"Truly remarkable in the insight they give into the relationship of the various fields cultivated under the name of Physics.... A vein of humor is met here and there reminding the reader of Heaviside, never offending one's taste. These features, together with the lightness of touch with which Mr. McCormack has rendered them, make the volume one that may be fairly called rare. The spirit of the author is preserved in such attractive, really delightful, English that one is assured nothing has been lost by translation."--Prof. Henry Crew, in _The Astrophysical Journal_. "A very delightful and useful book.... The author treats some of the most recondite problems of natural science, in so charmingly untechnical a way, with such a wealth of bright illustration, as makes his meaning clear to the person of ordinary intelligence and education.... This is a work that should find a place in every library, and that people should be encouraged to read."--_Daily Picayune_, New Orleans. "In his translation Mr. McCormack has well preserved the frank, simple, and pleasing style of this famous lecturer on scientific topics. Professor Mach deals with the live facts, the salient points of science, and not with its mysticism or dead traditions. He uses the simplest of illustrations and expresses himself clearly, tersely, and with a delightful freshness that makes entertaining reading of what in other hands would be dull and prosy."--_Engineering News_, N. Y. "The general reader is led by plain and easy steps along a delightful way through what would be to him without such a help a complicated maze of difficulties. Marvels are invented and science is revealed as the natural foe to mysteries."--_The Chautauquan_. "The beautiful quality of the work is not marred by abstruse discussions which would require a scientist to fathom, but is so simple and so clear that it brings us into direct contact with the matter treated."--_The Boston Post_. "A masterly exposition of important scientific truths."--_Scotsman_, Edinburgh. "These lectures by Dr. Mach are delightfully simple and frank; there is no dryness or darkness of technicalities, and science and common life do not seem separated by a gulf.... The style is admirable, and the whole volume seems gloriously alive and human."--_Providence Journal_, R. I. "The non-scientific reader who desires to learn something of modern scientific theories, and the reasons for their existence, cannot do better than carefully study these lectures. The English is excellent throughout, and reflects great credit on the translator."--_Manufacturer and Builder_. "We like the quiet, considerate intelligence of these lectures."--_Independent_, New York. "Professor Mach's lectures are so pleasantly written and illumined with such charm of illustration that they have all the interest of lively fiction."--_New York Com. Advertiser_. "The literary and philosophical suggestiveness of the book is very rich." _Hartford Seminary Record_.