The observer requires only to assure himself that this progression possesses certain characteristics, to feel assured that he is dealing with no accidental relations, but with true periodic changes. These characteristics are, chiefly, a steady (not uniform) progression from maximum to minimum, and vice versâ, a rapidity of change midway between maximum and minimumas compared with the rate of change near either of these stages, and a certain uniformity of character in the progression towards each stage (though the progression from minimum to maximum may not resemble in character the return towards the minimum). These characteristics Schwabe noted, and he felt satisfied that the numerical relations of the solar spots vary in a truly periodic manner. But he felt that further observations were necessary to convince the scientific world-always slow to recognise new truths, or to accept results not rendered palpable, so to speak, by an accumulation of evidence. He laboured on therefore for twenty more years, tracing the gradual increase and diminution of spots in frequency and in their general dimensions, until he had completed the observation of no less than three complete oscillations from maximum frequency through minimum back to maximum again. It began to be felt that Schwabe was establishing his case, and accordingly the scientific societies of Europe (somewhat tardily, it must be admitted) accepted the result which Schwabe had demonstrated,-viz., that the solar spots increase and diminish in frequency and size in a period of about ten years. The following remarks, addressed by the Rev. Mr. Main, then President of the Royal Astronomical Society, in awarding to Schwabe the Society's Gold Medal for 1857, justly express the importance of Schwabe's researches: 6 What the Council wish most emphatically to express is their admiration of the indomitable zeal and untiring energy which Herr Schwabe has displayed in bringing that research to a successful issue. Twelve years he spent to satisfy himself; six more years to satisfy, and still thirteen more to convince mankind. For thirty years never has the Sun exhibited his disc above the horizon of Dessau without being confronted by Schwabe's imperturbable telescope, and that appears to have happened on an average about 300 days a year. So that supposing he observed but once a day, he has made 9,000 observations, in the course of which he discovered 4,700 groups. This is, I believe, an instance of devoted persistence (if the word were not equivocal, I should say pertinacity) unsurpassed in the annals of astronomy. The energy of one man has revealed a phenomenon which had eluded even the suspicion of astronomers for 200 years.' But even after these thirty-one years of labour, Schwabe continued his observations. With the exception of a few weeks during which he was unwell, he has watched the Sun as pertinaciously as ever until the present time. The results of his labours (so far as the question of the periodicity of spot-frequency is concerned) are included in the table on p. 200. The careful study of Schwabe's results (combined with such scattered records as the observations of former astronomers supply) has led Professor Wolf, of Zurich, to the conclusion that a period of 11·11 years (or the ninth part of a century) is indicated, rather than a ten-yearly period. He also recognises the He finds,' says Sir John existence of minor periods. Herschel, that a perceptibly greater degree of apparent activity prevails annually, on the average, in the months of September to January, than in the other months of the year; and, again, by projecting all the results in a continuous curve, he finds a series of small undulations succeeding each other at an average interval of 7·65 months, or 0.637 of a year. Now, the periodic time of Venus (2254), reduced to a fraction of a year, is 0.616,— a coincidence certainly near enough to warrant some considerable suspicion of a physical connection.' A long period, estimated at about fifty-six years, has also been suspected. The most cursory examination of the numbers in the table given in the next page, suffices to indicate the peculiarity that the progression from minimum to maximum is more rapid than the progression from maximum to minimum. In other words, if we regard the periodic changes of spot-frequency in the light of a series of waves-the maxima corresponding to the crests of the waves and the minima to the troughs' the front slope of each wave is more abrupt than the rear slope. One of the most remarkable of the results following Schwabe's noble discovery was the recognition of an association between the Sun-spot period and magnetic disturbances on the Earth. In every part of the Earth the magnetic needle has at any given epoch a certain definite position, about which, under normal conditions, it would oscillate during the day. Both as regards inclination and direction. with respect to the compass-points (called the magnetic declination), this position may be regarded as determinate, at least for every fixed observatory; and, further, the intensity of the needle's directive power, that is, the energy with which, if slightly disturbed, it seeks to recover its position of rest, may also be regarded as determinate. From year to year all these magnetic elements undergo change; but with these changes we are not here concerned.* Changes of a much more * I may note here my belief that the recognition of the laws affecting the secular variation of the Earth's magnetism would be simplified if attention were primarily directed to the magnetic lines determined by the inclination of the needle, instead of to those which depend on the intensity of the directive action. Fully admitting the weight of General Sabine's arguments as to the importance of the intensity, and as to its being the more essentially magnetic element, as it were, (north and south, or vertical and horizontal, having no direct relation to the Earth's magnetic forces,) I yet cannot but regard the inclination as affording the more trustworthy means of determining the geographical features (so to speak) of terrestrial magnetism. As the matter is of some importance, I will exhibit the reasons on which I found this opinion, leaving others to judge whether they are or are not valid. The intensity, as well as the inclination and declination (which together form one magnetic feature,-the position of rest), is doubtless affected by local circumstances; so that, superadded, as it were, on the normal intensity, there is an intensity depending on local conditions; and so in like manner, as to the inclination and declination. Now, the question arises, which of these two features is likely to be most significantly affected by these doubtless slight peculiarities? In determining, for |