and Motion of Glaciers. 371 



so slowly, that the bending closely resembles what would take 

 place if the substance were viscous. The gradual nature of 

 the change of curvature may be inferred from an experiment 

 made by Professor Forbes on an apparently compact portion of 

 the Mer de Glace. He divided a distance of 90 feet transverse 

 to the axis of the glacier into spaces of 2 feet each, and ob- 

 served with a theodolite the gradual passage of this straight line 

 into a bent one. The row of pins bent gradually so as to form 

 a curve convex towards the lower extremity of the glacier ; their 

 deviations from a perfect curve were slight and irregular, nor 

 was any great dislocation to be observed throughout their whole 

 extent. After six days the summit of the curve formed by 

 the forty-five pins was 1 inch in advance of the straight chord 

 which united its two ends. It is not surprising if, with this ex- 

 tremely gradual change, the motion should have appeared to be 

 the result of viscosity. It may, however, be remarked, that the 

 slight and irregular variations to which Professor Forbes alludes, 

 and which are such as would occur if the motion were such as 

 we suppose it to be, are likely to throw much light upon the pro- 

 blem. It is also extremely probable that the motion, if effected 

 in the manner referred to, will be sometimes accompanied by 

 an audible crackling of the mass. To this we paid but little 

 attention when on the ground ; for the significance of this as 

 well as of many other points was first suggested by the experi- 

 ments made after our return. It is, however, we believe, a phe- 

 nomenon of common occurrence. Professor Forbes calls the 

 glacier a "crackling mass;" he speaks of the ice "cracking and 

 straining forwards;" and in that concluding passage of his 

 ' Travels ' which has excited such general admiration, he says of 

 the glacier, " it yields groaning to its fate." Other observers 

 make use of similar expressions. M. Desor also speaks of the 

 sudden change of the colour of the blue veins of the ice where a 

 portion of the central moraine near the Abschwung is cleared 

 away ; the observation is very remarkable. "Au moment," says 

 M. Desor, "oii on la met k decouvert, la glace des bandes bleues 

 est parfaitement transparente, Poeil y plonge jusqu'a une profon- 

 deur de plusieurs pieds, mais cette purete ne dure qu'un instant, 

 et I'on voit bientot se former des petites felures d'abord super- 

 ficielles, qui se combinent en reseau de maniere a enlever peu a 

 peu 5\ la glace bleue toute sa transparency. Ces felures propagent 

 ^galement dans les bandes blanches, et lorsqu'on approche PoreiUe 

 de la surface de la glace, on entend distinctement un leger bruit de 

 crepitation qui les accompagnc au moment dcleur formation." 

 These facts appcarto be totfilly at variance with the idea of viscosity. 

 In a chapter on the " Appearance of the larger Glaciers," in an 

 interesting httlc work by M. Mousson of Zurich, for which one 



