Page:Popular Science Monthly Volume 28.djvu/322

310 abnormal conditions of pressure, of wind-direction, of cloud-formation and movement."

The most remarkable and interesting feature of the development of tornadoes is the fact that they nearly always form southeast of a moving center of low pressure, and their tracks, scattered here and there, conform closely to the progressive direction of the main storm. For example, on February 19, 1884, forty-four tornadoes occurred in Georgia, Alabama, and South Carolina, but principally in Georgia and Alabama. They developed at a distance of from five hundred to two thousand miles from a storm-center that moved across the northern part of the United States, beginning at the northern extremity of the Rocky Mountains in Montana, thence southeasterly through Dakota, Minnesota, and Wisconsin to Northern Illinois and Indiana, northward through Michigan across Lake Huron, disappearing north of Quebec. This sudden sharp turn of the storm-center southward into Illinois and Indiana seems to have relation to the unprecedentedly large number of tornadoes that developed not far from the South Atlantic coast, extending inland as far as Southern Illinois and Indiana. This southward lunge of a mass of cold, moist air seems to have caused the abnormal conditions of temperature and dew-point, and the high winds necessary to cause the most tremendous exhibition of destructive tornado-power ever recorded by the Signal Service. This invariable location southeast of the storm-center is one of the main peculiarities of tornado development upon which the predictions depend.

One of the best illustrations of the advance made in definiteness in prediction during 1885 occurred on August 3d, in the instance of the tornado at Camden, New Jersey, and at Philadelphia, Pennsylvania. In October, 1885, the writer had sent a short communication to the press of the country, advocating that tornado-signals of either safety or danger be shown during certain seasons of the year at all telegraph stations in States in which tornadoes are frequent. The gentlemen of the press had generally favored the scheme, and one of the editors wanted to know if the tornado at Camden had been predicted. It occurred at 3.20 (seventy-fifth meridian time), and was very destructive, involving a loss of about half a million dollars' worth of property. The chart used by Lieutenant Finley shows that tornadoes were predicted and their location marked upon the map for the States of Delaware, Southeastern Pennsylvania, and for New Jersey. The tornadoes actually occurred in these States about eight hours from the time of the prediction, which was made on the basis of the 7 (seventy-fifth meridian time) telegraphic reports. On that day heavy wind-storms were predicted for Vermont, New Hampshire, Massachusetts, New York, Northern New Jersey, and Eastern Pennsylvania. Nature carried out these predictions with as fair a degree of accuracy and with as definite a conformity to location as could be expected at the present primary stage of this science.