Page:Popular Science Monthly Volume 28.djvu/324

312 altitudes and velocities of air-currents. It is estimated that in the center of the funnel the air sometimes attains the enormous speed of two thousand miles an hour. The whirling movement is almost invariably in an opposite direction from that taken by the hands of a clock.

The weather-predictions of the Signal Service are distinct from the tornado-predictions, which involve local treatment that severely tests the science of meteorology. It is true the tornado region follows the usual storm-center along parallel lines, but at a distance of several hundred miles. The tornadoes develop far from the storm-center, and generally under conditions of partial sunshine and cloudiness and high humidity or excess of moisture. The relation of tornado-prediction to the usual weather-service is only in regard to details of temperature, wind-direction, dew-point, etc., as furnished by the general weather reports. The prediction of the movement of the usual storm-center is by no means so difficult as the attempt to even approximately locate the general region where a series of tornadoes will occur, because of the narrow track in which the destructive power is manifested. The officers of the Signal Service are careful to make no rash promises. While knowledge of the phenomena is not entirely complete, yet the advancement of the science is so marked and positive that tornadoes can be predicted for certain parts of States with a degree of average certainty that will, if carried out by the establishment of a system of signals in 1886, prove of very great value to the people. Already insurance companies have been enabled to take millions of dollars of tornado risks, and the more complete knowledge of the average danger for given localities will set the questions of premiums and rates of insurance upon a basis that will be profitable for the people as well as for the companies. The danger in localities will be established by averages, and the amount of precaution necessary will be known, and may be expressed in trustworthy percentages. This will economize expenditure both for insurance and tornado-retreats underground. The protection to life will be a very marked feature of the results attained. The approach of the tornado along its almost inevitable path, of from southwest to northeast, can be seen for fully an hour above the surface of a flat prairie, thus enabling people to get far beyond the reach of its narrow but fearfully destructive path. With this we close the question of the prediction of tornadoes for certain parts of States.

Let us now examine the closer prediction made by the trained observer or tornado-reporter, as he sees the tornado-cloud in process of formation. The question as to whether the furious movement of the clouds is forming the funnel-shape so much dreaded can only be decided by careful study of the sights and sounds described by hundreds of observers. A very important characteristic of tornado air-currents is that the disturbance begins in the upper air. In the "North American Review" for September, 1882, Professor T. B.