If we watch the stages of gradually increasing wind, we find that as the strength rises the tendency is more and more to blow in gusts. Gradually these gusts get still more violent, and in their highest development come with a boom like the discharge of a piece of heavy ordnance. This is what sailors call “blowing in great guns,” and these are the gusts which blow sails into ribbons, and dismast ships more than any amount of steady wind. These gusts only last a few minutes, but they seem to be very closely allied to the simplest form of squalls. In a true, simple squall the wind generally need not be of the exceptional violence which causes “guns”; but after it has rather fallen a little, the blast comes on suddenly with a burst, and rain or hail, according to intensity, or other circumstances, while the whole rarely lasts more than five or ten minutes. At sea one often sees two or three squalls flying about at a time. Then we readily observe that over the squall there is firm, hard, cumulus cloud; that the disturbance only reaches a short distance above the earth’s surface; that the squall moves nearly in the same direction as the wind; and that there is little or no shift of the wind before or during the squall. We also see that the shape of the squall is merely that of an irregular patch, with a tendency rather to be longer in the direction of the wind than in any other quarter; and that the motion of the squall as a whole is much slower than that of the wind which accompanies the first blasts. If, at the same time, we watch our barometer closely, we find that if the squall is sufficiently strong, the mercury invariably rises--sometimes as much as one-tenth of an inch--and returns to its former level after the squall is over. No difference is observed in this sudden rise, whether the squall is accompanied with rain, hail, or thunder and lightning; and though we are unable exactly to explain why the wind sometimes takes this irregular method of blowing, we have still to do with a comparatively simple phenomenon.