What is a storm? Strictly speaking, it is any marked or unusual disturbance of the normal atmospheric conditions. There may be excessive wind: there may be cessation of the customary winds. Two great classes are found: cyclonic, or low area storms, and anti-cyclonic, or high area storms. The former may be accompanied by heavy rainfall or snow; the latter is usually noted for absence of either. It is with the low area storm that we must deal at present. This term is used to designate all storms which are marked by low barometer, and therefore it is clear that such are accompanied and partially occasioned by an unusual amount of moisture in the atmosphere. The resultant commotion is usually extensive, the storm centre traveling across the country; but occasionally the effects are perceptible only for a short distance, the storm centre either breaking up or ascending to the upper atmosphere. By a cyclonic storm is signified a storm characterized by unusually low barometer, and a wind system blowing spirally inward, as in a genuine cyclone. They usually affect only the lower strata of the air. Quite frequently they are broken up by striking low mountain ranges, such as the Alleghany system: and often pass Mount Washington without making their presence felt at the signal station on its summit. To what extent they are influenced by or are due to the upper air currents is therefore unknown, though not a few of the attendant phenomena indicate that the latter are of no little importance. Any one who has observed the waters at the junction of two streams, is familiar with the appearance of numerous tiny eddies or whirlpools formed at the point of junction. Such are perceptible also in every rapid stream, when the current, sheering sharply from a projecting point, is made in a measure to collide with itself. This is also the principle of the many tiny whirlwinds seen during the warm summer days: and such are also observable in winter, if there be snow enough to render their presence in the air clearly visible. Their results are most readily recognized in snow-drifts, where the wind meets some special obstruction. It does not often occur that a high fence is covered with a snow-drift: a great drift will be thrown up by it, but not against it: and the side next the fence will be curved inward, or concave. The wind strikes the fence and partially recoils, curving upward to pass over the fence. The drift is then built up between the wind and the current recoiling from the fence, and its inner curve shows the direction pursued by the rebounding current. Now, when opposing air-currents meet each other on a large scale, the immense whirlwind that is produced is called a cyclone or tornado. It follows then, that if we would find any regularity or law in these unusual disturbances, we must know if there exists any permanent condition of atmospheric currents that is favorable to their generation.