The movement of ice on Minnesota lakes. — Atwood. 253 
springs on the northern side the ice freezes firmly to the bottom 
and sides, consequently when the ice expands it moves in the di- 
rection of the least resistance, which in this case is to the south. 
Having studied the ice movement upon this and other lakes for 
many years in temperatures ranging from 40° above to 40° below 
zero, I find that ice after it has once formed into a solid body is 
subject to the same law which governs other bodies ; that is, that 
heat expands and cold contracts. The question naturally arises, 
if heat expands the ice in a certain direction why does it not con- 
tract to its first position when the temperature falls to its first de- 
gree of cold ? The correct solution of this question will show the 
true principle of the glacial movement, which has been so little 
understood b} r scientific men of all ages. To explain the move- 
ment of the ice in one direction, let us begin of a bright morning 
with the ice on the lake, say 20 inches thick and the temperature 
10° below zero. Remember that while the temperature is 10° be- 
low on the top side of the ice, the bottom lies upon the water 
with an unchanging temperature at or near 32° above zero. As 
the sun comes up and its rays strike the ice, the temperature rap- 
idly rises, causing the surface of the ice to expand, while the 
temperature of the lower surface remains the same. As the 
lower ice cannot stretch to keep up with the expansion of the up- 
per surface, innumerable cracks will be formed on the under sur- 
face, some reaching part wa} r through the ice and a few clear 
through. These cracks are wedge-shaped, like the capital letter A. 
They immediately fill with water and freeze, thus forming so much 
new ice and preventing contraction when the temperature falls. 
This movement will continue as long as the temperature rises, ac- 
companied by a roaring and cracking sound like a fierce battle, 
the ice meanwhile crowding upon the shore, often 12 to 15 inches 
a day. About 4 o'clock p. m. the temperature begins to fall and 
the top of the ice to contract, while the bottom remains station- 
ary. The contraction of the top causes V-shaped cracks to form 
from the top downward, the new ice which had filled the A-shaped 
cracks in the bottom preventing the body from resuming its first 
position. These cracks from the top fill with water and freeze un- 
til the next morning, when we have a body of ice which has been 
enlarged by the action of both heat and cold, and as the morning 
sun shines upon its surface the same process of expansion and 
contraction and the formation of new ice goes on. 
