35 
particles of quicksilver may be moved more easily upon one another than those 
of iron may. The pressure necessary to move a given portion of the surface of 
a body on any other given portion of the same body is called the force of 
shearing. A quicksilver glacier would descend because it shears easily, a cast 
iron one would not because it shears with difficulty. There must exist a 
relation between the shearing-force and the weight of a given volume of a 
glacier, so that it may just descend by its weight only. Now, it is possible 
to investigate mathematically what that relation is. That investigation has 
been made, and it results from it that in order that a glacier should descend 
at the slope at which the Mer de Glace descends, and as the Mer de Glace 
descends, the force requisite to shear one square inch of it over another square 
inch must be less than 21bs. But it requires from 75 lbs. to 120 lbs. to shear 
one square inch of ice over another square inch. The ice of the Mer de Glace 
cannot therefore descend by its weight only ; it does not shear easily enough. 
It must be ice of about the consistency of soft putty to descend by its weight 
only, — for that substance shears with a pressure of from 1 4 to 3 lbs. per 
square inch. Some other force, in addition to its weight, acting in the 
direction of its descent, must therefore act on a glacier, causing it to descend. 
This force must, moreover, be such as would produce those molecular dis- 
placements and strains which are actually observed in glacier ice ; and it 
must, to do that, be 34 times as great as the pressure which the weight of 
the glacier produces in the direction of its descent. What is that force ? 
The fact of the descent of a sheet of lead when placed upon the inclined surface 
of a roof, however low the pitch, has long been known ; I myself first observed 
it on the southern side of the roof of the Choir of the Bristol Cathedral, in 
1855. I have verified it by the following experiment : — I fixed a deal board 
9 feet long and 5 inches broad to the southern wall of my house, so as to form 
an inclined plane, and upon it I placed a sheet of lead, turning its edges down 
over the side edges of the board, and taking care that it should not bind 
upon them, but be free to move with no other obstruction than that which 
arose from its friction. The inclination of the board was 18° 32 the thickness 
of the lead | of an inch, its length 9 feet, and its weight 28 lbs. The lower 
end of the board was brought opposite to a window, and a vernier was con- 
structed, which could be read from within, and by which the position of the 
lead upon the board could be observed to the 100th of an inch. I began to 
measure the descent of the lead on the 16th of February, 1858, and recorded 
it every morning between 7 and 8 o'clock, and every evening between 6 and 
7 o'clock, until the 28th of June. I have preserved all these observations. 
In the night, between sunset and sunrise, the lead scarcely descended at all, 
