A MONTH WITH THE MUIR GLACIER. 71 



SUPPLEMENT TO CHAPTER III. 



For various reasons it is best to let this chapter stand as 

 it was originally written. But it is necessary to append a 

 summary of the results of subsequent observations by others, 

 especiall}^ as they have a most important bearing on several 

 questions of glacial theory. During the summers of 1890 

 and 1892 Professor Harry Fielding Reid with a corps of 

 competent assistants carefully surveyed the region and made 

 extensive additions to our knowledge, not only of this glacier, 

 but of glacial movements in general. 



The main facts, as determined by Professor Eeid, do not, 

 however, differ materially from ours. Our estimate of twelve 

 hundred square miles for the area of the Muir Glacier would, by 

 his calculations, be brought down to a thousand square miles. 

 He failed, however, to detect any motion in the glacier greater 

 than about ten feet per day. But it should be noted that he 

 did not measure the central, and consequently most rapidly 

 moving portion of the ice, but limited himself to calculating 

 the motion of those portions of the ice which he could traverse, 

 and upon which he could plant flags of observation. Thus, not- 

 withstanding his utmost efforts, in going out from both direc- 

 tions, about a quarter of a mile in width, as he informs me, re- 

 mained untraversed, and his attempts to take angles, after the 

 method pursued by us, upon the masses of ice themselves, failed 

 of success. This was probably due to the fact that his base- 

 line (near B in our map on page 53) was eight hundred feet 

 higher upon the mountain than ours, so that he did not have 

 the advantage which we had of seeing the domes and pinnacles 

 of the central and higher portion of the ice projected upon the 

 sky and the dark background of the mountains beyond. Hence 

 it does not appear that there is any occasion to question the ap- 

 proximate correctness of our figures as given on page 54. If, how- 

 ever, I were to revise the estimates of the average rate of move- 

 ment in the mass of ice, I should not place it quite so high as I 

 have done on page 55, especially since in that calculation no 

 allowance was made for the decrease of velocity toward the 

 bottom. Taking this into account, together with the com- 



