Popular Scientific Lectures

^ CORNELL UNJVERSITY LIBRARY BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND GIVEN IN 189I BY HENRY WILLIAMS SAGE BRAKY V-'^'. ^^bS" Popular scientific lectures 3 1924 012 322 354 All books are subject to recall after two weeks. Olin/Kroch Library .--DATE DUE ra:ii?ir^'-i« BY THE SAME AUTHOR. The Science of Mechanics. Translated from the Second German Edition by T. J. McCormack. 250 Cuts, 534 Pages. Half Morocco, Gilt Top. Price, $2.50. Popular Scientific Lectures. 313 Pages. 44 Cuts. Price, $1.00. Contributions to the Analysis of the Sensations. (In Preparation.) THE OPEN COURT PUBLISHING CO., 324 DEARBORN ST., CHICAGO. POPULAR SCIENTIFIC LECTURES ERNST MACH PROFESSOR OF PHYSICS IN THE UNIVERSITY OF PRAGUE TRANSLATED BY THOMAS J. McCORMACK WITH FORTY-FOUR CUTS AND DIAGRAMS CHICAGO THE OPEN COURT PUBLISHING COMPANY 1895 A- *5^'5 4'i^ copyright by The Open Court Publishing Co. chicago, illinois, 1894. ^•i> '# h(4 PREFACE. POPULAR lectures, owing to the knowledge they presuppose, and the time they occupy, can afford only a modicum of instruction. They must select for this purpose easy subjects, and re- strict themselves to the exposition of the simplest and the most essen- tial points. Nevertheless, by an appropriate choice of the matter, the charm and ihe. poetry of research can be conveyed by them. It is only necessary to set forth the attractive and the alluring features of a problem, and to show what broad domains of fact can be illuminated by the light radiating from the solution of a single and ofttimes unobtrusive point. Furthermore, such lectures can exercise a favorable influence by showing the substantial sameness of scientific and every-day thought. The public, in this way, loses its shyness towards scientific questions, and acquires an interest in scientific work which is a great help to the inquirer. The latter, in his turn, is brought to understand that his work is a small part only of the universal pro- cess of life, and that the results of his labors must redound to the benefit not only of himself and a few of his associates, but to that of the collective whole. I sincerely hope that these lectures, in the present excellent translation, will be productive of good in the direction indicated. E. Mach. Prague, December, 1894. TRANSLATOR'S NOTE. THE present lectures, extending ovar the period from 1864 to 1894, are here published in collected form for the first time. What few repetitions occur, in the way of examples and quotations, have been retained, as throwing additional light on the topics they are designed to illustrate. As the dates of the first five lectures are not given in the foot- ' notes they are here appended. The first lecture, ' On the Forms of Liquids," was delivered in 1868 and published with that "On Symmetry" in 1872 (Prague). The second and third lectures, on acoustics, were first published in 1865 (Graz) ; the fourth and fifth, on optics, in 1867 (Graz). They belong to the earliest period of Professor Mach's scientific activity, and with the lectures on electrostatics and education will more than realise the hope expressed in the author's Preface. The four remaining lectures are of a more philosophical character and deal principally with the methods and nature of scientific inquiry. In the ideas summarised in them will be found one of the most important contributions to the theory of knowledge made in the last quarter of a century. All the proofs of this translation have been read by Professor Mach himself. T. J. McCORMACK. La Salle, III., December, 1894. Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924012322354 TABLE OF CONTENTS. The Forms of Liquids . i The Fibres of Corti . 17 On the Causes of Harmony . 32 The Velocity of Light . ... 48 Why Has Man Two Eyes ? 66 On Symmetry . , . 89 On the Fundamental Concepts of Electrostatics . 107 On the Principle of the Conservation of Energy . 137 On the Economical Nature of Physical Inquiry .... 186 v' ,--, On Transformation and Adaptation in Scientific Thought , 214 On the Principle of Comparison in Physics ... 236 On Instruction in the Classics and the Mathematico-Physical Sciences . ... 259 — THE FORMS OF LIQUIDS. WHAT thinkest thou, dear Euthyphron, that the holy is, and the just, and the good ? Is the holy holy because the gods love it, or are the gods holy because they love the holy ? By such easy questions did the wise Socrates make the market-place of Athens un- safe and relieve presumptuous young statesmen of the burden of imaginary knowledge, by showing them how confused, unclear, and self-contradictory their ideas were. You know the fate of the importunate questioner. So called good society avoided him on the promenade. Only the ignorant accompanied him. And finally he drank the cup of hemlock—a lot which we ofttimes wish would fall to modern critics of his stamp. What we have learned from Socrates, however, our inheritance from him, — is scientific criticism. Every one who busies himself with science recognises how unsettled and indefinite the notions are which he has brought with him from common life, and how, on a minute examination of things, old differences are 2 THE FORMS OF LIQUIDS. effaced and new ones introduced. The history of science is full of examples of this constant change, de- velopment, and clarification of ideas. But we will not linger by this general consideration of the fluctuating character of ideas, which becomes a source of real uncomfortableness, when we reflect that it applies to almost every notion of life. Rather shall we observe by the study of a physical example how much a thing changes when it is closely examined, and how it assumes, when thus considered, increasing defi- niteness of form. The majority of you think, perhaps, you know quite well the distinction between a liquid and a solid. And precisely persons who have never busied themselves with physics will consider this question one of the easiest that can be put. But the physicist knows that it is one of the most difficult. I shall mention here only the experiments of Tresca, which show that solids subjected to high pressures behave exactly as liquids do ; for example, may be made to flow out in the form of jets from orifices in the bottoms of vessels. The supposed difference of kind between liquids and solids is thus shown to be a mere difference of degree. The common inference that because the earth is oblate in form, it was originally fluid, is an error, in the light of these facts. True, a rotating sphere, a few inches in diameter will assume an oblate form only if it is very soft, for example, is composed of freshly kneaded clay or some viscous stuff. But the earth, THE FORMS OF LIQUIDS. 3 even if it consisted of the rigidest stone, could not help being crushed by its tremendous weight, and must perforce behave as a fluid. Even our mountains could not extend beyond a certain height without crumbling. The earth may once have been fluid, but this by no means follows from its oblateness. The particles of a liquid are displaced on the ap- plication of the slightest pressure ; a liquid conforms exactly to the shapes of the vessels in which it is con- tained ; it possesses no form of its own, as you have all learned in the schools. Accommodating itself in the most trifling respects to the conditions of the vessel in which it is placed, and showing, even on its surface, where one would suppose it had the freest play, nothing but a polished, smiling, expressionless countenance, it is the courtier /ar excellence of the natural bodies. Liquids have no form of their own ! No, not for the superficial observer. But persons who have observed that a raindrop is round and never angular, will not be disposed to accept this dogma so unconditionally. It is fair to suppose that every man, even the weak- est, would possess a character, if it were not too difficult in this world to keep it. So, too, we must suppose that liquids would possess forms of their own, if the pressure of the circumstances permitted it, —if they were not crushed by their own weights. An astronomer once calculated that human beings could not exist on the sun, apart from its great heat, because they would be crushed to pieces there by their 4 THE FORMS OF LIQUIDS. own weight. The greater mass of this body would also make the weight of the human body there much greater. But on the moon, because here we should be much lighter, we could jump as high as the church- steeples without any difficulty, with the same muscular power which we now possess. Statues and "plaster" casts of syrup are undoubtedly things of fancy, even on the moon, but maple-syrup would flow so slowly there that we could easily build a maple-syrup man on the moon, for the fun of the thing, just as our children here build snow-men. Accordingly, if liquids have no form of their own with us on earth, they have, perhaps, a form of their own on the moon, or on some smaller and lighter heav- enly body. The problem, then, simply is to get rid of the effects of gravity; and, this done, we shall be able to find out what the peculiar forms of liquids are.

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