otherwise have been found. It was by means of much more refined apparatus that Fraunhofer, of Munich, was enabled to observe and discover 576 dark lines in the solar spectrum, although Wollaston, by means of the prism, had previously found only seven. Airy also, by adopting a modified mode of observation, discovered that all the different varieties of polarised light might be obtained by means of rock-crystal, according to the direction in which the ray was transmitted, and that a crystal of quartz is capable of exhibiting every variety of elliptical polarisation of light.

It was by the use of a more sensitive thermo-electric pile than the original one of Seebeck, that Melloni was enabled to make his discoveries in diathermancy. By means of the very greatly increased delicacy of the spectroscope as an instrument of chemical analysis over that of previous chemical tests, Swan was enabled to detect the widely diffused presence of excessively minute quantities of common salt. It was by the assistance also of greatly increased sensitiveness of the galvanometer which he employed, that Du Bois Reymond was enabled to make his discoveries of electric currents in animal tissues. As the molecular phenomena of nature are extremely minute in comparison with the power of the senses, and we are acquainted only with the more crude existences and actions, it is probable that much of the discovery of the future will be effected by means of still greater refinements and powers of our apparatus and means of observation.

c. By means of more intelligent and acute observation. Although this cannot be properly classed as a different method, it is so common an occurrence that I venture to place it under a separate heading. Many discoveries have been made by attentively observing effects which would otherwise have passed unnoticed; for it is 1 See p. 179.

DISCOVERY BY MEANS OF NEW MODES OF OBSERVATION. 575

often the case, that effects, when first obtained, are so extremely feeble that they can hardly be perceived at all. The movements of the magnetic needle, for instance, which revealed the existence of magnetic storms over large portions of the earth, were many of them so small as to be microscopically minute, and required the most acute observation in order to detect them.

d. By the combined action of many observers.-A large amount of discovery in astronomical, magnetical, and meteorological science, and the subject of cosmical spectrum analysis, has resulted from the combined intellectual action of many observers in different parts of the world. In consequence of the advice of M. Humboldt, the Imperial Academy of Russia, about the year 1830, established a chain of magnetic stations right across the entire Russian Empire; at Petersburg, Moscow, Helsingfors in Finland, Catherinburg, Kasan, Barnaoul, and Nertschinsk in Siberia, Sitka in Russian America, and even in Pekin. In the year 1835, similar stations were erected all over Germany; also at Stockholm, Copenhagen, Upsala, Milan, Munich, Dublin, and Greenwich; and at these places simultaneous observations of the three magnetic elements of intensity, declination and inclination, were taken six times in each year, at intervals of five minutes each, during twenty-four hours. Subsequently, also, stations were formed at Toronto, St. Helena, the Cape of Good Hope, Van Diemen's Land, Bombay, Simla, Singapore, and at Kelso in Scotland; and afterwards at Brussels, Prague, Algiers, Cadiz, Cairo, Lucknow, Travancore, and at Philadelphia and Cambridge, U.S. America; and the reduction and comparison of the observations made at all these places have resulted in the discovery of a large amount of valuable knowledge respecting terrestrial magnetism.

CHAPTER LVII.

DISCOVERY BY COMPARING AND CLASSIFYING KNOWN

TRUTHS.

EVERY truth of nature contains a vast deal more information than appears in it at first sight; and some of the additional truth may be evolved from it not only by varying the senses with which we observe it, but also by means of logical analysis and permutation of ideas.'

2

Every different truth, and class of truths, when compared in a new aspect, yield us new knowledge; and this may be viewed as the law of scientific discovery by means of the senses and intellect. It not only yields us new knowledge, but, if combined with additional truths, either yields more knowledge, or excites in our minds new hypotheses, which, by further experiment or observation, may also lead to new discoveries. We occasionally, without having recourse to new experiments, acquire new knowledge, either by comparing facts and arranging together all those of a similar kind; comparing collections of facts, and observing their similarities and differences; arranging a collection of facts in particular orders, and comparing the orders; or by classifying a collection of similar facts in different ways, &c. &c. Discoveries, however, are usually made by mentally operating upon new facts; and in either case a discovery is rarely made by one of these methods alone, but usually by means of several combined, or employed in succession, because of the very intimate relations and dependence of all our intellectual powers.

DISCOVERY BY MEANS OF COMPARISON AND CLASSIFICATION. 577

a. By simple comparison of facts or phenomena.The simple comparison of facts has enabled us to discover an immense amount of new knowledge. The effects of precession on the apparent places of the fixed stars were discovered by Hipparchus, in the year 128 before Christ, by comparison of his own observations with those of Timocharis, made 155 years previously. Comparison of the heavens with star maps and catalogues largely conduced to the discovery of the minor planets or asteroids, by enabling a new or movable star to be more easily recognised; it was in a great measure by means of such comparison that Astrea was found; the other asteroids also were largely discovered by comparing the heavens with Bode's 'Berlin Catalogue.' It was by making, classifying, and comparing a long series of observations during a number of years, of the spots on the sun, that Schwabe discovered regular periods of increase and decrease of the number of spots.

Graham, a philosophical instrument maker of London, and also astronomer, by making as many as 1,000 very careful observations of the movements of a freely-suspended magnetic-needle, and comparing them, discovered, in 1722, the diurnal magnetic variation. Canton also, by means of 4,000 such observations and similar comparison, discovered, about the year 1756, the yearly variation, and also that the daily variation was greater in summer than in winter. It was by means of comparison of the effects of electrified bodies upon each other, and upon neutral ones, that Du Fay, between the years 1733 and 1737, discovered the existence of two forms of electricity, and that similarly electrified bodies mutually repel, and dissimilarly electrified ones attract each other, and that bodies excited by either form of electricity attract neutral ones. It was by comparing the rates of chronometers in the presence and

P P

absence of magnets that Fisher, in 1818, discovered that their rate was affected by the proximity of a magnet. In 1820 Brewster, by comparing, discovered the coincidence of position between the geographical points of maximum cold and the terrestrial magnetic poles. Kreil, of Prague, in 1839, and Plantamour, of Geneva, in 1842, discovered, by comparison of the positions of the moon with the movements of the magnetic needle, that the former influenced the magnetic declination.

Comparisons of coincident phenomena and observations often render manifest new truths. It was observed by Mr. Carrington and by Mr. Hodgson, at different places simultaneously, that a very bright spot suddenly appeared upon a particular part of the sun's surface, and lasted about five minutes; other persons remarked that the self-recording magnetic needles at Kew Observatory were strongly affected at precisely the same moment; and it was further observed that strong electric currents were produced at the same time in the telegraph wires nearly all over the earth-all these circumstances indicating a terrestrial magnetic storm coincident with the solar outburst, although the distance between the sun and the earth is about ninety-two millions of miles. It was whilst endeavouring to obtain a fixed measure of the positions of lines in luminous spectra, by placing two spectra together and comparing them, that Kirchoff and Bunsen discovered that the black line D in the solar spectrum exactly coincided in position with the bright yellow arc of sodium; and, by passing sunlight through the flame or heated vapour of sodium, Kirchoff found that the dark line D was blacker than before, and thus was led to conclude that the black line D in the solar spectrum was caused by the white light of the sun having passed through vapour of sodium in the solar atmosphere, and thus had its yellow rays