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Page:Popular Science Monthly Volume 15.djvu/702

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By ROBERT H. SCOTT, M. A., F. R. S., Etc.

WHEN we are asked to give an account of the birth of a storm, we are reluctantly compelled to admit that our storms are, almost without exception, foundlings, and that, as the precise conditions to which they owe their origin are, for the most part, shrouded in uncertainty, warm discussions at times arise as to the parish whence they have set out on their wanderings.

Dove said long ago that storms were due to the interference of the polar current or the east wind with the equatorial current or west wind. He gave the winds these names, because on his views the east winds really consisted of air flowing from the north or south pole toward the equator, which was modified in the direction of its motion by its change of latitude; while west winds were really due to air endeavoring to make its way back to the pole from the equator, whose coarse was in its turn modified by its moving from lower to higher latitudes. To the conflict of these two grand currents, east and west winds, Dove attributed all our storms; but he did not attempt to explain how the currents came into collision.

These views, however correct on their cosmical principles, have been superseded, of late years at least, as regards the explanation of our winds, by the modern views of the relation between the wind and the distribution of barometrical pressure; but, unfortunately, we still remain in comparative ignorance of the ultimate causes to which this distribution of pressure, or the rise and fall of the barometer, are due. To give some conception of the existing difference of opinion on these fundamental principles of our science, I may say that while some authorities maintain that the force of the wind in a hurricane is caused by the amount of barometrical disturbance which accompanies it, others hold that the fall of the barometer at the center is itself, in great measure, due to the centrifugal force of the revolving mass of air.

Of the various theories which have been propounded to account for storms, which are generally more or less cyclonic in their character, I shall only mention four:

1. Some authorities, and among them our own countryman the Rev. Clement Ley, attribute the formation and subsequent progress of a storm to the condensation of moisture, but they apparently ignore the fact that many of our very heaviest rains do not give rise to cyclonic disturbances of serious character. For instance, when on April 10 and 11, 1878, 4·6 inches of rain fell at Haverstock Hill, we had no

  1. Founded on a lecture delivered by the author at the London Institution, February 3, 1879.