t

stars by means of the spectroscope, and when Warren De la Rue set up a photoheliograph at Kew, we see that a progress in the same direction as before, in the evolution of our conception of the universe, was being made. Without definite expression at any particular date, it came to be an accepted fact that not only do earthly dynamics apply to the heavenly bodies, but that the laws we find established here, in geology, in chemistry, and in the laws of heat, may be extended with confidence to the heavenly bodies. Hence arose the branch of astronomy called astronomical physics, a science which claims a large portion of the work of the telescope, spectroscope, and photography. In this new development it is more than ever essential to follow the dictum of Tycho Brahe-not to make theories until all the necessary facts are obtained. The great astronomers of to-day still hold to Sir Isaac Newton's declaration, "Hypotheses non fingo." Each one may have his suspicions of a theory to guide him in a course of observation, and may call it a working hypothesis. But the cautious astronomer does not proclaim these to the world; and the historian is certainly not justified in including in his record those vague speculations founded on incomplete data which may be demolished to-morrow, and which, however attractive they may be, often do more harm than good to the progress of true science. Meanwhile the accumulation of facts has been prodigious, and the revelations of the telescope and spectroscope entrancing.

12. The Sun.

One of Galileo's most striking discoveries, when he pointed his telescope to the heavenly bodies, was that of the irregularly shaped spots on the sun, with the

dark central umbra and the less dark, but more extensive, penumbra surrounding it, sometimes with several umbræ in one penumbra. He has left us many drawings of these spots, and he fixed their period of rotation as a lunar month.

It is not certain whether Galileo, Fabricius, or

They all did

Scheiner was the first to see the spots. good work. The spots were found to be ever varying in size and shape. Sometimes, when a spot disappears at the western limb of the sun, it is never seen again. In other cases, after a fortnight, it reappears at the eastern limb. The faculæ, or bright areas, which are

seen all over the sun's surface, but specially in the neighbourhood of spots, and most distinctly near the sun's edge, were discovered by Galileo. A high telescopic power resolves their structure into an appearance like willow-leaves, or rice-grains, fairly uniform in size, and more marked than on other parts of the sun's surface.

Speculations as to the cause of sun-spots have never ceased from Galileo's time to ours. He supposed them to be clouds. Scheiner said they were the indications of tumultuous movements occasionally agitating the ocean of liquid fire of which he supposed the sun to be composed.

A. Wilson, of Glasgow, in 1769,2 noticed a movement of the umbra relative to the penumbra in the transit of the spot over the sun's surface; exactly as if the spot were a hollow, with a black base and grey shelving sides. This was generally accepted, but later investigations have contradicted its universality. Regarding the cause of these hollows, Wilson said :—

Whether their first production and subsequent numberless changes depend upon the eructation of elastic vapours from below, or upon eddies or whirlpools commencing at the surface, or upon the dissolving of the luminous matter in the solar atmosphere, as clouds are melted and again given out by our air; or, if the reader pleases, upon the annihilation and reproduction of parts of this resplendent covering, is left for theory to guess at.3

Ever since that date theory has been guessing at it. The solar astronomer is still applying all the instruments of modern research to find out which of these

Rosa Ursina, by C. Scheiner, fol.; Bracciani, 1630. • R. S. Phil. Trans., 1774.

3 Ibid, 1783.

suppositions, or what modification of any of them, is nearest the truth. The obstacle-one that is perhaps fatal to a real theory-lies in the impossibility of reproducing comparative experiments in our laboratories or in our atmosphere.

Sir William Herschel propounded an explanation of Wilson's observation which received much notice, but which, out of respect for his memory, is not now described, as it violated the elementary laws of heat.

Sir John Herschel noticed that the spots are mostly confined to two zones extending to about 35° on each side of the equator, and that a zone of equatoreal calms is free from spots. But it was R. C Carrington who, by his continuous observations at Redhill, in Surrey, established the remarkable fact that, while the rotation period in the highest latitudes, 50°, where spots are seen, is twenty-seven-and-a-half days, near the equator the period is only twenty-five days. His splendid volume of observations of the sun led to much new information about the average distribution of spots at different epochs.

Schwabe, of Dessau, began in 1826 to study the solar surface, and, after many years of work, arrived at a law of frequency which has been more fruitful of results than any discovery in solar physics. In 1843 he announced a decennial period of maxima and minima of sun-spot displays. In 1851 it was generally accepted, and, although a period of eleven years has been found to be more exact, all later observations, besides the earlier ones which have been hunted up for

1 Observations on the Spots on the Sun, etc., 4°; London and Edinburgh, 1863.

2 Periodicität der Sonnenflecken. Astron. Nach. XXI., 1844, p. 234.

the purpose, go to establish a true periodicity in the number of sun-spots. But quite lately Schuster1 has given reasons for admitting a number of co-existent periods, of which the eleven-year period was predominant in the nineteenth century.

In 1851 Lamont, a Scotchman at Munich, found a decennial period in the daily range of magnetic declination. In 1852 Sir Edward Sabine announced a similar period in the number of "magnetic storms" affecting all of the three magnetic elements-declination, dip, and intensity. Australian and Canadian observations both showed the decennial period in all three elements. Wolf, of Zurich, and Gauthier, of Geneva, each independently arrived at the same conclusion.

It took many years before this coincidence was accepted as certainly more than an accident by the oldfashioned astronomers, who want rigid proof for every new theory. But the last doubts have long vanished, and a connection has been further traced between violent outbursts of solar activity and simultaneous magnetic storms.

The frequency of the Aurora Borealis was found by Wolf to follow the same period. In fact, it is closely allied in its cause to terrestrial magnetism. Wolf also collected old observations tracing the periodicity of sun-spots back to about 1700 A.D.

Spoerer deduced a law of dependence of the average latitude of sun-spots on the phase of the sun-spot period.

All modern total solar eclipse observations seem to show that the shape of the luminous corona surrounding the moon at the moment of totality has a special

1 R. S. Phil. Trans. (ser. A), 1906, p. 69-100.