Page:Popular Science Monthly Volume 43.djvu/137

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the Colombos were numbered. Even the house in which the family lived is pointed out. Then follows the story of Columbus's journey to Portugal, his weary waiting in Spain, his voyages, discoveries, misfortunes, and last days spent in pleading with the unappreciative Ferdinand. The tale is related in very simple but graphic fashion, with many touches of humor, while the varied illustrations constantly keep fresh the flavor of the time. Only those anecdotes are given that come from authentic sources, and the recent labors of Mr. Henry Harrisse and Signor Stalieno have added so largely to the fund that there are enough to make the narrative sufficiently life like. Xo attempt is made to screen the failings of Columbus—his pursuit of wealth, his curious theories, and the evil which is chargeable to him as an exponent of his time, the establishment of slavery in the New World. On the other hand, these are not enlarged until they obscure his courageous project and unflagging zeal. He still remains "the most conspicuous figure in the history of his age." He crossed the sea of darkness, and we rightly honor him for his great achievement.

The Visible Universe. By J. Ellard Gore, F. R. A. S. London: Crosby Lockwood & Son. New York: Macmillan & Co. Pp. 346. Price, $3.76.

Although astronomers have not yet solved the problem of celestial construction, the author of this volume refrains from adding any new conjecture to the list. He examines critically all the explanations worth serious mention, and this task may well have '. served to keep him within the dry land of | fact. Besides the theoretical discussions, the book contains the latest observations of the position of stars and nebula; and, so far as known, their motions and chemical composition.

Five principal objections have been brought against the nebular theory; most of these have been well answered by M. Roche. According to M. Wolf, two points are yet undetermined—how large planets were formed from the nebulous mass, and how the equatorial and orbital inclinations were produced. M. Faye, however, finds the fifth objection—the retrograde motion of the satellites of Uranus and Neptune—destructive of Laplace's theory and advances another hypothesis in his work, Sur l'Origine du Monde, with which Mr. Gore agrees. In this he assumes that the earth was formed before the sun, and that its internal heat sufficed for the evaporation of water and for the uniform vegetation that existed for æons of time. Laplace did not explain the origin of the primitive nebula, therefore Dr. Croll considered the hypothesis incomplete and furnished a cause in his impact theory. Two dark bodies endowed with enormous velocity collided in space and produced a perfect nebula!

A contention which promises no settlement is the duration of the sun's heat in past time. Noted physicists allow only twelve millions of years as the maximum period on the gravitation theory. This is insufficient for the geologists, who demand a hundred millions for the denudation of rocks. Dr. Croll's careful estimate is ninety millions; while biologists ask for a still longer period for the evolution of species. Most astronomers concur in the theory of Helmholtz that the heat of the sun is caused by the shrinkage of its mass through gravitation. To this philosopher also is due the vortex-ring idea—that matter consists of whirling portions of the luminiferous ether. This wondrous fluid, supposed to fill interstellar space and act as a medium for the transmission of light, is enormously elastic and wholly unlike matter, since planetary motion is not retarded by it as it would be by the most attenuated gas.

The spectroscope, which has revealed so much of the constitution of the stars, shows also another defect in the nebular theory, unless chemists may come to the rescue. The spectra of various nebulae give only hydrogen and one other unknown element. If the solar system was evolved from a nebulous mass by condensation, whence the dozen elements of the sun and the sixty-five of our own planet? It has been suggested that all our elements may be further resolved into one original element. In anticipation of its discovery this has been named protyle.

Lockyer's hypothesis was that the upper reaches of the atmosphere contained particles of magnesium, manganese, iron, and carbon, and that nebulæ were swarms of meteoritic dust. His observations in regard to