Part 6

The navigation laws had, too, a certain external attraction about them which induced many men, not actually Protectionists, to believe in their necessity. The principle of the navigation laws was to impose such restrictions of tariff and otherwise as to exclude foreign vessels from taking any considerable part in our carrying trade. The law was first enacted in Oliver Cromwell's day, at a time when the Dutch were rivals on the sea, and when it was thought desirable to repress, by protective legislation, the energy of such experienced seamen and pushing traders. The navigation law was modified by Mr. Huskisson in 1823, but only so far as to establish that which we now know so well as the principle of reciprocity. Any nation which removed restrictions from British merchant marine was favored with a similar concession. The idea also was that these navigation laws, keeping foreigners out of England's carrying trade, enabled her to maintain always a supply of sailors who could at any time be transferred from the merchant marine to the royal navy, and thus be made to assist in the defence of the country.

Of course, the ship-owners themselves upheld the navigation laws, on the plea that, if the trade were thrown open by the withdrawal of protection, their chances would be gone; that they could not contend against the foreigners upon equal terms; that their interests must suffer, and that Great Britain would in the end be a still severer sufferer, because, from the lack of encouragement given to the native traders and the sailors, England would one day or another be left at the mercy of some strong power which, with wiser regulations, would keep up her protective system and with it her naval strength.

Nevertheless, the ship-owners and the Protectionists and those who raised the alarm-cry about England's naval defences were unable to maintain their sophisms in the face of growing education and of the impulse given by the adoption of free trade. In 1849 the navigation laws were abolished. We believe there are very few ship-owners who will not now admit that the prosperity of their trade has grown immensely, in place of suffering, from the introduction of the free-trade principle in navigation as well as in com and sugar.

(1846) THE DISCOVERY OF NEPTUNE, Sir Oliver Lodge

Among modern astronomical discoveries none has been regarded as more important than that of Neptune, the outermost known planet of the solar system. It was a rich reward to the watchers of the sky when this new planet swam into their ken. This discovery was hailed by astronomers as "the most conspicuous triumph of the theory of gravitation." Long after Copernicus even, the genius of philosophers was slow to grasp the full conception of a spherical earth and its relations with the heavenly bodies as presented by him. So it was also with the final acceptance of Newton's demonstration of the universal law of gravitation (1685), whereby he showed that "the motions of the solar system were due to the action of a central force directed to the body at the centre of the system, and varying inversely with the square of the distance from it." After making this discovery, Newton himself, with the aid of others, especially of the French mathematician Picard, labored for years to verify it, and still further verification was necessary before it could be fully comprehended and accepted by the scientific world. The discovery of the asteroids or small planets revolving in orbits between those of Mars and Jupiter, aided in confirming the Newtonian theory, which the discovery of Uranus, by Sir William Herschel (1781), had done much to establish.

From the time of Sir William Herschel the science of stellar astronomy, revealing the enormous distances of the stars--none of them really fixed, but all having real or apparent motions--was rapidly developed. The discovery of stellar planets, at almost incalculable distances, still further changed the aspect of the heavens as viewed by astronomers, and when the capital discovery of Neptune was made those men of science were well prepared for studying its nature and importance. These matters, as well as the simultaneous calculation of the place of Neptune by Adams and Leverrier, and its actual discovery by Galle, are set forth by Sir Oliver Lodge in a manner as charming for simplicity as it is valuable in its summary of scientific learning.

The explanation by Newton of the observed facts of the motion of the moon, the way he accounted for precession and nutation and for the tides; the way in which Laplace explained every detail of the planetary motions--these achievements may seem to the professional astronomer equally, if not more, striking and wonderful; but of the facts to be explained in these cases the general public is necessarily more or less ignorant, and so no beauty or thoroughness of treatment appeals to it or excites its imagination. But to predict in the solitude of the study, with no weapons other than pen, ink, and paper, an unknown and enormously distant world, to calculate its orbit when as yet it had never been seen, and to be able to say to a practical astronomer, "Point your telescope in such a direction at such a time, and you will see a new planet hitherto unknown to man"--this must always appeal to the imagination with dramatic intensity, and must awaken some interest in the dullest.

Prediction is no novelty in science; and in astronomy least of all is it a novelty. Thousands of years ago Thales, and others whose very names we have forgotten, could predict eclipses, but not without a certain degree of inaccuracy. And many other phenomena were capable of prediction by accumulated experience. A gap between Mars and Jupiter caused a missing planet to be suspected and looked for, and to be found in a hundred pieces. The abnormal proper-motion of Sirius suggested to Bessel the existence of an unseen companion. And these last instances seem to approach very near the same class of prediction as that of the discovery of Neptune. Wherein, then, lies the difference? How comes it that some classes of prediction--such as that if you put your finger in fire it will be burned--are childishly easy and commonplace, while others excite in the keenest intellects the highest feelings of admiration? Mainly, the difference lies, first, in the grounds on which the prediction is based; second, in the difficulty of the investigation whereby it is accomplished; third, in the completeness and the accuracy with which it can be verified. In all these points, the discovery of Neptune stands out as one among the many verified predictions of science, and the circumstances surrounding it are of singular interest.

Three distinct observations suffice to determine the orbit of a planet completely, but it is well to have the three observations as far apart as possible so as to minimize the effects of minute but necessary errors of observation. When Uranus was found old records of stellar observations were ransacked with the object of discovering whether it had ever been unwittingly seen before. If seen, it had been thought, of course, to be a star--for it shines like a star of the sixth magnitude, and can therefore be just seen without a telescope if one knows precisely where to look for it and if one has good sight--but if it had been seen and catalogued as a star it would have moved from its place, and the catalogue would by that entry be wrong. The thing to do, therefore, was to examine all the catalogues for errors, to see whether the stars entered there actually existed, or whether any were missing. If a wrong entry were discovered, it might of course have been due to some clerical error, though that is hardly probable considering the care spent in making these records, or it might have been a tailless comet, or possibly the newly found planet.

The next thing to do was to calculate backward, to see whether by any possibility the planet could have been in that place at that time. Examined in this way the tabulated observations of Flamsteed showed that he had unwittingly observed Uranus five distinct times; the first time in 1690, nearly a century before Herschel discovered its true nature. But more remarkable still, Le Monnier, of Paris, had observed it eight times in one month, cataloguing it each time as a different star. If only he had reduced and compared his observations, he would have anticipated Herschel by twelve years. As it was, he missed it. It was seen once by Bradley also. Altogether it had been seen twenty times.

These old observations of Flamsteed and those of Le Monnier, combined with those made after Herschel's discovery, were very useful in determining an exact orbit for the new planet, and its motion was considered thoroughly known. For a time Uranus seemed to travel regularly, and as expected, in the orbit which had been calculated for it; but early in the present century it began to be slightly refractory, and by 1820 its actual place showed quite a distinct discrepancy from its position as calculated with the aid of the old observations. It was thought at first that this discrepancy must be due to inaccuracies in the older observations, and they were accordingly rejected, and tables prepared for the planet based on the newer and more accurate observations only. But by 1830 it became apparent that it did not coincide with even these. The error amounted to about 20". By 1840 it was as much as 90", or a minute and a half. This discrepancy is quite distinct, but still it is very small; and had two objects been in the heavens at once, the actual Uranus and the theoretical Uranus, no unaided eye could possibly have distinguished them or detected that they were other than a single star.

The errors of Uranus, though small, were enormously greater than other things which had certainly been observed; there was an unmistakable discrepancy between theory and observation. Some cause was evidently at work on this distant planet, causing it to disagree with its motion as calculated according to the law of gravitation. If the law of gravitation held exactly at so great a distance from the sun, there must be some perturbing force acting on it besides all the known forces that had been fully taken into account. Could it be an outer planet? The question occurred to several, and one or two tried to solve the problem, but were soon stopped by the tremendous difficulties of calculation.

The ordinary problem of perturbation is difficult enough: Given a disturbing planet in such and such a position, to find the perturbations it produces. This was the problem that Laplace worked out in the Mécanique Céleste.

But the inverse problem--given the perturbations, to find the planet that causes them--such a problem had never yet been attacked, and by only a few had its possibility been conceived. Friedrich Bessel made preparations for solving this mystery in 1840, but he was prevented by fatal illness.

In 1841 the difficulties of the problem presented by these residual perturbations of Uranus excited the imagination of a young student, an undergraduate of Cambridge--John Couch Adams by name--and he determined to make a study of them as soon as he was through his tripos. In January, 1843, he was graduated as senior wrangler, and shortly afterward he set to work. In less than two years he reached a definite conclusion; and in October, 1845, he wrote to the astronomer-royal, at Greenwich, Professor Airy, saying that the perturbations of Uranus could be explained by assuming the existence of an outer planet, which he reckoned was now situated in a specified latitude and longitude.

We know now that had the astronomer-royal put sufficient faith in this result to point his big telescope at the spot indicated and begin sweeping for a planet, he would have detected it within 1-3/4º of the place assigned to it by Adams. But anyone in the situation of the astronomer-royal knows that almost every post brings absurd letters from ambitious correspondents, some of them having just discovered perpetual motion, or squared the circle, or proved the earth flat, or discovered the constitution of the moon or of ether or of electricity; and in this mass of rubbish it requires great skill and patience to detect such gems of value as may exist.

Now this letter of Adams's was indeed a jewel of the first water, and no doubt bore on its face a very different appearance from the chaff of which I have spoken; but still Adams was unknown: he had been graduated as senior wrangler, it is true, but somebody must be graduated as senior wrangler every year, and a first-rate mathematician is not produced every year. Those behind the scenes--as Professor Airy of course was, having been a senior wrangler himself--knew perfectly well that the labeling of a young man on his taking his degree is much more worthless as a testimony to his genius and ability than the general public is apt to suppose.

Was it likely that a young and unknown man should have solved so extremely difficult a problem? It was altogether unlikely. Still, he should be tested: he should be asked for explanations concerning some of the perturbations which Professor Airy had noticed, and see whether he could explain these also by his hypothesis. If he could, there might be something in his theory. If he failed--well, there was an end of it. The questions were not difficult. They concerned the error of the radius vector. Adams could have answered them with perfect ease; but sad to say, though a brilliant mathematician, he was not a man of business. He did not answer Professor Airy's letter.

It may seem a pity to many that the Greenwich equatorial was not pointed at the place, just to see whether any foreign object did happen to be in that neighborhood; but it is no light matter to derange the work of an observatory, and alter the plans laid out for the staff, into a sudden sweep for a new planet on the strength of a mathematical investigation just received by post. If observatories were conducted on these unsystematic and spasmodic principles they would not be the calm, accurate, satisfactory places they are.

Of course, if anyone had known that a new planet was to be found for the looking, any course would have been justified; but no one could know this. I do not suppose that Adams himself felt an absolute confidence in his attempted prediction. So there the matter dropped. Adams's communication was pigeonholed, and remained in seclusion eight or nine months.

Meanwhile, and quite independently, something of the same sort was going on in France. A brilliant young mathematician, Urban Jean Joseph Leverrier, born in Normandy in 1811, held the post of astronomical professor at the École Polytechnique, founded by Napoleon. His first published papers directed attention to his wonderful powers; and the official head of astronomy in France, the famous Arago, suggested to him the unexplained perturbations of Uranus as a worthy object for his fresh and well-armed vigor. At once he set to work in a thorough and systematic way. He first considered whether the discrepancies could be due to errors in the tables or errors in the old observations. He discussed them with minute care, and came to the conclusion that they were not thus to be explained away. This part of the work he published in November, 1845.

He then set to work to consider the perturbations produced by Jupiter and Saturn to see whether they had been accurately allowed for, or whether some minute improvements could be made sufficient to destroy the irregularities. He introduced several fresh terms into these calculations, but none of them of sufficient importance to do more than partly explain the mysterious perturbations. He next examined the various hypotheses that had been suggested to account for them. Were they caused by a failure in the law of gravitation or by the presence of a resisting medium? Were they due to some large but unseen satellite or to a collision with some comet?

All these theories he examined and dismissed for various reasons. The perturbations were due to some continuous cause--for instance, some unknown planet. Could this planet be inside the orbit of Uranus? No, for then it would perturb Saturn and Jupiter also, and they were not perturbed by it. It must, therefore, be some planet outside the orbit of Uranus, and in all probability, according to Bode's empirical law, at nearly double the distance from the sun that Uranus is. Finally he proceeded to determine where this planet was, and what its orbit must be to produce the observed disturbances.

Not without failures and disheartening complications was this part of the process completed. This was, after all, the real tug of war. Many unknown quantities existed: its mass, its distance, its eccentricity, the obliquity of its orbit, its position--nothing was known, in fact, about the planet except the microscopic disturbance it caused in Uranus, several thousand million miles away from it. Without going into further detail, suffice it to say that in June, 1846, he published his last paper, and in it announced to the world his theory as to the situation of the planet.

Professor Airy received a copy of this paper before the end of the month, and was astonished to find that Leverrier's theoretical place for the planet was within 1° of the place Adams had assigned to it eight months before. So striking a coincidence seemed sufficient to justify a Herschelian sweep for a week or two. But a sweep for so distant a planet would be no easy matter. When seen through a large telescope it would still only look like a star, and it would require considerable labor and watching to sift it out from the other stars surrounding it. We know that Uranus had been seen twenty times, and thought to be a star, before its true nature was discovered by Herschel; and Uranus is only about half as far away as Neptune.

Neither at Paris nor at Greenwich was any optical search undertaken; but Professor Airy wrote to ask M. Leverrier the same old question that he had fruitlessly put to Adams: Did the new theory explain the errors of the radius vector or not? The reply of Leverrier was both prompt and satisfactory--these errors were explained, as well as all the others. The existence of the object was then for the first time officially believed in. The British Association met that year at Southampton, and Sir John Herschel was one of its sectional presidents. In his inaugural address, on September 10, 1846, he called attention to the researches of Leverrier and Adams in these memorable words:

"The past year has given to us the new [minor] planet Astræa; it has done more--it has given us the probable prospect of another. We see it as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration."

It was nearly time to begin to look for it. So the astronomer-royal thought on reading Leverrier's paper. But as the national telescope at Greenwich was otherwise occupied, he wrote to Professor Challis, at Cambridge, to know whether he would permit a search to be made for it with the Northumberland equatorial, the large telescope at Cambridge University, presented to it by one of the Dukes of Northumberland.