Page:Popular Science Monthly Volume 43.djvu/406

392 higher latitudes—such was the simplest expression of Dove's theory given in text-books.

Under this provisory hypothesis meteorology made an immense progress, and some five-and-thirty years ago, Loverrier in France, and Fitzroy in England, ventured for the first time to foretell weather twenty-four hours in advance, or at least to send out warnings as to the coming storms. This bold step brought meteorologists face to face with a quite new problem. From the air pressure, the temperature, the moisture, and the winds observed at a certain hour of the day at various spots and telegraphed to a central station, they had to infer the next probable state of weather. So, leaving aside the great problems of atmospheric circulation, they directed their attention to the changes of weather rather than to the causes of the changes. For this purpose purely empirical laws were of great value. When the meteorologist saw on a weather chart a region of low atmospheric pressure, with winds blowing in spirals round and toward its center, he named it, by analogy with real cyclones, a "cyclonic disturbance" or a "cyclone," giving the name of "anticyclone" to the region of high atmospheric pressure—and he studied the tracks of both disturbances in their advance across the oceans and the continents. He did not inquire for the moment into the causes of the disturbances; he took them as facts, and, following Buys Ballot's law, he said that the wind will blow as a rule from the region of high barometic pressure (the anticyclone) to the region of low pressure (the cyclone), with a certain deflection to the right or to the left. Immense researches were made to study the routes followed by the centers of barometrical minima, and we now have splendid atlases showing the normal tracks of cyclones across the Atlantic Ocean, over Europe and the States, in Japan, in the Indian Ocean, and so on, at various seasons of the year. With these empirical data meteorologists attained such a perfection in their weather forecasts that in five cases out of six their previsions are now correct, while the coming gales are even foretold with a still greater accuracy.