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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,” 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.”
Ever since that date theory has been guessing at it. The solar astronomer is still applying all the instruments of modern research to find * Rosa Ursina, by C. Scheiner, fol.; Bracciani, 1630.
out which of these 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 equatorial 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
* Observations on the Spots on the Sun, etc., 4°; London and Edinburgh, 1863.
solar physics." In 1843 he announced a decennial period of maxima and minima of sunspot 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 the purpose, go to establish a true periodicity in the number of sun-spots. But quite lately Schuster” 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 old-fashioned astronomers, who want rigid proof for every new theory. But the last * Periodicität der Sonnenflecken. Astron. Nach. XXI.,
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 17oo 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 distinct character during the time of a sun-spot maximum, and another, totally different, during a sun-spot minimum. A suspicion is entertained that the total quantity of heat received by the earth from the sun is subject to the same period. This would have far-reaching effects on storms, harvests, vintages, floods, and droughts; but it is not safe to draw conclusions of this kind except from a very long period of observations. Solar photography has deprived astronomers of the type of Carrington of the delight in devoting a life's work to collecting data. It has now become part of the routine work of an observatory. In 1845 Foucault and Fizeau took a daguerreotype photograph of the sun. In 1850 Bond produced one of the moon of great beauty, Draper having made some attempts at an even earlier date. But astronomical photography really owes its beginning to De la Rue, who used the collodion process for the moon in 1853, and constructed the Kew photoheliograph in 1857, from which date these instruments have been multiplied, and have given us an accurate record of the sun's surface. Gelatine dry plates were first used by Huggins in 1876. It is noteworthy that from the outset De la Rue recognised the value of stereoscopic vision, which is now known to be of supreme accuracy. In 1853 he combined pairs of photographs of the moon in the same phase, but under different conditions regarding libration, showing the moon from slightly different points of view. These in the stereoscope exhibited all the relief resulting from binocular vision, and looked like a solid globe. In 1860 he used successive photographs of the total solar eclipse stereoscopically, to prove that the red prominences belong to the sun, and not to the moon. In 1861 he similarly combined two photographs of a sunspot, the perspective effect showing the umbra like a floor at the bottom of a hollow penumbra;