Page:Popular Science Monthly Volume 40.djvu/713

Rh that Ferrel undertook, to be stopped only by the illness that caused his death.

In 1882 Ferrel accepted a professorship in the Signal Service, producing while there several special reports of high value, among which his Recent Advances in Meteorology should have first mention. He also lectured to the officers of the Signal Corps at Washington, and it is from these lectures that he subsequently prepared his Popular Treatise on the Winds, the most comprehensive statement of theoretical meteorology in the English language. He resigned this professorship in 1886, in his seventieth year. He had before this accumulated a competence from judicious investments of the small earnings of earlier years.

Ferrel's name is chiefly connected with his original investigations in meteorology. The first of these was made at Nashville, as stated above, but a more serious study was made in his Motions of Fluids and Solids relative to the Earth's Surface, prepared shortly after going to Cambridge, and published in Runkle's Mathematical Monthly. This is regarded by a most competent critic as "the starting-point of our knowledge of the mechanics of the atmosphere." It is here that he first clearly states the important law that "in whatever direction a body moves on the earth's surface there is a force arising from the earth's rotation which tends to deflect it to the right in the northern hemisphere, but to the left in the southern." This was published in May, 1858, six months before it was discussed, with the same result, in the French Academy of Sciences. Space can not be given here to show the great importance of this principle in meteorology, but if the reader desires to follow it to its applications he should consult the Treatise on the Winds, named above. As to the importance of the principle, let any one attempt to explain the motions of the wind and the distribution of atmospheric pressures without it, and he will soon see the service rendered to meteorology by Ferrel in its introduction. The essential quality of this principle may perhaps be briefly stated.

The general conception of the theory of the winds refers them to convectional movements, arising from the action of gravity on parts of the atmosphere of different temperatures. According to this, the poles, where the temperatures are low, should have high pressures, and the occurrence of low pressures there has been a stumbling-block to more than one writer on the subject; indeed, hardly an English text-book can be named that will lead the student around this difficulty. The consideration introduced by Ferrel is to the effect that the actual distribution of pressure does not depend only on differences of temperature, but also on the motions excited by reason of the pressure differences. The condition of steady motion, under which the winds are impelled by an