Some aerolites, instead of being shattered into fragments, have been observed to fall to the Earth intact, and bury themselves in the ground. Numerous instances have been observed during the last century, and masses of meteoric stones have been found in positions which clearly indicate that they must have fallen from the sky. Chemists have made analyses of the elements in these remarkable bodies, and have found them to contain iron, magnesium, silicon, oxygen, nickel, cobalt, tin, copper, &c. The spectrum of these aerolites, raised to incandescence, has been studied by Vogel and by the Swedish observer, Bernhardt Hasselberg (born 1848), who detected the presence of hydrocarbons, which are also present in cometary spectra.
When the existence of aerolites as celestial bodies was first recognised, Laplace suggested that they had been ejected from volcanoes on the Moon. This theory, although supported by Olbers and other astronomers, was soon rejected. Next, it was suggested that they were ejected from the Sun, and Proctor believed them to come from the giant planets. A very detailed discussion of the subject is to be found in Ball’s ‘Story of the Heavens’ (1886), in which he expresses views in harmony with those of the Austrian physicist Tschermak. Ball demonstrated that the meteors which fall to the Earth cannot have come from any other planet, nor from the Sun. Accordingly, he concluded that they were originally ejected by the volcanoes of the Earth many ages ago, when they were active enough to throw up pieces of matter with a velocity great enough to carry them away from the Earth altogether. Such meteors would, however, intersect the terrestrial orbit at each revolution.
The alternative theory to this, supported by Schiaparelli and Lockyer, is that the aerolites are merely larger members of the meteor-swarms, which have been deflected from their paths. The chief objection to this theory is the absence of connection between the meteoric showers and the falls of aerolites and bolides. Only on one occasion was a meteoric stone observed to fall during a shower. On November 27, 1885, during the shower of Andromedid meteors from Biela’s comet, a large bolide, weighing more than eight pounds, fell at Mazapil, in Mexico. This, however, was the only case hitherto observed; and it may have been merely a coincidence.
CHAPTER IX.
THE STARS.
The most remarkable progress in astronomy during the past century has been in the department of sidereal science, or the study of the Suns of space, observed for their own sakes, and not merely for the purpose of determining the positions of the Sun and Moon, and to assist navigation. Thanks to Herschel, the nineteenth century witnessed the steady development of stellar astronomy, combined with many important discoveries and investigations.
The one pre-Herschelian problem in sidereal astronomy was the distance of the stars. Owing to its bearing on the Copernican theory, the problem was attacked by the astronomers of the seventeenth and eighteenth centuries. Herschel made numerous attempts to detect the parallax of the brighter stars, but failed. Meanwhile there had been many illusions. Piazzi believed that his instruments—which in reality were worn out and unfit for use—had revealed parallaxes in Sirius, Aldebaran, Procyon, and Vega; Calandrelli, another Italian, and John Brinkley (1763-1835), Astronomer-Royal of Ireland, were similarly deluded; and in 1821 it was shown by Friedrich Georg Wilhelm Struve (1793-1864), the great German astronomer, that no instruments then in use could possibly be successful in measuring the stellar parallax. A few years later, however, Fraunhofer brought the refractor to a degree of perfection surpassing all previous efforts. In 1829 he mounted for the observatory at Königsberg a heliometer, the object-glass of which was divided in two, and capable of very accurate measurements. This heliometer eventually revealed the parallax of the stars in the able hands of Friedrich Wilhelm Bessel.
Friedrich Wilhelm Bessel was born at Minden, on the Weser, south-west of Hanover, on July 22, 1784. His father was an obscure Government official, unable to provide a university education for his son. Bessel’s love of figures, together with an aversion to Latin, led him to pursue a commercial career. At the age of fourteen, therefore, he entered as an apprenticed clerk the business of Kuhlenkamp & Sons, in Bremen. He was not content, however, to remain in that humble position. His great ambition was to become supercargo on one of the trading expeditions sent to China; and so he learned English, Spanish, and geography. But he never became a supercargo. In order to be fully equipped for such a position, he determined to learn how to take observations at sea, and his acquaintance with observation aroused a desire to study astronomy. He constructed for himself a sextant, and by means of this, along with a common clock, he determined the longitude of Bremen.
Such enthusiasm could not be long without its reward. For several years Bessel remained a clerk, and the hours devoted to study were those spared from sleep. He studied the works of Bode, Von Zach, Lalande, and Laplace, and in two years was able to compute the orbits of comets by means of mathematics. From some observations of Halley’s comet at its appearance in 1607, Bessel calculated its orbit, and forwarded the calculation to Olbers, then the greatest authority on cometary astronomy. Olbers was delighted at this work, and he sent the results to Von Zach, who published them. The self-taught young astronomer had accomplished a piece of work which fifteen years before had taxed the skill and patience of the French Academy of Sciences.
In 1805, Harding, Schröter’s assistant at Lilienthal, resigned his position for a more promising one at Göttingen. Olbers procured for Bessel the offer of the vacant post, which the latter accepted. At Lilienthal Bessel received his training as a practical astronomer. He remained in Schröter’s observatory until 1809. Although only twenty-five years of age, he had become so well known in Germany that in that year he was appointed Professor of Astronomy in the University of Königsberg, and was chosen to superintend the erection of the new observatory there. Within a few years a clerk in a commercial office had worked his way from obscurity to fame.
In 1813 the Königsberg Observatory was completed, and here Bessel worked for thirty-three years, until his death, on March 17, 1846. It was only about ten years before his death that he commenced his search for the stellar parallax, with the aid of Fraunhofer’s magnificent heliometer. He determined to make a series of measures on a small double star of the fifth magnitude in the constellation Cygnus, named 61 Cygni, the large proper motion of which led him to suspect its proximity to the Solar System. From August 1837 to September 1838 he made observations on 61 Cygni, and he found that there was an annual displacement which could only be attributed to parallax. In order to have no mistake, he made another year’s observations, which confirmed the results he arrived at previously, and all doubt was removed by a third series. The resulting parallax was 0·3483″, corresponding to a distance of 600,000 times the Earth’s distance from the Sun. This was confirmed some years later by C. A. F. Peters at Pulkowa, and still later by Otto Struve, who estimated the distance at forty billions of miles. Meanwhile, F. G. W. Struve, working at Pulkowa, found a parallax of 0·2613″ for Vega, but this was afterwards found to be considerably in error. Accordingly, Struve does not rank with Bessel as a successful measurer of star-distance. But independently of Bessel, another accurate measure had been made by Thomas Henderson, the great Scottish astronomer.