PROCEEDINGS OF THE AMERICAN PHILOSOPHICAL SOCIETY HELD AT PHILADELPHIA FOR PROMOTING USEFUL KNOWLEDGE. VOL. XXXVII. JANUARY TO DECEMBER, 1898. LIBRARY OF THE UNIVERSITY OF CALIFORNIA PHILADELPHIA : THE AMERICAN PHILOSOPHICAL SOCIETY. UNIVERSITY Dr. T. J. J. See and Mr. Sydney Geo. Fisher, newly elected. members, were presented to the Chair and took their seats. Acknowledgments of election to membership were read from Messrs. Charles De Garmo, Arnold E. Ortmann, Thomas J. J. See, Alden Sampson, Sydney George Fisher, Benjamin Kendall Emerson, Francis L. Patton, Edward S. Holden, and Ethelbert Dudley Warfield. An invitation was received from the Academy of Natural Sciences of Philadelphia, to participate in a memorial meeting commemorative of Harrison Allen, M.D., and George H. Horn, M.D., to be held at the Academy on December 31, at 8 o'clock. The Judges and Tellers of the annual election reported the following officers elected for the ensuing year: President. Frederick Fraley. E. Otis Kendall, William Pepper, Coleman Sellers. Persifor Frazer, I. Minis Hays, Frederick Prime, Curators. J. Cheston Morris, Benjamin Smith Lyman, Henry Pettit. Treasurer. Councillors for three years. Henry C. Baird, Isaac J. Wistar, Jacob M. DaCosta. Dr. I. Minis Hays was nominated for Librarian for the ensuing year. Prof. Cleveland Abbe read a paper on Altitude of the Aurora Borealis." "The Accepted Mr. Sachse and Mr. Cook offered some remarks in discussion. Dr. T. J. J. See presented a paper for the Transactions on "The Evolution of the Stellar Systems," which was discussed by Prof. Doolittle, Prof. Snyder, Dr. See and Prof. Abbe. The Society was adjourned by the presiding member. THE ALTITUDE OF THE AURORA ABOVE THE EARTH'S SURFACE. BY CLEVELAND ABBE. (Read January 7, 1898.) During the past three centuries numerous observers and physicists, astronomers and magneticians have endeavored to contribute to our knowledge of the altitude of the region whence the auroral light. proceeds, and still the greatest diversity of opinion seems to prevail on this subject. Some observers have seen the light in such positions between themselves and neighboring objects as to demonstrate that the aurora, like the lightning, descends to the very surface of the earth and may even be entirely confined to the lowest stratum: prominent among these are Captain Parry, Sir James Clark Ross and Sir John Ross, his uncle, Dr. Walker and Prof. J. P. Lesley. Others, such as Dr. Richardson, Sir John Franklin, Silbermann, have seen it so located among the clouds that its origin must be placed at or below their level and, therefore, within a few thousand feet of the earth's surface. On the other hand, those who have calcu lated the altitudes of specific beams and arches by trigonometrical or equivalent methods have generally found figures indicating altitudes between twenty and a hundred miles. Perhaps the highest altitudes that have been deduced were the following: Dalton, 150 miles; Loomis, 400 to 600; Bergman, 468; Boscovich, 825; Fournerius, 1006; Twining, 1100; Boller, 2000 kilometers, or 1243 miles. Those who delight in numerical calculations accept these larger altitudes and content themselves with saying that the altitude of the aurora ranges from 50 miles upward to 1000. The experimental physicists, by studying the analogies between the auroral light and the discharge of electricity through vacuum tubes, have shown that the auroral phenomena harmonize in part at least with those observed in vacua such as might occur at moderate altitudes. Thus, Miller and De La Rue give altitudes of from ten to forty miles. Espy and Bache maintained that observers a few miles apart did not and could not have observed the same arches. The most careful observers have in many cases defended the accuracy of the observations made under circumstances that admit of no doubt that the auroral light in the free atmosphere often emanates from points within a few yards of the observer. Lemstrom has sought to reconcile the diverse conclusions by maintaining that while many auroras are quite high up and belong to the upper air, yet those in extreme northern latitudes most generally belong to the lowest strata and follow the unevenness of the ground, appearing as glows around the mountain top, or as rays directed toward prominent objects. The object of the present paper is to study some of the numerous observations, calculations and opinions bearing on the nature and the altitude of the auroral light. We shall not especially consider the electrical origin, or the source of the electricity, but simply acquiesce in the universal conviction that it really is one form of electrical discharge, our main object being to ascertain whether we can in any way definitely fix its locus in the atmosphere. The most instructive method of procedure consists in taking up the consideration of a number of authorities in chronological order, by which means one is led to appreciate the slow progress of knowledge and the difficulty which many investigators have felt, from time to time, in giving up preconceived views without having anything better to accept in their place. There is nothing more diffi |