Their exertions served to define more closely the circumstances of stellar movement. The crucial question could now be put, whether they are governed by the force that binds the planets to the sun, or by some other form of attractive influence. In other words, is the law of gravitation universal? An answer could only be obtained experimentally, by computing, on gravitational principles, the paths of the best-known pairs, and then trying the fit. If the stars, as time went on, kept near their predicted places, the unity of nature in this respect might be safely inferred; although considerable discrepancies might in any case be expected, owing to errors of measurement minute in themselves, but large relatively to curves reduced by distance to hair-breadth dimensions.
This kind of inquiry was fairly started in 1827, when Savary computed the orbit of Xi Ursæ. His success made it almost certain that the pair moved under the planetary regimen, conformed to, there is no reason to doubt, by all binaries. John Herschel, although not the first, was the most effective early investigator of stellar orbits. His method, described before the Royal Astronomical Society January 13, 1832, and approved by the award of its Gold Medal in 1833, went to the root of the matter. The author declared it a mere waste of time to attempt to deal, by any refined or intricate process of calculation, with data so uncertain and irregular as those at hand. “Uncertain and irregular,” it must be repeated, because referred to a scale on which tenths of a second assume large proportions. He accordingly discarded, as mere pedantic trifling, such analytical formulæ as those employed by Savary and Encke, and had recourse to a graphical process, in which “the aid of the eye and hand” was used to “guide the judgment in a case where judgment only, and not calculation, could be of any avail.” The operation which he went on to explain was commended by Sir George Airy for its “elegance and practical utility.” Nothing more appropriate could have been devised than this plan, at once simple, ingenious, and accommodating, for drawing a curve representative of the successive relative positions of double stars. Its invention effectively promoted acquaintance with their orbits; most of those at present known having, indeed, been calculated with its aid.
In 1821, Herschel travelled, in Babbage’s company, through Switzerland and Italy. His only recorded adventure was an ascent of Monte Rosa. In the following year he visited Holland with James Grahame, the learned author of a “History of America”; and on the removal of South’s observatory to Passy, he again went abroad, starting with Babbage, but returning alone. This time he made a number of scientific acquaintances. His father’s name worked like a spell. “I find myself,” he said, “for his sake, received by all men of science with open arms.” His modesty forbade him to remember that his own merits were already conspicuous. In Paris, Arago and Fourier showed him all possible attentions; he was welcomed at Turin “like a brother” by Plana, “one of the most eminent mathematicians of the age;” at Modena, Amici was, if possible, still more cordial. “He is the only man,” Herschel told his aunt, “who has, since my father, bestowed great pains on the construction of specula.” “Among other of your inquiring friends,” he continued, “I should not omit the Abbé Piazzi, whom I found ill in bed at Palermo, and who is a fine, respectable old man, though, I am afraid, not much longer for this world. He remembered you personally, having himself visited Slough.”
On July 3 Herschel “made the ascent of Etna, without particular difficulty, though with excessive fatigue.” On the summit, reached before sunrise, by “a desperate scramble up a cone of lava and ashes, one thousand feet high,” he found himself “enveloped in suffocating sulphurous vapours”; and “was glad enough to get down,” after having made a reading of the barometer in concert with the simultaneous observations of the brothers Gemellaro at Catania and Nicolosi. The same night he arrived at Catania “almost dead” from the morning’s arduous climb, “and the dreadful descent of nearly thirty miles, where the mules could scarce keep their feet.”
In traversing Germany, he deviated to Erlangen, where Pfaff was engaged in translating Sir William Herschel’s writings; and visited Encke, Lindenau, and Harding, at Seeberg, Gotha, and Göttingen. With Göttingen he had a special tie through his creation, in 1816, an honorary member of the University; and at Göttingen, too, he hoped to meet Gauss—a man of strange, and—to the lay mind—unintelligible powers. “Gauss was a god,” one of his fellow-mathematicians said of him; but the “god” was on this occasion absent—feasting with the “blameless Ethiopians,” perhaps, like the Homeric deities when wanted. He was reported “inconsolable” for the lost opportunity, which seems never to have recurred.
From Munich Herschel wrote to his aunt, in view of his approaching visit to Hanover:—“I hope you haven’t forgotten your English, as I find myself not quite so fluent in this language (German) as I expected. In fact, since leaving Italy, I have so begarbled my German with Italian that it is unintelligible both to myself and to everyone that hears it: and what is very perverse, though when in Italy I could hardly talk Italian fit to be heard, I can now talk nothing else, and whenever I want a German word, pop comes the Italian one in its place. I made the waiter to-day stare (he being a Frenchman) by calling to him, ‘Wollen Sie avere la bontà den acet zu apportaren!’ But this, I hope, will soon wear off.”
His next foreign holiday was spent in France. He had designed a new instrument for measuring the intensity of the sun’s radiations, and was eager to experiment with it alternately at high and low levels, for the purpose of determining the proportion of solar heat absorbed by the earth’s atmosphere. This method was employed with fine effect by Professor Langley on Mount Whitney in 1881. Herschel carried his “actinometer” to the top of the Puy de Dôme in September 1826, and waited at Montpellier for “one day of intense sunshine,” in order to procure his second term of comparison. The Puy de Dôme, with its associated three hundred summits, strongly allured him. “I have been rambling over the volcanoes of Auvergne,” he wrote from Montpellier, September 17, “and propose, before I quit this, to visit an extinct crater which has given off two streams of lava at Agde, a town about thirty miles south of this place on the road to the Spanish frontier. Into Spain, however, I do not mean to go—having no wish to have my throat cut. I am told that a regular diligence runs between this and Madrid, and is as regularly stopped and robbed on the way.”
This exploratory turn alarmed Miss Herschel. “I fear,” she replied, “you must often be exposed to great dangers by creeping about in holes and corners among craters of volcanoes.” He was, nevertheless, only dissuaded by his mother’s anxious remonstrances from pursuing their study in Madeira and Teneriffe.
In the autumn of 1827, Babbage accompanied him to Ireland. The young Astronomer Royal, Sir W. R. Hamilton, was unluckily absent at the time of their visit; but he sent Herschel, by way of compensation, one of his brilliant optical essays, and a correspondence sprang up from which a lasting friendship developed.
Herschel’s scientific occupations at home were meanwhile various and pressing. He co-operated in the foundation of the Astronomical Society, and became in 1821 its first foreign secretary. In 1824 he undertook the more onerous duties of secretary to the Royal Society, and rented a house in Devonshire Street for the three years of his term of office. Astronomy, it might have been feared, should be at least temporarily shelved; yet he informed his aunt, April 18, 1825, “A week ago I had the twenty-foot directed on the nebulæ in Virgo, and determined the right ascensions and polar distances of thirty-six of them. These curious objects I shall now take into my especial charge—nobody else can see them.”