ice, and so darkening the air, that houses within a few paces were totally invisible. During the summer of 1834, which was unusually sultry, tornados and hail storms of limited extent, visited almost every state in the Union, from Maine to Louisiana; several of them attended with great loss of lives and property. In most cases, they pursued a course from west to east. About the 20th of March, 1832, a tornado occurred in the southern portion of Tennessee, lat. 35° N. which differed in some respects from any of the preceding. After several remarkably warm days for the season, accompanied with a southerly wind, a tornado came on suddenly, without the slightest warning, about seven o'clock, P.M. It excited great astonishment, as the whole day had been warm, serene, and clear, with the exception of a slight haziness, up to the moment when the crashing noise of falling timber announced the approaching storm from the west. The temperature was immediately reduced from about 70°, to several degrees below 32° F. and attended with a rapid fall of snow, which continued for several hours; but the tornado blew with violence only about ten minutes. The next morning, the wind was cold and north westerly, and continued northerly for two days. This storm seems to have been owing to the prevalence of a cold upper current from the I shall close this imperfect history of thunder storms with the account of an extraordinary and sudden coldness which occurred in the island of Cuba, on the 24th of May, 1809, that was evidently caused by a descent of air from the upper regions. Cornelius Roberts, a sugar and coffee planter, who resided forty years on the island, informed the author, "that after a hurricane had been blowing from the south east and south, from the 22nd, until eleven o'clock, A. M. of the 24th, a calm followed for an hour, when the wind prevailed from the north west. At the same time, the atmosphere became extremely dark, like night, accompanied with a roaring in the air, tremour of the earth, and intense cold. Everything green was killed, and became black, as if a fire had passed over the country for several miles in breadth, and about sixty miles in length:" which proves that a mass of air may descend from above and refrigerate the lower atmosphere of the tropical regions, as in the middle latitudes, though such a phenomenon is extremely rare. CHAPTER III. Barometer. IT has been long known that the fluctuations of the barometer are intimately connected with all the phenomena of meteorology; that during winter, in the middle and higher latitudes, its depressions are followed with rain, snow, and tempestuous weather; while its rising is accompanied with cold, frosty, and dry weather; and that during summer its falling forebodes storms of thunder, lightning, and rain, which are often attended with violent hurricanes; while its rising indicates clear and serene weather. Yet the theory of its variations has never been clearly reduced to the simplicity of established principles. Without stopping to examine the relative merits of those who have devoted their attention to this difficult and important problem, it may be observed without injustice to any, that we are chiefly indebted to the labours of Dr. Dalton for the true mode of investigating it. The most important facts connected with the barometer may be reduced to the following propositions : 1. The mean height of the mercurial column at the level of the sea, is the same all over the earth, amounting to the average of about thirty inches. 2. The density or specific gravity of the atmosphere, increases gradually from the equator to the regions of lowest mean temperature, corresponding with the decrease of temperature; the consequence of which is, that the height of the atmosphere must diminish from the equator to the poles. 3. Within the tropics, the temperature is always nearly the same: the range of the barometer is also small, varying from two lines to a quarter of an inch. 4. The range of temperature augments from the equator to the regions of maximum cold: the variation of the barometer augments in a corresponding ratio, amounting to about three inches in the coldest latitudes. The range of temperature is much greater in *It is however but seldom, that the temperature falls below 0° in the States, south of Philadelphia and New York; so that the usual annual range of temperature does not much exceed that of England, which is about 80°, according to Mr. Daniell; BAROMETER. is also very great, as the thermometer often rises to 90° in September and October, during the hottest part of the day, and falls below 32° at night, making the daily range of temperature 60° F. Still more extraordinary are the changes from heat to cold in those extensive regions of North America beyond the limits of the United States, which have been explored by Scoresby, Parry, Franklin, Ross, and other British navigators. We are informed by Captain Back, that on the 17th of January, 1834, the thermometer stood 70° F. at 6 A. M. on the Great Slave Lake, lat. 62° 46′; but rose to 45° in the afternoon of the same day,—making a diurnal range of 115°. He also states, that on the preceding day, the temperature rose to 52°; while the long summer days were often oppressively warm. Hence it is, that as we approach the polar regions, we find the winds so irregular and variable, coinciding with that is, from 11° to 90°. Hence it is, that the mean range of the barometer is about the same in Great Britain that it is in the middle States of America, as observed by Dr. Dalton forty years ago. Perhaps there is no part of the world where the fluctuations of temperature are more frequent than in England; the reason of which is obvious from its insular and geographical position, being situated about midway between the burning plains of Africa to the south, and the frozen regions of the north; while on the east and north east it is influenced by Sweden, Poland, Russia, and the cold elevated plains of Tartary; and by the Atlantic ocean on the west; all of which contribute their share successively in forming the climate of Britain. |