=Africa.= In Africa is to be found the hottest region of the world, the great Desert of Sahara, upon whose sands beats down the fierce tropical sun with merciless intensity. Here shade temperatures of 130° are frequently experienced. Only those bred to extreme tropical desert heat can long live under such conditions. In a portion of the desert lying between Egypt and the Red Sea the temperature has been known not to fall below 113° for a period of ten days, while on several nights the lowest temperature reached was 118°, with a practically calm air. Africa lies with about one half of its immense area on each side of the equator, and the greater part of its territory inside the Tropical Zone. Except in a few isolated cases on high mountains, temperatures as low as zero never are experienced. =Europe.= The warmest portion of Europe is in the region round and about the Mediterranean Sea. The coldest places in all Europe are in the western part of Russia and in the northern part of the Scandinavian Peninsula. Here the average of the coldest days of winter is 50° below zero. =Asia.= It is difficult to determine in what part of Asia the highest temperature occurs, as data from many parts are meager. It is known however that extremely hot weather prevails in India and Arabia. Siberia, however, experiences the coldest weather to be found anywhere in the world. At Werchojansk, in that country, a temperature of 90.4° below zero was observed in January, 1884, while the average temperature for the whole month was 69.4° below zero. The coldest weather of the world is not found at the North or the South Pole, as many suppose, but rather at the center of vast continents, far from the modifying influence of oceans. =Australia.= In extreme heat the interior of Australia is fairly comparable with northern Africa, Persia, Afghanistan, and northern India, where every year maximum temperatures of 115° occur, and where, at times, an extreme heat of 120° or 125° is experienced in the shade. We now know that the forceful, dominating peoples come out of the regions where the heat is not so great as to debilitate, nor the cold so fierce as to deaden the mental and the physical faculties; but rather from the region of the thoroughfare of the great circum-polar storm tracks, where there are frequent changes of weather from sunshine to clouds, and where there is a fairly wide difference in temperature between night and day and between winter and summer. For the best coördination of the mental and the physical faculties, so as to produce the most efficient composite of man, the temperature should range between 45° and 50° at night and between 65° and 70° during the day, with about sixty-five to seventy per cent. of relative humidity. Some day we will artificially create the exact conditions of temperature and moisture needed for patients in hospitals and sanitaria. Science is persistently seeking means to increase comfort and prolong life. CHAPTER XV