44 REPORT — 1855. 



are nearly double that distance from the Atlantic. The south wind perhaps never 

 occupies at one time the whole breadth of the country from western Texas to eastern 

 Florida. The south wind is rapidly propagated from the west along the northern 

 shores of the Gulf of' Mexico, but it is almost as rapidly destroyed on its western 

 edges by the cold upper current descending along the eastern slopes of the Rocky 

 Mountains, and penetrating, as a surface wind, this warm current from the Caribbean 

 Sea. In this manner the western edges of the south wind are raised into the upper 

 current, and drifted towards the east. Thus the winter storms of the United States 

 are alwaj's succeeded by a cold wind from a westerly direction. The cause of the 

 violence of the west wind in winter was then shown. The weather during summer 

 was regulated by the same principles, but the north-west wind then lost its power, 

 in consequence of its being warm and elastic. The thunder-storms and tornadoes 

 generally drifted from west to east in the middle States, and from north-west in the 

 northern States. This arose from the clouds being formed in the upper current, and 

 drifting towards the east at the very time that the south wind was prevailing. The 

 thunder and tornado clouds usually drifted in the south wind over the States bor- 

 dering on the Gulf of Mexico. The hurricane-clouds also drifted in the southern 

 stream of warm air, and were often propagated along the Atlantic coast. The fluc- 

 tuations of the barometer were attributed to the fluctuations of density of the air at 

 the surface of the earth. This was Dalton's hypothesis, which he thought explained 

 the fluctuations of the barometer more consistently than any which had been offered. 

 It did not explain all in Britain, but it explained a great deal, — the apparent excep- 

 tions were all grouped together very consistently. 'J"he height of the barometer is 

 inversely as the temperature, or rather moisture, for the latter is a more permanent 

 cause of high temperature. Diagrams were exhibited to illustrate this connexion 

 between the rise or fall of temperature and the fall or rise of mercury. By adopting 

 the arbitrary scale of 5° of heat as equal to one-tenth of an inch of mercury, which 

 indicated the south wind to be about 10,000 feet in height, a great parallelism between 

 the curve of temperature and inverted curve of the barometer was exhibited. A more 

 perfect explanation of the fluctuations of the barometer at Alabama could not be 

 given. The south wind being lighter, depressed the barometer at every place where 

 the temperature was raised. The low barometer extended in a long line from the 

 Gulf of Mexico to the lakes, and travelled to the east as the rains and high tempera- 

 ture did. The grand exception to fluctuations of the barometer being occasioned by 

 fluctuations in the density of the air at the surface of the earth, arises in the West 

 Indian hurricane, when a depression of two inches was sometimes observed to take 

 place. The only theory which successfully met this phaenomena was that of Prof. 

 Espy, in which the wind blowing towards a central space rose in consequence of the 

 extrication of latent caloric, by the condensation of moisture through the expansion 

 of the air causing a reduction of temperature below the dew-point. Prof. Espy 

 maintains that the whole force generated during hurricanes can be accounted for by 

 the effects of heat, — Prof. Hare, that part is due to the electrical agency. In the 

 case of the sea-breeze, a considerable body of air is put and kept in motion by slight 

 differences in the weights of adjoining columns of air. Were such differences of the 

 atmospheric conditions as the chart of the 10th of November exhibited between the 

 mouth of the Mississippi and Montreal, tremendous disturbances would ensue. When 

 the distance is great, the power is diffused in moving the whole body of air betwixt 

 the stations. The expenditure of power in this diffused manner may be compared 

 to the flow of the Mississippi over the last 1400 miles of its course, where the fall is 

 less than three inches to a mile. On the other hand, when the Niagara tumbles 

 over its great precipice, it expends much power at once. The hurricane might be 

 regarded as an aerial cataract, only the air being forced upwards. If a slight fall of 

 rain produced such remarkable effects as are noticed on the passage of the squall 

 cloud, what must be the power evoked by the evolution of latent caloric in hurricanes ! 

 Six inches of rain have been known to fall during some hurricanes. The caloric set 

 free by the condensation of this amount of water over every square mile is equal to 

 that which would be generated in the burning of 2,620,000 tons of coal, allowing 

 1 lb. of coal to evaporate 13 lbs. of water. The clouds of the hurricane interrupt the 

 ominous calm as suddenly as the smooth flow of the stream is changed at the brink 

 of the cascade. 



