We are indebted to M. Buys Ballot, a Dutch meteorologist, for an invaluable generalization, the importance of which it is almost impossible to over-estimate. This distinguished _savant_ says:—“It is a fact above all doubt that the wind that comes is nearly at right angles to the line between the places of highest and lowest barometer readings. The wind has the place of lowest barometer at its left hand, and is stronger in proportion as the difference of barometer readings is greater.” These facts have been variously stated by other writers; for example: “Stand with your back to the wind, and the barometer will be lower on your left hand than on your right;” “Facing the wind the centre of depression bears in the right-hand direction,” statements which can be verified at any time by a brief study of the “Weather Charts” now published in the daily journals. The value of the law consists in its connecting the surface winds of our planet with the actual pressure of the air itself, and it admits of the following tabulation:— +-------------+-------------+-------------+-------------+ | The wind is | The wind is | The wind is | The wind is | | NORTHERLY | SOUTHERLY | EASTERLY | WESTERLY | | when the | when the | when the | when the | | BAROMETER | BAROMETER | BAROMETER | BAROMETER | | is, in the | is, in the | is, in the | is, in the | | N. ⎫ | N. ⎫ | N. High. | N. Low. | | & ⎬ about | & ⎬ about | S. Low. | S. High. | | S. ⎭ equal. | S. ⎭ equal. | E. ⎫ | E. ⎫ | | E. Low. | E. High. | & ⎬ about | & ⎬ about | | W. High. | W. Low. | W. ⎭ equal. | W. ⎭ equal. | +-------------+-------------+-------------+-------------+ which can be verified by the reader from the daily Weather Charts in the newspapers. The above are deductions from Buys Ballot’s Law, still further impressed on the memory by taking four outline maps of the British Isles, inserting the names of Thurso, Penzance, Yarmouth, and Valentia, with barometer readings of the kind above named at each place, and then drawing a large arrow in red ink across the centre of each map in the direction appropriate to the readings.