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They seem to be made either of aluminium, or more probably magnalium.

We have returned to the last Discovery type with improvements; the magnalium sole plates of our own crampons are retained but shod with 1/2-inch steel spikes; these plates are rivetted through canvas to an inner leather sole, and the canvas is brought up on all sides to form a covering to the 'finnesko' over which it is laced—they are less than half the weight of an ordinary ski boot, go on very easily, and secure very neatly.

Magnalium takes the place of aluminum in the crankcase, because it is not only lighter but stronger and can be cast very thin.

Magnalium is a silver-white alloy of aluminum with from 5 to 20 per cent of magnesium, forming a metal even lighter than aluminum and strong enough to be used in making high-speed gasoline engines.