It cannot be too clearly and emphatically stated that the _proved_ fact of a glacier conforming to the law of semi-fluid motion is a thing totally different from the _alleged_ fact of its being viscous. Nobody since its first enunciation disputed the former. I had no doubt of it when I repaired to the glaciers in 1856; and none of the eminent men who have discussed this question with Professor Forbes have thrown any doubt upon his measurements. It is the assertion that small pieces of ice are proved to be viscous[A] by the experiments made upon glaciers, and the consequent impression left upon the public mind--that ice possesses the "gluey tenacity" which the term viscous suggests--to which these observations are meant to apply. FOOTNOTES: [A] "The viscosity, though it cannot be traced in the parts _if very minute_ nevertheless _exists_ there, as unequivocally proved by experiments on the large scale."--Forbes in 'Phil. Mag.,' vol. x., p. 301. HEAT AND WORK. (19.) [Sidenote: CONNEXION OF NATURAL FORCES.] Great scientific principles, though usually announced by individuals, are often merely the distinct expression of thoughts and convictions which had long been entertained by all advanced investigators. Thus the more profound philosophic thinkers had long suspected a certain equivalence and connexion between the various forces of nature; experiment had shown the direct connexion and mutual convertibility of many of them, and the spiritual insight, which, in the case of the true experimenter, always surrounds and often precedes the work of his hands, revealed more or less plainly that natural forces either had a common root, or that they formed a circle, whose links were so connected that by starting from any one of them we could go through the circuit, and arrive at the point from which we set out. For the last eighteen years this subject has occupied the attention of some of the ablest natural philosophers, both in this country and on the Continent. The connexion, however, which has most occupied their minds is that between _heat_ and _work_; the absolute numerical equivalence of the two having, I believe, been first announced by a German physician named Mayer, and experimentally proved in this country by Mr. Joule. [Sidenote: MECHANICAL EQUIVALENT OF HEAT.]