70 THE FORMS OF WATER IN 



168. Look at these numbers. The first broad fact 

 they reveal is the advance in the rate of motion from 

 first to last. There are however small irregularities ; 

 from 23 inches at the 17th stake we fall to 21 inches at 

 the 18th ; from 23 inches at the 19th we fall to 21 inches 

 at the 20th ; from 25 inches at the 21st we fall to 22 

 inches at the 22nd and 23rd ; but notwithstanding these 

 small ups and downs, the general advance of the rate 

 of motion is manifest. Now there may have been some 

 slight displacement of the stakes by melting, sufficient 

 to account for these small deviations from uniformity 

 in the increase of the motion. But another solution is 

 also possible. We shall afterwards learn that the gla- 

 cier is retarded not only by its sides but by its bed ; 

 that the upper portions of the ice slide over the lower 

 ones. Now if the bed of the Mer de Glace should have 

 eminences here and there rising sufficiently near to the 

 surface to retard the motion of the surface, they might 

 produce the small irregularities noticed above. 



169. We note particularly, while upon the ice, that the 

 26 th stake, like the 10th stake in our last line, stands 

 much nearer to the eastern than to the western side 

 of the glacier ; the measurements, therefore, offer a fur- 

 ther proof that the centre of this portion of the glacier 

 is not the place of swiftest motion. 



§ 23. Unequal Motion of the tivo Sides of the 

 Mer de Glace. 



1 70. But in neither the first line nor the second were 



