In stormy weather, the column of water in a water-barometer where the scale is very large may be seen to pulsate with every change of pressure from the air-waves at the surface. The winds are nothing more than the rush of air from the regions of high pressure to fill up the spaces where low pressure prevails. Thus, if the column of mercury should stand 28.6 inches high at London, with a gradual rise as we travelled northward, until the barometer-reading was 29 inches at Edinburgh at the same time, this would indicate that a region of depression existed over the former place, and we should expect a rush of air in the form of wind blowing upon London from the north. When the barometrical readings taken simultaneously at stations distributed over a wide area are compared, the distribution of atmospheric pressure can be ascertained, and it is possible to tell from this the force and direction of the winds prevailing within this area, and generally also the weather which is likely to be experienced. The greater the inequality of pressure, the greater will be the rush of air to the centre of depression, and the stronger will be the wind. The wind, however, does not flow in a straight line from a region of high to a region of low pressure. The surrounding air from all quarters has a tendency to flow in, and, as with water, which rushes to the centre of a funnel when it is flowing out at the bottom, a gyratory movement is the result. The wind blows round a centre of depression in this way, always curving inward towards the centre; and in the northern hemisphere, this gyratory movement of the wind is always in a direction against the hands of a watch, while the contrary is the case in the southern hemisphere. These principles of the relation of the winds to atmospheric pressure hold good without exception over all the world. They were first definitively stated in America twenty-five years ago; but Professor Buys Ballot of Utrecht first drew attention to them in Europe, and the law expressing them is now generally recognised as Buys Ballot’s law.