NAVIGATION 293 
given later in the report. These observations show that the 
thickness increases steadily until the month of June, when a 
maximum of seventy-four inches was measured. The conditions 
under which this was obtained were very favourable for the 
ice, and only in similarly protected northern harbours does it 
attain such a thickness. In the larger bays and along the un- 
protected coasts, where the ice freezes later, and is frequently 
broken up by gales during the winter, the thickness rarely 
exceeds three or four feet. This thinner ice makes up the 
greater part of that found in the spring-time covering the 
waters of Hudson bay and strait. 
As the ice continues to increase until june, winter conditions 
continue well into that month, and it is not until its last days 
that the heat of the sun is sufficiently strong and sustained to 
begin the melting process. With the advent of July this process 
is well under way, and the daily change in the condition and 
amount of the ice is then marvellous, so much so, that where 
everything was fast frozen in the beginning of the month, by 
the middle not a vestige of ice remains. 
If a single thickness of sheet ice covered these northern 
waters they would be completely clear early in July, but unfor- 
tunately much of the floating ice is ' rafted ' or piled up, sheet 
on sheet, and the whole cemented solidly together to form large 
masses often twenty feet or more in thickness. This rafting is 
caused by the pressure formed by large masses of ice driven 
together, or against ice attached to the shores, which causes the 
ice along the margins to break and buckle, cake on cake. These 
pressure areas are often of considerable size, and usually are 
many times longer than broad. They serve as a framework to 
hold together large fields of single sheet ice. When the thinner 
ice melts, these pressure masses remain, and are dangerous to 
shipping until the water has become sufficiently warm to melt 
the ice cementing the cakes together; then they are harmless, 
