Chapter I: Foreword

SINCE these lectures were delivered in the Royal Institution of Great Britain there has been much advance in our knowledge of astronomy. The simultaneous advance in other sciences allied with astronomy has been, perhaps, even more remarkable. I am glad to avail myself of the opportunity afforded by a new issue of “The Earth’s Beginning” to draw attention to certain recent developments of science which relate in a very striking way to the subject of this volume, namely, the famous Nebular Theory of the origin of the solar system. It appears to me that these recent developments tend to reduce greatly, even if they do not altogether remove, the chief outstanding difficulty which has hitherto retarded the acceptance of the Nebular Theory.

I have explained in Chapter VI. those views of Helmholtz which have for so long provided the received explanation of the maintenance of solar heat. Calculation shows that if the sun’s heat has been maintained by the contraction of the primæval nebula—and this was the supposition of Helmholtz—the orb of day cannot have radiated with its present intensity for a period much longer than twenty million years.

But from the evidence of geology it must now be admitted that the existence of our earth, indeed even that part of its existence during which it has been the abode of life, has endured for a period far in excess of that which this calculation would allow. It therefore seems to follow that the theory of Helmholtz does not provide an adequate explanation of such an amazing phenomenon as the continuance of a sufficient supply of sunbeams throughout the vast periods demanded by geological phenomena.

There is another entirely different line of reasoning by which Professor John Joly has recently taught us the immense antiquity of our earth. His argument is based upon an estimate of the time that must have elapsed since the waters of the ocean, which had previously been sustained in the great vapours of the atmosphere, were deposited in the ocean beds. When the earth had become sufficiently cool to permit of the vapours now forming the ocean passing from the gaseous to the liquid form, the oceans descended from the heavens above to the earth beneath in the form of _fresh_ water. In the lapse of subsequent ages the sea has become salt because ordinary river water, which always contains some small quantity of salt in solution, is continually bearing salt down to the sea. No doubt water is constantly being abstracted from the sea by evaporation, but only _fresh_ water is thus removed, so in this cycle of change the salt in the sea must be gradually accumulating. Thus, day by day, though no doubt extremely slowly, the sea has been growing more and more salt.

Professor Joly has made an estimate of the quantity of salt daily added to the sea by all the rivers of the globe. He has also made an estimate of the total quantity of salt which is at present contained in the sea. He has thus the means of forming an estimate of the number of years necessary for the sea to have become converted from its primæval freshness to its present saltness. His result is not a little astonishing. The saltness of the sea could not be accounted for unless the rivers had been running into the sea for at least a hundred million years. This period is five times as long as the total period during which the sun could have been shining if the Helmholtzian view were correct.

Of course, there are many elements of uncertainty in such a calculation. We have assumed that the total flow of the rivers is practically constant, and that our estimate fairly represents the average salinity of river water. We have also made a large assumption in supposing that we have accurately estimated the total volume of salt in the oceans. But taken in conjunction with the geological evidence already referred to, taken in conjunction with the immense periods of time that have been required for the evolution of life on the globe by the process of natural selection, the conclusion arrived at is inevitable. It seems impossible to doubt that the sun must have been shining and that our solar system must have existed in practically the same form as it is at present for periods enormously greater than would have been possible if the heat of the sun had been sustained by the solar contraction only.

The difficulty here indicated has been not unjustly considered the most serious difficulty with which the development of modern physical and astronomical science has been confronted. The time during which the sun must have lasted, according to the received explanation of the source of its heat and the time during which the sun has actually lasted, as shown by the facts of geology, present a wide discrepancy. Science demands that some reconciliation must be effected, yet how is that to be accomplished? There is only one possible solution of the problem. It is obvious that there must have been some vast reserve of heat in the sun in comparison with which the quantity of heat yielded by the contraction may be deemed insignificant. Until this new source of solar energy had been discovered, our knowledge of the physics of the solar system lay under a reproach, which it was the bounden duty of men of science to endeavour to remove.

During the last few years lines of research carried on in various directions have, in a most unexpected manner, thrown much light on the origin of the sun’s heat, and, indeed, we may now say that the great difficulty which has for so long troubled us no longer exists in a serious form.

Recent discoveries show that matter possesses stores of energy which, if not actually boundless, are enormously in excess of what had been previously deemed possible. These stores of energy are available for supplying the heat of the sun, and it is easy to show that they are amply sufficient to furnish the necessary sunbeams for even the longest periods during which the claims of geology maintain that the sun must have been shining.

The researches of Professor Sir J. J. Thomson have shown how corpuscles of matter are sometimes moving with velocities enormously greater than those of any celestial body with which astronomy had made us acquainted. The case of high corpuscular velocity which is most generally known is that presented by radium, the particles from which are being continually shot forth in myriads. It is quite true that each of these corpuscles is excessively small, and it may be useful to give the following illustration bearing on the subject. Think of a number represented by unity followed by eighteen cyphers, or more concisely as 10^{18}, and think of a line a kilometre long. If that line were divided into 10^{18} parts, each of those parts would represent the diameter of a corpuscle of radium. If that line were multiplied by 10^{18}, the result would be a line so long that a ray of light would require a period of no less than 100,000 years to pass from one end to the other.

These corpuscles of radium are, no doubt, excessively small, but the velocity with which they are moving is comparable with the velocity of light. When a material object is moving with a velocity of that magnitude the energy it contains in virtue of that velocity is indeed startling. A very small grain of sand would, if moving with the velocity of light, contain, in virtue of that motion, the equivalent of more heat than could be produced by the combustion of a ton of the best coal. The late Dr. W. E. Wilson showed that if an excessively minute percentage of radium should be found to exist in the sun, it would completely account for the sustentation of the solar heat, and the Hon. R. Strutt has shown that the minute quantities of radium which he has proved to exist in terrestrial rocks would enormously protract the earth’s cooling. These discoveries have, in fact, completely changed the outlook on the problem of the sun’s heat, and, though no doubt much has yet to be done before the whole subject is cleared up, the great difficulty may be regarded as vanquished. Thus, the discovery of radium, and the wonderful phenomena associated therewith, has pointed out a possible escape from one of the gravest difficulties in science.

The most notable fact which emerges from the modern study of the structure of the heavens is the ever-increasing significance and importance of the spiral nebulæ. The following pages will have failed in their object if they have not succeeded in emphasising the fact that the spiral nebula is, next to a fixed star itself, the most characteristic type of object in the material universe. With every increase in the power of the telescope, and with every development of the application of photography to celestial portraiture, the importance of the spiral structure in nebulæ becomes of ever-increasing interest.

But I revert to this subject here for the purpose of taking notice of a suggestive paper by Mr. C. Easton in the “Astrophysical Journal,” Vol. XII., No. 2, September, 1900, entitled “A New Theory of the Milky Way.” This paper advances the striking view that the Milky Way is itself a spiral nebula, and certainly the considerations adduced by Mr. Easton seem to justify his remarkable conclusion.

It is first to be noticed that the Milky Way extends as an irregular band completely round the heavens, and that it follows very nearly the course of a great circle. The curious convolutions of the Milky Way, the varying star densities of its different parts, would, as shown by Mr. Easton, be completely accounted for if the Milky Way were a mighty spiral. We view the ordinary celestial spirals from the outside at an immense distance in space. We view the Milky Way from a position within the circuit of the nebula. It has, however, been shown by Mr. Easton that the centre of the Spiral Nebula is not exactly at the sun. The centre of the Milky Way is near that superb region of the galaxy which lies in Cygnus.

Thus, the significance of the spiral structure in the universe becomes greatly enhanced. The spirals abound in every part of the heavens; they are placed in every conceivable position and in every possible plane; they have every range in size from comparatively small objects, whose destiny is to evolve into a system like our solar system, up to stupendous objects which include a myriad of such systems. There is now the further interest that as the sun and the solar system are included within the Milky Way, and as the Milky Way is a spiral, this earth of ours is itself at this moment a constituent part of a great spiral.

Finally, I would say that, so far as I have been able to understand the subject, it appears to me that every advance in our knowledge of the heavens tends more and more to support the grand outlines of the Nebular Theory as imagined by Kant and Laplace.

R. S. B.

_May 1, 1909._

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CONTENTS

CHAPTER PAGE

I. —INTRODUCTION 1

II. —THE PROBLEM STATED 21

III. —THE FIRE-MIST 39

IV. —NEBULÆ—APPARENT AND REAL 52

V. —THE HEAT OF THE SUN 75

VI. —HOW THE SUN’S HEAT IS MAINTAINED 95

VII. —THE HISTORY OF THE SUN 112

VIII. —THE EARTH’S BEGINNING 122

IX. —EARTHQUAKES AND VOLCANOES 158

X. —SPIRAL AND PLANETARY NEBULÆ 191

XI. —THE UNERRING GUIDE 207

XII. —THE EVOLUTION OF THE SOLAR SYSTEM 246

XIII. —THE UNITY OF MATERIAL IN THE HEAVENS 261
AND THE EARTH

XIV. —THE FIRST CONCORD 294

XV. —THE SECOND CONCORD 308

XVI. —THE THIRD CONCORD 324

XVII. —OBJECTIONS TO THE NEBULAR THEORY 337

XVIII. —THE BEGINNING OF THE NEBULA 348

XIX. —CONCLUDING CHAPTER 361

APPENDICES 369

INDEX 382

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LIST OF ILLUSTRATIONS

FIG. PAGE

An English Sunset tinged by Krakatoa _Frontispiece_
(colour)

1. Immanuel Kant (from an old print) 7

2. A Faint Diffused Nebulosity 17

3. The Crab Nebula 19

4. Jupiter 25

5. Nebulous Region and Star-cluster 33

6. The Great Nebula in Orion 41

7. The Dumb-bell Nebula 45

8. The Crossley Reflector 49

9. The Cluster in Hercules 53

10. Spectra of the Sun and Capella 62

11. Spectrum of Nebula in Orion and Spectrum 64
of White Star

12. Solar Spectra with Bright Lines and Dark 69
Lines during Eclipse

13. The Nebulæ in the Pleiades 71

14. The Sun 81

15. I. Spectrum of the Sun. II. Spectrum of 85
Arcturus

16. Brooks’ Comet and Meteor Trail 89

17. Argus and the surrounding Stars and 103
Nebulosity

18. Trifid Nebula in Sagittarius 105

19. To illustrate the History of the Sun 113

20. Solar Corona 117

21. The Great Comet of 1882 119

22. Special Thermometer for use in Deep 129
Borings

23. At the Bottom of the Great Bore 140

24. Three consecutive Shells of the Earth’s 145
Crust

25. Earthquake Routes from Japan to the Isle 171
of Wight

Showing Localities of Earthquakes 175
(colour)

26. Showing Coasts invaded by the Great 179
Sea-waves from Krakatoa

The Early Stage of the Eruption of 180
Krakatoa (colour)

27. Spread of the Air-wave from Krakatoa to 183
the Antipodes

28. The great Spiral Nebula 193

29. How to find the great Spiral Nebula 196

30. A group of Nebulæ 199

31. A Ray Nebula 201

32. Portion of the Milky Way 205

33. A Spiral Nebula seen Edgewise 211

34. A foreshortened Spiral 212

35. Edge-view of a Spiral boldly shown 213

36. To illustrate Moment of Momentum 223

37. Saturn 233

38. The Ring Nebula in Lyra 249

39. Lunar Craters: Hyginus and Albategnius 255

40. A remarkable Spiral 257

41. A clearly-cut Spiral 259

42. The H and K Lines in the Photographic 276
Solar Spectrum

43. Spectrum of Comet showing Carbon Lines 290

The Solar Spectrum (colour) 290

44. Spectrum of the Sun during Eclipse 291

45. A Spiral presented Edgewise 296

46. The Plane of a Planet’s Orbit 298

47. A Right Angle divided into Ten Parts 301

48. Illustration of the Second Concord 309

49. Orbits of the Earth, Eros and Mars 313

50. I. A Natural System. II. An Unnatural 318
System

51. An elongated irregular Nebula 329

52. Two-branched Spiral 345

53. Cluster with Stars of the 17th Magnitude 353

54. Spectrum of Nova Persei (1901) 359

55. The Apteryx: a Wingless Bird of New 365
Zealand

56. Skeleton of the Apteryx, showing 366
Rudimentary Wings

57. Spirals in other Departments of Nature: 367
Foraminifer

58. Ditto ditto Nautilus 367

59. To illustrate a Theorem in the 369
Attraction of Gravitation

60. First Law of Motion exemplifies Constant 375
Moment of Momentum

61. A useful Geometrical Proposition 376

62. Acceleration of Moment of Momentum 376
equals Moment of Force

63. Moment of Momentum unaltered by 380
Collision

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THE EARTH’S BEGINNING.