Mr. Mitchell is not only an accomplished astronomer, in every respect qualified to be the interpreter of the mysteries of his science to the popular mind, but, if we may judge from the style of his book, is a fine, frank, warm-hearted, enthusiastic man. On every page he gives evidence of really loving his pursuit. By a certain sensitiveness of imagination, and quickness of sensibility, every thing he contemplates becomes alive in his mind, and an object in which he takes a personal interest. This gives wonderful distinctness to his exposition of natural laws, and his delineation of the characters and pursuits of men of science. His Copernicus, Kepler, Gallileo and Newton are not dry enumerations of qualities, but vivid portraits of persons. He seems in close intellectual fellowship with them as individuals, and converses of them in the style of a friend, whose accurate knowledge is equalled by his intense affection. So it is with his detail of the discovery of a new law, or fact in science. His mind "lives along the line" of observation and reasoning which ended in its detection, and he reproduces the hopes, fears, doubts, and high enthusiasm of every person connected with the discovery. His delineation of Kepler is especially genial and striking. By following this method he infuses his own enthusiasm into the reader, bears him willingly along through the most abstruse processes of science, and at the end leaves him without fatigue, and ready for a new start. In the treatment of scientific discoveries, by minds like Mr. Mitchell's, we ever notice an unconscious personification of Nature, as a cunning holder of secrets which only the master-mind can wrest from her after a patient siege. The style of our author glows in the recital of the exploits of his band of astronomers, as that of a Frenchman does in the narration of Napoleon's campaigns. This is the great charm of his book, and will make it extensively popular, for by it he can attract any reader capable of being interested in a tale of personal adventure, ending in a great achievement. We can hardly bring to mind a popular lecturer or writer on science, who has this power to the extent which Mr. Mitchell possesses it. He himself has it by virtue of the mingled simplicity and intensity of his nature.