406. The theory advocated by Professor Forbes was enunciated by himself in these words:--"A glacier is an imperfect fluid, or viscous body, which is urged down slopes of certain inclination by the natural pressure of its parts." In 1773 Bordier wrote thus:--"As the glaciers always advance upon the plain, and never disappear, it is absolutely essential that new ice shall perpetually take the place of that which is melted: it must therefore be pressed forward from above. One can hardly refuse then to accept the astonishing truth, that this vast extent of hard and solid ice moves as a single piece downwards." In the passage already quoted he speaks of the ice being pressed as a fluid from above. These constitute, I believe, Bordier's contributions to this subject. The quotations show his sagacity at an early date; but, in point of completeness, his views are not to be compared with those of Rendu and Forbes. 407. I must not omit to state here that though the idea of viscosity has not been espoused by M. Agassiz, his measurements, and maps of measurements, on the Unteraar glacier have been recently cited as the most clear and conclusive illustrations of a quality which, at all events, closely resembles viscosity. 408. But why, with proofs before him more copious and characteristic than those of any other observer, does M. Agassiz hesitate to accept the idea of viscosity as applied to ice? Doubtless because he believes the notion to be contradicted by our every-day experience of the substance. 409. Take a mass of ice ten or even fifteen cubic feet in volume; draw a saw across it to a depth of half an inch or an inch; and strike a pointed pricker, not thicker than a very small round file, into the groove; the substance will split from top to bottom with a clean crystalline fracture. How is this brittleness to be reconciled with the notion of viscosity? 410. We have, moreover, been upon the glacier and have witnessed the birth of crevasses. We have seen them beginning as narrow cracks suddenly formed, days being required to open them a single inch. In many glaciers fissures may be traced narrow and profound for hundreds of yards through the ice. What does this prove? Did the ice possess even a very small modicum of that power of stretching, which is characteristic of a viscous substance, such crevasses could not be formed. 411. Still it is undoubted that the glacier moves like a viscous body. The centre flows past the sides, the top flows over the bottom, and the motion through a curved valley corresponds to fluid motion. Mr. Mathews, Mr. Froude, and above all Signor Bianconi, have, moreover, recently made experiments on ice which strikingly illustrate the flexibility of the substance. These experiments merit, and will doubtless receive, full attention at a future time. § 61. _Regelation Theory._