310 
NATURE 
[ Fed. 16, 1871 
Ci SSS 
the ice, which it loosens momentarily from their mutual 
cohesion, and allows to be re-arranged under the influence 
of gravity. Heat, he says, is the condition, gravity the 
cause of the motion which takes place, molecule by mole- 
cule rather than in masses. It seems very doubtful, 
however, if this theory is more tenable than the one it is 
intended to supersede. If heat entering the glacier loosens 
the molecules in its passage and enables them to move in- 
sensibly into new positions, it isdifficult to understand what 
causes the numerous longitudinal and transverse fissures 
of a glacier, the production of which is often attended by 
loud reports, and which indicate movements of masses, not 
of molecules. And how could molecular motion lead to 
that heavy grinding of the ice over its bed, which scores 
and wears down the hardest rocks, and whitens great rivers 
with the finely triturated mud? 
None of the opponents of Canon Moseley have noticed 
what seems to the present writer to be a radical fallacy in 
his argument about “shearing force.” He assumes that, 
whatever the bulk or weight of the glacier, or of any por- 
tion of it to which the formula of the shearing force may 
be applied, the whole mass shears at once by the action 
of gravity on the same mass, and does not recognise the 
possibility of one portion of a glacier acting by its 
weight to shear another and much smaller portion. 
But this must inevitably occur; for, owing to the 
excessive irregularity of the bed in which every glacier 
moves, the mass must be every where in varying 
states of tension and compression, and must contain at 
each instant certain lines and planes of least resistance, 
the extent of which lines and surfaces may be very small 
compared with the dimensions of the glacier itself. At 
any moment, therefore, the whole descending weight of a 
portion of the glacier containing perhaps thousands of 
cubic yards of ice, may act so as to cause the shearing of 
a few superficial feet where the tension is greatest. This 
being effected, a partial equilibrium is produced there ; 
but the points or surfaces of greatest tension are shifted, 
and another small shear or fracture occurs ; and by this 
process and the continued regelation of fractured surfaces 
brought into contact, it may easily be seen that the glacier 
as a whole would be gradually moulded to its bed, which 
it would descend as surely as if it were a viscous mass. 
Another source of motion not taken into account either by 
Canon Moseley or Mr. Croll is the irregular melting away 
of the under surface of the glacier by terrestrial heat, which 
would often form unsupported hollows till a fracture oc- 
curred, and every such fracture must result ina downward 
motion of a portion of the glacier. The observed difference 
of the rate of motion between winter and summer, day 
and night, is more probably due to the different quantities 
of water which descend the crevasses into the bed of the 
glacier at those periods, than to any direct action of the 
heat. It is well known that in the higher portions of a 
glacier the supply of water from melting snow diminishes 
during the night, as it does in a still greater degree during 
the winter; and the large quantity of water that flows 
beneath every glacier in the summer must greatly assist its 
motion, both by melting away its lower surface, and by, 
to some extent, buoying it up. 
Mr. Matthews’s important experiment of the bar of ice 
which gradually curved by its own weight, should be tried 
again in an atmosphere kept at the freezing point. This 
would settle the question whether heat is an essential con- 
dition for the curvature or motion of ice by gravitation ; 
but so far as the facts lead us at present, the arguments 
of Canon Moseley and Mr, Croll by no means rove that 
glaciers do not descend by the force of gravity alone. 
ALFRED R. WALLACE 
[The publication of this article has been delayed. It 
was in our hands before the appearance of Mr. Ball’s 
paper in the Philosophical Magazine for February, where 
a view almost identical with Mr. Wallace’s is ably 
advocated.—ED, ] 


AN ACCOUNT OF THE ECLIPSE AS SEEN 
FROM VILLASMUNDA BY AN UNSCIEN- 
TIFIC OBSERVER 
Sees set in authority over the branch of the Eclipse 
Expedition stationed at Agosta having decided 
against depending only upon observations to be made 
from the Observatory there, deputed. Mr. Ranyard to 
proceed to another point upon the line of totality, and 
selected me as his coadjutor. Accordingly we set off, 
accompanied by Jarvis and Burgoyne, two of Colonel 
Porter’s Sappers, at half-past nine in the morning of 
the eventful day; and, after driving some eight miles 
inland, we attained about eleven o’clock a point which 
appeared to my companion to present advantages for our 
object. Leaving the road, we went into the middle of a 
field of springing oats, on the highest point of a rocky 
ridge at an elevation of 6o0ft. above sea level, and of 52oft. 
above the g/acis of Fort Agosta, where were posted the 
rest of our friends. The spot which Mr. Ranyard 
selected as the most suitable lay about a hundred 
s 
s| 
ys 
3 
Catania 

Cc. Murro di Porco 
SY 
Avola ° 
Fic. 1. 
A our position; B the sun; CC, CC the lines of cloud; D the road to 
Agosta. 
yards from a roadside farmhouse, called Casa Vecchia, 
upon the property of that friend of Science, the Mar- 
chese di Sangiuliano, and about two miles distant 
from the village of Villasmunda. A keen wind was 
blowing with considerable violence from the north-west, 
and the situation we had chosen being exposed to its 
full fury, we at first felt very uneasy with regard to 
our probable success, for we feared every moment that 
the telescope would be overturned and injured. A happy 
thought, however, soon extricated us from our dilemma. 
Causing our luckless coachman (who wept true Sicilian 
tears over the imaginary danger to his springless vehicle) 
to drive it, in the cause of Science, over the rock-sprinkled 
field, we utilised our carriage as a temporary shelter for 
the precious instrument, and were ready some time before 
