76 



Royal Society : — 



Each plate would under these circumstances descend faster than 

 the one beneath ; and supposing the adherence of the lowest plate to 

 the board to be the same as that of the plates to one another, then, of 

 any number of blocks similarly placed and subject to the like varia- 

 tion of temperatures, the thickest or deepest would descend, at its 

 surface, the fastest ; and if there were a block of different depths in 

 different parts, the deepest parts would descend the fastest. The 

 differential motion thus set up would not be appreciable in a block 

 of ice of different thicknesses in different parts if its dimensions 

 were no larger than the block experimented on by Schumacher, but 

 in a glacier it would be appreciable. 



To bring Schumacher's block to the proportions of a glacier, it 

 must be converted into a slab twelve feet long, twenty inches wide, and 

 two inches thick. It would then represent on a scale of the 1500th 

 part, a glacier 2500 feet wide, 250 feet deep, and 18,000 feet long, 

 which are something like the dimensions of the Mer de Glace from 

 2300 feet below Montanvert to the Tacul. If we suppose it to be 

 placed at the same inclination of 18^° at which the lead was, its 

 under surface being coated with lead so as to have the same friction 

 on board as the lead had, then it may be calculated that if it had 

 experienced the same variations of temperature as the lead did, its 

 average daily descent, measured in inches, would have been 



February. 



March. 



April. 



May. 



June. 



•26666 



•36816 



•43022 



•57334 



•58368 



If, now, we conceive its inclination to change from 18|° to that of 

 the Mer de Glace, which is about 5°, and its dimensions to become 

 actually those of that glacier, then, supposing the glacier to experience 

 the same elevations and depressions of temperature as the lead did, 

 its average daily descents in inches would be 



February. 



March. 



April. 



May. 



June. 



104-56 



144-06 



168-74 



224-87 



228-92 



which rates of motion are probably twelve times greater than the 

 actual rates of motion of the glacier ; showing that variations of the 

 temperature of the glacier twelve times less than those of the lead, 

 would be sufficient to produce its actual descent ; or that it would 

 descend as it actually does, if the resistances opposed to its descent were 

 twelve times greater than the resistances opposed to the descent of 

 the lead — If its descent were resisted by a friction, for instance, having 

 twelve times the coefficient of that of the lead on the board, or such as 

 would cause it to rest without slipping on an incline having twelve 

 times the tangent of the inclination of the board ; or if the variations 

 of temperature were less and the resistance greater in any proportion 

 which would retard the descent twelve times as much. So that 

 we may suppose in the case of the glacier a far greater resistance in 



