422 ON THE CONTRACTION AND EXPANSION OF ICE. 
rate, on an average, of .00000 765 per degree per foot, or more than 
twice the extent of its previous movement. The temperature again 
rose to 32°, and the ice expanded at the rate of its Jast contraction 
to its original or maximum dimensions. 
From the 8th until the 29th of February the ice obeyed the various 
fluctuations of temperature (considerating its increasing thickness) 
with great regularity, ever maintaining the latter ratio of .00000 765 
per degree per foot. A continuation of a high temperature from the 
22nd to the 24th of February did not affect it in its uniform rate of 
movement ; neither did the beams of the mid-day sun, at a tempera- 
ture of 45°, which I allowed to act on it for some hours, cause further 
expansion than it manifested at a temperature of 34°. 
The permanent and greatest length of the ice seemed to tally with 
a thermometrical reading of 34°; the thermometer was suspenced 
about a foot above the ice level, and probably was two degrees higher 
than the atmospheric temperature at the surface. It will be remem- 
bered that the ice at this temperature was ever the same length, and 
the different ratios of movement were owing to the change in the 
character of the ice after the thaw of the 5th February, which gave 
it a greater shrinking, and consequently a greater expanding capacity. 
Ice, at a low temperature, is extremely sensitive and brittle. On 
one occasion during my experiments, the temperature in my shed was 
plus four; outside the north wind read zero. Being anxious to lower 
the temperature within the shed, I desired my assistant to take a board 
off the roof. He did so, and in a few minutes the current of cold air 
from the north caused my ice to crack into two pieces, with a loud 
report. The ice at the time was perfectly isolated, and floating clear 
of the main field. 
It has been often remarked on Rice Lake, that when ice attains a 
great thickness, it does not seem to move about with the same vio- 
lence, or to the same extent, as it did when it was comparatively thin. 
It is a well known fact, that the greatest “‘shoves’’ occur when the 
ice is from four to ten inches im thickness. My experiments confirmed 
this fact. I found on my experimental ice, that when it increased in 
thickness it became tardy in its movements. In fact, the rapidity 
with which ice expands or contracts is inversely as its thickness. If 
ice three inches in thickness takes half an hour to move a given distance 
corresponding to a change of temperature, ice twenty-four inches in 
thickness will take four hours to expand to the same extent. Should 
