Meteorology in its connection with agriculture / by Joseph Henry.

  • Henry, Joseph, 1797-1878.
Date:
1858
Catalogue details

Licence: Public Domain Mark

Credit: Meteorology in its connection with agriculture / by Joseph Henry. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    according to the distance to which they extend northward before they commence to descend to the earth. If the sun continued on the equator during the year, and there were no obstacles to the free motion of these currents, they would he constant in intensity and direction around the whole earth ; hut the change in declination of the sun, and the obstacles opposed by continents and mountain chains modify in an important degree the simplicity of this motion. When the sun ascends to the north, it carries with it the whole circulating system of the atmosphere, causes the northeast trade- winds to invade the southern part of the United States, and the inferior currents, which give rise to the southwest wind, to flow in summer over a large portion of our territory. The latter, charged with the vapor from the Atlantic and the Grulf of Mexico, impart warmth and fertility to all parts of the surface on which they descend. The higher currents, which produce the west and northwest winds, flow in summer above us, to descend further to the north. Their course, however, is marked by the almost invariable direc- tion of the upper clouds and of the summer thunder storms, which, in the greater part of the United States, pass from the west to the east. The curving course of the returned currents, when the sun is south of the equator, is, perhaps, best marked by the direction of the hurricanes, which exactly follow the path we have described as that of the atoms of air in the general circulation so often referred to. This will be seen by examining the storm tracks on one of the maps of the lamented Eedfleld. It is evident, from theory as well as from every day observation, that the currents of the belt of the northern hemisphere in which the United States is situated must be subject to many perturbing influences, and that this region is well entitled to the denomination of the zone of variable winds. While the great circulation which we have described is going on, particularly above us, every rain that occurs, and every variation of temperature, tends to disturb its regularity at the surface of the earth. According to the views here presented, the following winds of the United States belong to the general circulation, namely, the southwest, west, northwest, north, and northeast; while those from the opposite quarters of the horizon are principally due to abnormal atmospheric disturb- ances. We say principally, because a portion of the surface north- east trade wind in summer probably blows over Florida and the lower part of Louisiana. These views have been strengthened by a series of observations collected by M. De Done, from which it is shown that the winds from the western half of the horizon, as indicated by the clouds, preponderate over those from the east, as indicated by the wind vane at the surface; or, in other words, that there is a greater tendency to a movement, even in our latitude, in the upper strata of air from the western half of the horizon, and in the lower from the eastern—a result in conformity with the general principles we have endeavored to explain. The circulation in the region of variable winds may often be inverted, and the compensa- tion take place by means of winds in different parts of the hemi- sphere. It must be evident, from mechanical principles, that, to