Very closely connected with atmospheric pressure, as shown us by the barometer, is the study of the direction of winds. If we take a map of the British Isles and the neighbouring countries, and put down upon them the barometer readings from a great number of observing stations, and then join together the different places which show the same barometric pressure, we shall find that these lines of equal pressure--technically called 'isobars'--are apt to run much nearer together in some places than in others. Clearly, where the isobars are close together it means that in a very short distance of country we have a great difference of atmospheric pressure. In this case we are likely to get a very strong wind blowing from the region of high pressure to the region of low pressure, in order to restore the balance. If, further, we had information from these various observing stations of the direction in which the wind was blowing, we should soon perceive other relationships. For instance, if we found that the barometer read about the same in a line across the country from east to west, but that it was higher in the north of the islands than in the south, we should then have a general set of winds from the east, and a similar relation would hold good if the barometer were highest in some other quarter; that is, the prevailing wind will come from a quarter at right angles to the region of highest barometer, or, as it is expressed in what is known as 'Buys Ballot's law,' 'stand with your back to the wind, and the barometer will be lower on your left hand than on your right.' This law holds good for the northern hemisphere generally, except near to the equator; in the southern hemisphere the right hand is the side of low barometer. The instruments for wind observation are of two classes: vanes to show its direction, and anemometers to show its speed and its pressure. These may be regarded as two different modes in which the strength of the wind manifests itself. Pressure anemometers are usually of two forms: one in which a heavy plate is allowed to swing by its upper edge in a position fronting the wind, the amount of its deviation from the vertical being measured; and the other in which the plate is supported by springs, the degree of compression of the springs being the quantity registered in that case. Of the speed anemometers, the best known form is the 'Robinson,' in which four hemispherical cups are carried at the extremities of a couple of cross bars. For the mounting of these wind instruments the old original Observatory, known as the Octagon Room, has proved an excellent site, with its flat roof surmounted by two turrets in the north-east and north-west corners, and raised some two hundred feet above high-water mark. [Illustration: THE ANEMOMETER ROOM, NORTH-WEST TURRET.]