Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/406

392 FORBES theory of glacier motion : A glacier is an imperfect fluid or a viscous body, which is urged down slopes of a certain inclination ly the mutual pressure of its parts. In reply to the strongly urged objection that ice is by its nature a brittle solid, and not sensibly possessed of any viscous quality, he asserted that ice is only hard and crystalline at a temperature of 30 F. or lower, and that at 32 it is a plastic solid possessing a slight but perfectly sensible genuine molecular plasticity. When the urgency of the forces to which it is subjected causes this limit to be over powered, a general bruise takes place, producing innumer able internal fissures, with finite displacements in certain planes, and subsequent restoration of continuity by " time and cohesion"; hence the blue bands of the conchoidal veined structure. When the urgency is still greater the mass acts like a solid, and crevasses open. In the case of glacier ice an additional element of plasticity exists, for glacier ice is scarcely ever coherent. It is usually a congeries of irregular polyhedrons tightly wedged together, possessing a rude flexibility, and always traversed by an infinity of capillary fissures. Hence a glacier is really a compound of ice and water, more or less plastic according to its wetness and infiltration, a fact proved by its more rapid motion in summer when saturated with water, and its relative retar dation in winter when the surface water remains frozen. Forbes s " viscous theory " was sharply criticized, but it is now universally accepted, subject to some modifications which do not affect the groundwork of the structure, and rather elucidate than disprove his conclusions. But besides scientific criticism, he was compelled to defend himself which he did with complete success against in sinuations, not only of plagiarism, but of an endeavour to suppress the rights of others to priority of discovery. During the winter which followed this eventful summer, Forbes was busily engaged in writing his Travels through the Alps of Savoy, a charming contribution to Alpine literature, in which graphic descriptions of scenery and mountaineering are happily blended with accounts of his scientific researches. Its last lines were written almost on the eve of his marriage with Alicia, eldest daughter of the late George Wauchope, Esq. They started on their mar riage tour to Switzerland, but had only got so far as Bonn, when Forbes was attacked by a dangerous illness, and it was not until August that they reached the Alps. He was then so far restored as to be able to resume his obser vations on the glaciers of Chamounix and Grindelwald, but as the winter approached lie was pronounced unequal to the work of the following session at Edinburgh. His request for permission to appoint a substitute was readily granted, and he set out with Mrs Forbes for Italy. Naples and Rome were their headquarters until the end of May, and the summer months were passed at the Italian lakes and in Switzerland. Forbes returned to Edinburgh in the autumn of 1844, and carried on his work during the winter months, but his health had suffered a rude shock, from the effects of which he never entirely recovered, The succeeding summer was spent with his family in the Highlands of Scotland ; but the vacation of 1846 found him again on the Mer de Glace, engaged in observations on the ablation of the surface of the glaciers, and the dif ference of movement between the surface and the under lying ice. His next and last visit to the Alps in 1850 was entirely occupied in putting the finishing touches to his survey of the Mer de Glace, which was for some years after its publication the only correct Alpine map in existence. At the close of the session of 1850-51 he started for Norway with the double purpose of observing a total eclipse of the sun visible at Bergen, and of comparing the phenomena of the arctic glaciers with those of the Alps. These objects were not attained without great fatigue and exposure ; Forbes returned from Norway with health much impaired, and had scarcely recommenced his work when he was attacked by haemorrhage, the precursor of a long and dangerous illness. All through December Le lay between life and death, but in January he was able to be removed to Clifton, which he made his headquarters for two years. During this enforced leisure he was con stantly engaged in writing a very able " Dissertation on the Progress of Mathematical and Physical Science," for the pages of the Encyclopedia, Britannica, and in preparing for the press a work on Norway and its Glaciers, similar in character to his Glaciers of the Alps, and scarcely yield ing to it in picturesqueness of narrative or careful observa tion. In the summer of 1853 he was called from Clifton to Oxford to receive the honorary degree of D.C.L. Dur ing the three sessions of Forbes s absence, his class work had been conducted by Professor Kelland, but the session of 1854-1855 found him again at his post, and he worked with but little interruption from illness until 1859, assiste I latterly by Dr Balfour Stewart. In that year the current of his life was turned into another channel. Owing to the translation of Sir David Brewster to the principalship of the Edinburgh university, the principalship of the United College of St Andrews became vacant ; Forbes offered him self as a candidate, and was successful. His commission bears date December 2, 1859, but he did not resign his professorship until the following April. The degree it LL.D. was then conferred on him by the university which his own labours had so greatly benefited and adorned. His new post, although it relieved him of the fatigue of constant lecturing, was no sinecure. The Scottish University Commission were then in full session, and in their investigations of the affairs, financial and other, of his own college, and of the university, the responsibility of supplying information and suggesting reforms t el largely on him. A zealous reformer he always was, and he had not left behind him at Edinburgh that dauntless spirit of "Thorough " which saw only one straight road to a right principle, and pursued it, not without collisions, ami often painful ones. The duties of his office were performed with scrupulous industry. His own college is indebted to him for a laborious examination and classification of its ancient charters. He found the collegiate church of St Salvator in a condition of tasteless neglect; he left it partially restored and greatly beautified. He occasionally lectured on glaciers, climate, heat, and others of his favourite subjects, and even strained his failing powers to complete his researches on the thermal conductivity of iron. For the latter, shortly before his death, the Keith medal was awarded to him. In the spring of 1867 Forbes s health gradually declined, and towards the end of September he set out with his wife and three daughters to Cannes, and afterwards went on to Hyeres. His health now continued to improve until the middle of January, when he was again attacked by haemor rhage brought on by fatigue. He never sat up again, and while in that painful utate the presidency of the Royal Society of Edinburgh was offered to him and declined. It was evident that a summer at Hyeres was impossible, PO he was conveyed, together with one of his daughters who was almost in the same sad condition as himself, to Clifton, where he lingered for eight months, awaiting his end with patient calmness, sustained by the deep and fervent religious faith which had characterized his boyhood, and which a life of active scientific research had strengthened rather than impaired. He died on the 31st of December 1868. Forbes received the honorary degree of D.C.L. from Oxford, ami that of LL. D. from the university of Edinburgh. The, Royal Society of London awarded him their Rumford medal for his discovery of the polarization of heat, and their Royal medal for a paper on the in-