[B] In a letter to myself, published in the 17th volume of the 'Philosophical Magazine,' Professor Forbes writes as follows:--"In 1846, then, I abandoned no part of the theory of the veined structure, on which as you say so much labour had been expended, except the admission, always yielded with reluctance, and got rid of with satisfaction, that the congelation of water in the crevices of the glacier may extend in winter to a great depth." THE VEINED STRUCTURE AND THE DIFFERENTIAL MOTION. (28.) [Sidenote: DIFFERENTIAL MOTION GREATEST AT EDGES.] I have now to examine briefly the explanation of the structure which refers it to differential motion--to a sliding of the particles of ice past each other, which leaves the traces of its existence in the blue veins. The fact is emphatically dwelt upon by those who hold this view, that the structure is best developed nearest to the sides of the glacier, where the differential motion is greatest. Why the differential motion is at its maximum near to the sides is easily understood. Let A B, C D, Fig. 47, represent the two sides of a glacier, moving in the direction of the arrow, and let _m a b c n_ be a straight line of stakes set out across the glacier to-day. Six months hence this line, by the motion of the ice downwards, will be bent to the form _m a' b' c' n_: this curve will not be circular, it will be flattened in the middle; the points _a_ and _c_, at some distance on each side of the centre _b_, move in fact with nearly the same velocity as the centre itself. Not so with the sides:--_a'_ and _c'_ have moved considerably in advance of _m_ and _n_, and hence we say that the difference of motion, or the differential motion, of the particles of ice near to the side is a maximum. [Illustration: Fig. 47. Diagram illustrating Differential Motion.] During all this time the points _m a' b' c' n_ have been moving straight down the glacier; and hence it will be understood that the sliding of the parts past each other, or, in other words, the differential motion, _is parallel to the sides of the glacier_. This, indeed, is the only differential motion that experiment has ever established; and consequently, when we find the best blue veins referred to the sides of the glacier because the differential motion is there greatest, we naturally infer that the motion meant is parallel to the sides. [Sidenote: STRUCTURE OBLIQUE TO SIDES.]