174 Notice of Prof. Forbes' s Travels in the Alps. 



cially against all the obstacles that have been named, and over the jag- 

 ged and rough projections of rocky defiles. It is found that the freez- 

 ing of the water that percolates through the fissures of the ice pene- 

 trates but a small distance in summer, and in the winter, when the 

 dilatation by freezing is the greatest, the motion of the glacier down- 

 ward is the least, whereas upon the dilatation theory it should then be the 

 greatest. In the summer season the motion is the most rapid, and then 

 there is no internal congelation, and very little upon the surface. At 

 seven thousand nine hundred feet above the sea, on the Montanvert, in 

 the month of September, 1842, although in severe weather, the ice 

 was, as usual, charged with water, except on the surface, where it was 

 dry, but a little way down water was easily obtained by breaking the 

 ice in a pool frozen over under a stone. The progress of the glacier 

 downward was always retarded by the increase of cold, and augment- 

 ed by a thaw. The snows which cover the ice appear to preserve in it 

 an equality of temperature, as they do in the ground ; for Prof. Agas- 

 siz himself found the ice at all depths within a fraction of 32°. 



The supposed sliding motion of a solid mass of ice by gravity is in- 

 consistent with the rapid progress of the middle of the mass, (taken in 

 the direction of its length,) for it has been distinctly proved by Prof. 

 Forbes, that the sides next to the rocky walls move the most tardily, 

 and in no part of the summer is the ice frozen fast to the mountain sides. 



It has been supposed by Saussure and others, that the heat of the 

 earth causes a fusion of the bottom of the glaciers, lubricating their 

 path, and thus facilitating their motion ; but this effect must be extremely 

 small, and has been considered, upon the theory of Fourier, equal only 

 to melting a quarter of an English inch of ice in the space of a year. It 

 would, therefore, be entirely inadequate to smoothing off the nether as- 

 perities of the glacier, which must impede its motion. Prof. Forbes, after 

 setting aside the previous theories, announces his own, as follows : " A 



GLACIER IS AN IMPERFECT FLUID, OR A VISCOUS BODY, WHICH IS URGED 

 DOWN SLOPES OF A CERTAIN INCLINATION, BY THE MUTUAL PRESSURE OF 



its parts," (by gravity, of course.) This is illustrated by the motion of 

 a moderately thick mortar, or of melted tar, or treacle, down an inclined 

 plane, fashioned in the form of a trough, and these materials will be found 

 to be governed by their own viscosity and the friction on the channel. 



" That glaciers are semi-fluid is not an absurdity.'''' Even water is 

 not quite mobile ; it requires a slope of 6 inches to a mile, or t ^^tt 

 part, to make it run freely, while some other fluid might require a slope 

 of -j^y, and many bodies may be heaped up at an angle of many de- 

 grees before their parts will begin to slide over each other. 



" That a substance apparently solid may, under a great pressure, 

 begin to yield ; yet that yielding or sliding of the parts over one anoth- 



