资料题：Why do Eskimo build their houses partly under the ground（　）。

In the Arctic Circle(北极圈), it is not that Eskimos lack ability or industry, but the surroundings restrict constructive effort to the barest necessities of existence.This effectually retards progress to higher development.Agriculture is impossible all along the thousands of miles of the north shore.The only woodis such as drifts in.Other than his driftwood, the only available building materials are snow,ice, stone and bones of animals.All of these have been used for habitations(住所)and storage places, differing in various timbers according to the requirements and skill of the workers.The lack of necessary timbers to build walls and span wide spaces is probably one reason why these tribes construct their houses at least partly beneath the surface of the ground.This device also makes the houses more impervious to the cold.Most of us are inclined to think that the Eskimos always lives in an igloo( 圆顶建筑)or snow house.This is not entirely true.After the long cold winter, the family is very likely to move, when the weather permits, into a tent of sealskin.The actual construction of such a tent is similar to that used by other, more southerly tribes and will be described later.The snow house,however, is an interesting and unique habitation.Our summer campers will not build with snow,but the delicate ant is worth recording and some of our winter camps in the mountains might try to make now houses.