102 Professor Forbes's Twelfth Letter on Glaciers. 



consequently the velocity of motion had increased in the 

 space of 170 feet traversed by the veined structure, in the 

 ratio of no less than 29 to 10. 



I also examined the condition of motion of the embayed 

 ice in the position C on the ground plan, of which a section 

 (on a scale much larger than the last) is given in fig 3. 

 It is made through the visual line, or in the direction C, C 2 

 of the plan. Two pins were fixed at C 1 by excavating a 

 niche in the nearly vertical face of ice ; one was placed ver- 

 tically, the other horizontally. At C 2, 40 feet higher and 

 69 feet more distant, a vertical pin was placed. The diagonal 

 from C to C 1 was also accurately measured with a line. 

 After more than 24 hours interval it was found that the two 

 marks at C 1 had not moved towards the right hand, or in 

 the direction of motion of the glacier by the smallest 'percep- 

 tible quantity ; and that mark C2 had advanced by only a 

 small fraction of an inch (0*2) ; but on the other hand the 

 vertical pin at C 1 had approached the station C hy two inches ; 

 shewing that the motion is here entirely directed outwards 

 and upwards against the bank at C, exactly as an hydrostatic 

 pressure acting on a plastic mass would occasion. The result 

 was the more interesting because it was altogether unex- 

 pected. It had not occurred to me that the embaying of the 

 ice could be so complete as to leave no appreciable effect 

 due to the drag of the central ice in the direction of the de- 

 clivity. 



The precise analogy of the phenomena now described to 

 what obtain when the motion of a stream of water is inter- 

 rupted by lateral obstacles, will suggest itself to almost 

 every one ; but to take an illustration which is not imaginary, 



