ee t. A great number of experiments have verified the remarkable circum- 

 stance, that a greater quantity of rain falls upon a low site than upon one a little elevated 

 above it. Thus a rain-guage placed at the bottom of a hill, will collect a larger amount 

 of water in a given time than another placed upon the summit. Dr. Heberden found that 

 the annual depth of rain at the top of Westminster Abbey was 12*099 inches ; at a lower 

 altitude, on the top of a neighbouring house, it was 18*139 inches ; and on the ground, in 

 the garden of the house, it was 22*608 inches. M. Arago gives a similar result, from 

 observations made during ten years at Paris. On the terrace of the Observatory the 

 annual depth was 50*471 centimetres, or 19*88 inches; while thirty yards below, in the 

 court of the building, it was 56*371 centimetres, or 22*21 inches. Comparing, however, 

 an extensive tract of mountainous country with a low level district, the annual fall of rain 

 in the former greatly exceeds that in the latter, though contrary to the natural pre- 

 sumption suggested by the fact, that the lower regions of the atmosphere are much more 

 saturated with vapour than the upper. At Keswick in Cumberland a mountainous 

 district the average annual depth of rain is 67*5 inches, while on the sea-coast it is not 

 half that amount. On the Great St. Bernard it is 63*13 inches, and at Paris only 21*26. 

 The description of Judea by the sacred writer, contrasting it with the flat lands of Egypt, 

 though not intended to be philosophic, is in harmony with the teaching of science respect- 

 ing the important part performed by mountains in the general economy of the earth : 

 " For the land whither thou goest in to possess it, is not as the land of Egypt, from 

 whence ye came out ; but the land whither ye go to possess it, is a land of hills and val- 

 leys, and drinketh water of the rain of heaven." By arresting the course of the clouds, 

 and producing a condensation of aqueous vapour when a warm current of air lights upon 

 their cold summits, the elevations contribute to precipitate the moisture of the atmo- 

 sphere, often amid a terrible display of electric phenomena a blaze of fiery honours, 

 and the echo of heart-thrilling sounds. 



The annual amount of rain is the greatest between the tropics, and diminishes in 

 general with the distance from the equator ; but the number of rainy days is greater in 

 high latitudes than in the torrid zone, owing to the showers in the latter region being 

 more violent and prolonged. From north latitude 12 to 43 the mean number of rainy 

 days is 78 ; from 43 to 46 the mean is 103 ; from 46 to 50 it is 134; and from 50 

 to 60 it is 131. By a comparison of observations made during twenty years at Salem 

 and Cambridge in Massachusetts, with observations in twenty cities of Europe, it appears 



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