Whoever has truly felt the aerial perspective of Turner has attained a delicate sense of the pictorial significance of the bridge; for, as we look through his floating mists, we descry, amid Nature's most evanescent phenomena, the span, the arch, the connecting lines or masses whereby this familiar image seems to identify itself not less with Nature than with Art. Among the drawings which Arctic voyagers have brought home, many a bridge of ice, enormous and symmetrical, seems to tempt adventurous feet and to reflect a like form of fleecy cloud-land; daguerreotyped by the frost in miniature, the same structures may be traced on the window-pane; printed on the fossil and the strata of rock, in the veins of bark and the lips of shells, or floating in sunbeams, an identical design appears; and, on a summer morning, as the eye carefully roams over a lawn, how often do the most perfect little suspension-bridges hang from spear to spear of herbage, their filmy span embossed with glittering dew-drops! * * * * * INTERNAL STRUCTURE AND PROGRESSION OF THE GLACIER. It is not my intention, in these articles, to discuss a general theory of the glaciers upon physical and mechanical principles. My special studies, always limited to Natural History, have but indifferently fitted me for such a task, and quite recently the subject has been admirably treated from this point of view by Dr. Tyndall, in his charming volume entitled "Glaciers of the Alps." I have worked upon the glaciers as an amateur, devoting my summer vacations, with friends desirous of sharing my leisure, to excursions in the Alps, for the sake of relaxation from the closer application of my professional studies, and have considered them especially in their connection with geological phenomena, with a view of obtaining, by means of a thorough acquaintance with glaciers as they exist now, some insight into the glacial phenomena of past times, the distribution of drift, the transportation of boulders, etc. It was, however, impossible to treat one series of facts without some reference to the other; but such explanations as I have given of the mechanism of the glacier, in connection with its structure, are presented in the language of the unprofessional observer, without any attempt at the technicalities of the physicist. I do not wonder, therefore, that those who have looked upon the glacier chiefly with reference to the physical and mechanical principles involved in its structure and movement should have found my Natural Philosophy defective. I am satisfied with their agreement as to my correct observation of the facts, and am the less inclined to quarrel with the doubts thrown on my theory since I see that the most eminent physicists of the day do not differ from me more sharply than they do from each other. The facts will eventually test all our theories, and they form, after all, the only impartial jury to which we can appeal. In the mean