The state of the air at any time as regards heat, moisture, wind, rain, clouds, and electricity. The pressure of the atmosphere is an important factor in bringing about atmospheric changes, because air always flows from a region of high pressure to one of low pressure. The pressure of the atmosphere at any place is obtained from the readings of the barometer; but the direction and force of the wind depend upon the relative distribution of pressure at a given time over a large extent of country, and not upon the actual reading of the barometer at the particular place. Simultaneous observations are made at as many different stations as possible within the given area, and are communicated to a central station, where the readings are compared and the distribution mapped out. The direction of the wind can then be inferred from the fact that it blows from where the pressure is high to where it is low. The place where for the time being the pressure is lowest is said to be the seat of a barometric depression, and the heaviest fall of rain generally takes place in the neighborhood of such a depression. In the Weather Bureau of the United States, 83 per cent, of the forecasts given twenty-four hours previously have proved correct.