PROFESSOR DARWIN ON THE TIDES.*
PROFESSOR DARWIN gave the substance of this volume in a series of lectures delivered at the Lowell Institute of Boston, U.S. It is no reflection on his gift of popular exposition if we conjecture that even in a Boston audience, which justly possesses a high reputation for intelligence, there may have been found some who remembered, with a touch of sympathy, the legendary fate of Aristotle. That philosopher is said to have drowned himself because he failed to understand the causes of the tides or currents of the Euripus. It is certainly useless to attempt an epitome of explanations which are themselves highly compressed, and which have the help of illustrative diagrams. When we have got beyond the initial fact that the moon is one tide-generating force, • The Tides and Kindred Phenomena in the Solar Syittin. By George Howard Darwin. London: John Murray.
and the sun the other, that these two are in the pro- portion of two to one (about), and that these two influences combine with or counteract each other according to the posi- tion of the two bodies, we have left the easy part of our route behind us. (The reason of the proportion can be shortly stated : in consideration of its mass the sun should be twenty- live and a half million times more potent than the moon ; in consideration of its distance fifty-nine million times weaker ; hence the resulting force of something less than a half.) We must be content, therefore, with: leaving the substance of Professor Darwin's book with a general commendation to our readers, while we give some illustrations of its general interest and value, from some of what may be called its side issues.
The history of human speculation about the tides is not extensive, the reason being, of course, that the chief seats of ancient civilisation were near the tideless Mediterranean.
Strabo has preserved for us the speculations of two Greek physicists, one of whom observed the phenomenon in the Atlantic, the other in the Red Sea. It is interesting to observe that the descriptions given by both are verified by facts. In modern times Kepler and Galileo have discussed the subject, the latter attributing the tide-movement to the rotation of the earth. The foundations on which the now accepted theory is built were laid by Newton, and the chief builder thereon is Laplace, with whose name Professor Darwin associates those of Bernouilli, a French astronomer of the first half of the eighteenth century, and Lord Kelvin. Mankind, however, has not waited for the philosophers to construct a practically sufficient system of tide-calculation. The phenomena are constant and easily observed, and while mathematicians re- mained constant to the "high a priori road," persons actually interested in navigation recorded their experiences. Tide.
tables were thus constructed by methods which, as their results were of a commercial value, were kept secret. "At length," as Dr. Whewell writes in his History of the Induc-
tive Sciences, "men of science began to perceive that such calculations were part of their business; and that they were
called upon, as the guardians of the established theory of the universe, to compare it in the greatest possible detail with the facts." Such a system of prediction is obviously not only useful, but necessary.
To utilise the tide-force itself has naturally occurred to persons of a practically speculative turn. Most of us have done so in a small way, though we may not have risen to the grandiose conception of making the tides do the moving work of the world when its coal-supplies have been exhausted. Professor Darwin thinks that "there are other far more accessible funds on which to draw." One special application was, he tells us, embodied in the suggestion that "the rise and fall of old hulks on the tide would afford serviceable power." It was pointed out to the in. venter that a hulk of ten thousand ton displacement would generate only twenty horse-power by its rise and fall, "and he recognised that the idea was valueless." "it is the only instance," writes Professor Darwin, "of which I have ever heard where an inventor was deterred by the impracticability of his plan." We may conjecture that a multitude cf vexatious experiences of the inventing tribe has suggested this somewhat cynical observation.
Another interesting part of the subject is to be found in similar phenomena, some of them on the smallest, and others on the grandest, scale. Professor Darwin devotes his second chapter to " Seiches in Lakes," a seiche being a local name for the small tide-like risings and fallings of the water which have been long known to take place in the Lake of Geneva. They have been systematically observed for some years by Professor Foret of Lausanne. The account of them given in this volume is of no small interest,—one of the causes is probably to be found in miniature earthquakes, so slight that they would not be otherwise observed. This account also we cannot attempt to epitomise, but we may take occa- sion to quote some part of the remarks with which Professor Darwin introduces it :—
" People are nowadays too apt to think that science can only be carried to perfection with elaborate appliances, and yet it is the fact that many of the finest experiments have been made with cardboard, cork, and sealing-wax. The principal reason for elaborate appliances in the laboratories of universities is that a teacher could not deal with a large number of students if he had to show each of them how to make and set up his apparatus, and a student v!ould not be able to go through a large field of study if he had to spend days in preparation. Great laboratories have, indeed, a rather serious defect, in that they tend to make all but the very best students helpless, and thus to dwarf their powers of resource and inventiveness. The mass of scientific work is un- doubtedly enormously increased by these institutions, but the number of really great investigators seems to remain almost un- affected by them. But I must not convey the impression that, in my opinion, great laboratories are not useful. It is obvious, indeed, that without them science could not be taught to large numbers of students, and, besides, there are many investigations in which every possible refinement of apparatus is necessary. But I do say that the number of great investigators is but little in- creased by laboratories, and that those who are interested in science, but yet have not access to laboratories, should not give up their study in despair."
These are wise words especially worth noting in these days when individual effort of all kinds is in danger of being crushed by huge Trusts and combinations.
But the most interesting part of the whole volume is to be found when we come to the "Kindred Phenomena in the Solar System." From the axiom that "all moving systems which are subject to friction gradually come to rest" are deduced various conclusions as to the past and future of the earth and moon and other planets and satellites, conclusions which are supplemented by some very fascinating conjectures. If the earth's rotation is made slower by tide-friction, then we may look back to a past where it was much more rapid than it is now. This past is, indeed, very remote. The historical period of the earth's life is a quite insignificant fraction of it. (Hence our ability to calculate back to the eclipses of antiquity on the basis of present-time measures without anything more than "a small outstanding discrepancy.") But a retrospect over millions of years takes us back to a time when the rotation was very rapid indeed. And so— here the theory comes in—we arrive at a time when the earth spun round so swiftly that it was disrupted, and the fragments thus cast off became consolidated into the moon.
From the earth and moon we pass by reasoning of analogy to considerations of the other planets of the solar system, and even, beyond these, of the sidereal bodies.
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