Illustrative of the same point, we have the elaborate and extensive series of measurements executed by M. Wild under the direction of M. Agassiz upon the glacier of the Aar in 1842, 1843, 1844, and 1845, which exhibit on a grand scale, and in the most conclusive manner, the character of the motion of this glacier; and also show, on close examination, an analogy with fluid motion which neither M. Agassiz nor Professor Forbes suspected. The former philosopher publishes a section in his 'Systeme Glaciaire,' entitled 'Migrations of the Centre;' in which he shows that the middle of the glacier is not always the point of swiftest motion. The detection of this fact demonstrates the attention devoted by M. Agassiz to the discussion of his observations, but he gives no clue to the cause of the variation. On inspecting the shape of the valley through which the Aar glacier moves, I find that these "migrations" follow the law established in 1857 upon the Mer de Glace, and enunciated at page 286. To sum up this part of the question:--The _idea_ of semi-fluid motion belongs entirely to Rendu; the _proof_ of the quicker central flow belongs in part to Rendu, but almost wholly to Agassiz and Forbes; the proof of the retardation of the bed belongs to Forbes alone; while the discovery of the locus of the point of maximum motion belongs, I suppose, to me. FORBES'S THEORY. (16.) The formal statement of this theory is given in the following words:--"A glacier is an imperfect fluid, or viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." The consistency of the glacier is illustrated by reference to treacle, honey, and tar, and the theory thus enunciated and exemplified is called the 'Viscous Theory.' It has been the subject of much discussion, and great differences of opinion are still entertained regarding it. Able and sincere men take opposite sides; and the extraordinary number of Reviews which have appeared upon the subject during the last two years show the interest which the intellectual public of England take in the question. The chief differences of opinion turn upon the inquiry as to what Professor Forbes really meant when he propounded the viscous theory; some affirm one thing, some another, and, singularly enough, these differences continue, though the author of the theory has at various times published expositions of his views. [Sidenote: "FACTS AND PRINCIPLES."]