192 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
IX. I am unwilling to multiply deductions which every intelligent reader will 
draw for himself; but one more I must add. It very clearly appears that the varia- 
tions of velocity due to season are greatest where the variations of temperature of 
the air are greatest, as in the lower valleys ; but it also appears from Remark VIII., 
that variations of temperature below 0° centigrade, or 32° Fahrenheit, produce 
almost inappreciable changes in the rate of motion of the ice. Hence, from this 
circumstance alone, we should deduce that in the higher parts of the glacier (where, 
for example, it freezes almost every night in summer) the variations of velocity 
should be least, and indeed comparatively small at different seasons. This is well 
illustrated by comparing the summer motions of the stations D, A and C, mentioned 
in the first, part of this section, with their annual motion, which exhibit a much 
slighter excess in favour of the summer period than in the lower stations which we 
are now discussing. The same thing was observed by M. Agassiz’s surveyors on the 
glacier of the Aar, who at first saw, in this not very great inequality, an objection to 
my theory. On a more searching investigation, however, the objection disappears, as 
in their later writings they have acknowledged. Their position of observation far up 
on the glacier of the Aar, in a spot having a mean temperature near the freezing point 
if not lower, had a summer daily motion of 7'99 inches, and a mean daily motion 
during the whole year of 6 - 41 inches*. Now at station C, or the Pierre Platte, on 
the Mer de Glace, the mean motion for July 1842 was 10 inches, and for the whole 
year, 1842-43, it was 8 56 inches. It is quite evident that the motion of any point in 
the midst of a glacier is controlled by that of those which precede and follow it, and 
that it does not necessarily result, either that all must at once suffer a similar in- 
crease or diminution of speed, or that the times of maxima and minima, or even 
the general form of the annual curve, shall be the same. This leads to an important 
practical result which we shall follow out in the next section. 
§ 8. Summary of the Evidence adduced in favour of the Plastic or Viscous Theory of 
' Glacier Motion. 
It is often difficult to obtain a calm and full hearing for any new theory or experi- 
mental investigation ; not because there is any antipathy to novelty, or that experi- 
ment is undervalued, but simply because, in an age of bustle and struggle for pre- 
eminence, each man is so busy with his own reputation, or the means of increasing it, 
that he has no leisure to attend to the claims of others; to which may, perhaps, be 
added, that in the general diffusion of knowledge and acquirement, each reader, find- 
ing something in every course of experiment or reasoning which he knew, or thinks 
he knew before, is apt to run off with the chain of ideas which that one familiar link 
suggests, and losing patience to follow an argument of which he thinks he can, by 
his ov/n penetration, anticipate the close. He sits in judgment on errors which are 
of his own invention, and confronts the author with arguments and opinions already 
* Comptes Rendus, Dec. 9, 1844. 
