Page:The Worst Journey in the World volume 2.djvu/209

456 during summer temperatures on the Barrier the surface is fairly good, that between +5° and +15° less good, and between +15° and +25° best. The worst is from +25° upwards, the worst of all being round about freezing point.

As the temperature became high the amount of ice melted by this sliding friction was excessive. It was then that we found ice forming upon the runners, often in almost microscopic amounts, but nevertheless causing the sledges to drag seriously. Thus on the Beardmore we took enormous care to keep our runners free from ice, by scraping them at every halt with the back of our knives. This ice is perhaps formed when the runners sink into the snow to an unusual depth, at which the temperature of the snow is sufficiently low to freeze the water previously formed by friction or radiation from the sun on to a dark runner.

In very low temperatures the snow crystals become very small and very hard, so hard that they will scratch the runners. The friction set up by runners in such temperatures may be known as rolling friction, and the effect, as experienced by us during the Winter Journey and elsewhere, is much like pulling a sledge over sand. This rolling friction is that of snow crystal against snow crystal.

If the barometer is rising you get flat crystals on the ice, if it is falling you get mirage and a blizzard. When you get mirage the air is actually coming out of the Barrier. Thus far Wright's lecture.

Since we returned I have had a talk with Nansen about the sledge-runners which he recommends to the future explorer. The ideal sledge-runner combines lightness and strength. He tells me that he would always have metal runners in high temperatures in which they will run better than wood. In cold temperatures wood is necessary. Metal is stronger than wood with same weight. He has never used, but he suggests the possible use of, aluminium or magnesium for the metal. And he would also have wooden runners with metal runners attached, to be used alternately, if needed.

The Discovery Expedition used German silver, and it failed: Nansen suggests that the failure was due to the