Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/405

 P E B E S 391 Forbes strongly opposed the idea of a musical professor ship, urging that the money would be more usefully employed in providing retiring allowances for superan nuated professors. But controversy ran high, litigation ensued, and after years of contention a part of the Reid bequest was assigned by the law courts to the chair of music. Next year another large sum, the Straton fund, had to be dealt with, and here again Forbes was instru mental in inducing the senate to devote what was left of the Reid bequest, combined with the Straton fund, to the foundation of fellowships to be held by distinguished students after graduation. In these and other controversies the uncompromising energy with which he strove to carry out his views of what was right could not fail to bring him into collision with men whose opinions were as decided, and their wills as strong as his own. When two such unbend ing natures as Sir William Hamilton and James Forbes came into contact as they did more than once the shock was a rough one ; but although they differed widely in opinions, each has left a permanent mark for good on the university. It would be impossible to enter here into the subjects or method of Forbes s class lectures ; nor is it necessary to give more than a brief notice of the important researches which shared with his class work the winter mouths. They were the subject of voluminous private correspond ence with such men as Whewell, Brewster, Peacock, Airy, Faraday, Arago, Melloni, Cauchy, and many others, and their results were embodied in a long list of communi cations to various scientific societies, ranging over a great variety of subjects. For a paper on &quot; The Transparency of the Atmosphere and the Laws of Extinction of the Sun s Rays passing through it,&quot; the Royal Society of London awarded him their Royal Medal. Another, on &quot; The Selective Absorp tion of the Sun s Light in passing through Steaoi,&quot; was one of the first steps in the direction of spectrum analysis. In 1832 he described the experiments by which he suc ceeded in producing a spark by means of a natural magnet. He was the author of valuable memoirs on the thermal springs of the Pyrenees, the extinct volcanoes of the Vivarais (Ardeche), and the geology of the Cuchullin and Eildon hills. Soon after his elevation to the chair Forbes resumed his researches in heat, begun some years before. He commenced experiments with Melloni s thermo-multi- plier, measured ths refractive index of rock salt with heat from various sources, luminous and non-luminous, and pursued a course of investigations which led him to his most brilliant discovery, the polarization of non-luminous heat, by transmission through tourmaline and thin mica plates, and by reflection from the latter. By employing mica for depolarization he succeeded in showing the double refraction of non-luminous heat, a fact of which this experi ment remains the only proof. He also produced circularly polarized heat by two internal reflections, using Fresnel s rhombs made of rock salt. By these researches the identity of thermal and luminous radiations was finally established. In 1846 he made careful arrangements for the measurement of underground temperatures, and by sinking his thermo meters in three different sets of surface materials, he obtained an absolute determination of the thermal conductivity of trap- tufa, sandstone, and pure loose sand. His experiments on the conductivity of metals occupied the closing years of his life. By a thoroughly original method he obtained quantitative measurements of the absolute thermal conduc tivity of iron at various temperatures, and showed that this is diminished (contrary to the assumption of Fourier) by increase of temperature, thus following the known laws of electrical conductivity. Forbes generally devoted a part of each vacation to excursions with his family in various parts of Scotland, and to geological tours among his native mountains, in the course of which, at times in company with some scientific friend, more often alone, he traversed on foot the greater part of the Highlands. In 1856 he acquired a beautiful cottage near Pitlochrie iu Perthshire, where the succeeding summers were chiefly spent. From 1835 to 1851 Forbes usually passed some months on the Continent, travelling, as he expressed it, not as an amusement, but as a serious occupation, with De Saussure before him as a model And it may be almost doubted whether science did not profit as much by his summer travels as by his winter work. His first tour was devoted to the geology of the Pyrenees and of the Vivarais the latter of which he afterwards revisited. In 1837, after a short sojourn at Bonn, for the purpose of study, he travelled throughout Germany and Austria, making frequent experiments on terrestrial mag netism and other subjects. In 1839, among the crags and glaciers of Monte Viso, of the Pelvoux range, and of the Alps of Cogne, he learned to be an active and intrepid moun taineer; and during the intervals of his glacier investiga tions he did good service in opening up Alpine districts before scarcely known to Englishmen, by a series of excur sions extending over the whole of the Pennine chain. But of all his journeys and discoveries, those made by him among the mountains of Savoy are most popularly known; and their results have associated his name for ever with the glaciers of the Alps. In 1840, while presiding over the physical section of the British Association at Glasgow, he met M. Agassiz of Neuchatel who, after spending several years iu the study of glacial phenomena, had just published his fitudcs sitr les Glaciers, and agreed to visit Switzerland with him the following year. Accord ingly, at the close of the session of 1840-41, Forbes joined Agassiz and his party at the well-known &quot; Hotel des Neu- chatelois,&quot; a small hut on the moraine of the Lauter Aar Gletscher. After a fortnight spent in exploration and observation, in the course of which Forbes called attention for the first time to the veined structure of glacier ice, they ended the campaign by the passage of the Ober Aar Joch, and the ascent of the Jungfrau. This was Forbes s appren ticeship in glacier observation, a field of inquiry then jointly occupied by two rival theories, the gravitation or sliding theory of De Saussure, and the dilatation theory of Charpentier. After a close analysis of these Forbes not only showed conclusive physical reasons for rejecting both, but pointed out the impossibility of forming a sound theory of glaciers until the internal structure of these anomalous bodies had been investigated, and the character and velocity of their motions ascertained. These important data Forbes resolved to obtain for him self, and the summer of 1841 found him at Chamounix, engaged in a close examination of the Mer de Glace. He attacked the problem for the first time as a question of pure physics, executed accurate measurements with instru ments of precision, -and at once established the fact that the motion of a glacier averages roughly a foot in 24 hours, and that such motion is continuous. As his observations went on. he successively discovered that the centre of the glacier moves faster than the sides, and the surface than the ice vertically below it, that its velocity increases directly with the steepness of its bed, and that the motion of a portion of the glacier when embayed behind a rock is greatly diminished, and the whole mass distorted, without any solution of continuity taking place. Convinced by these discoveries that the resemblance between a glacier and a river, already noticed by Captain Hall, Msgr. Rendu, and others, was more than a mere analogy, he deduced from the above mentioned data the following