When of larger dimensions, so that the boundaries of the revolving column of dust and air are beyond the range of vision, they are known as “dust storms,” and in spite of the temporary discomforts they cause, are gladly welcomed, on account of the relief they bring from the suffocating heat that originated them. When on the larger scale that is met with in the equatorial zone their violence is well-nigh incredible--trees are torn up, houses levelled, crops destroyed, and massive bodies, such, for example, as large anchors lying on the quays of a dockyard, trundled along as if they were straw hats in an ordinary gust of wind. From a sanitary point of view these storms are usually beneficial by their effect in clearing and cooling the air; but this is unfortunately only small and temporary, so that they are of little interest to the hygienist. In any case the prophylaxis against their effects is purely mechanical, and consists in crawling, if possible, into the nearest cave or cellar. Over the great Asiatic continent, especially north of the Himalayas, a strong northerly current is produced about mid-summer by the area of low pressure caused by the intense heating of the Siberian steppes, which, owing to the length of the days at these high latitudes, are exposed to the sun’s rays for practically the whole twenty-four hours, and the current thus initiated makes its influence felt for hundreds of miles to the south of its point of origin, and no doubt reacts upon and modifies other periodical forces of the same character. A useful law to remember is that discovered by Professor Buys Ballot, which is to the effect that, if you stand with your back to the wind, the barometer will be lower in your left hand than in your right, in the Northern Hemisphere, and _vice versâ_ south of the Equator. The effect of ocean currents bringing with them masses of hot or cold water from other latitudes has already been alluded to, a familiar example being the mildness of our own climate under the influence of the Gulf Stream. In the Southern Hemisphere currents of cold water sweep up from the Antarctic regions along the western shores of the great masses of land, counter-currents of warm water flowing down from the Equator along their eastern sides. In the Northern Hemisphere the reverse is the case, the cold Arctic currents clinging to the eastern, and the warm equatorial to the western shores of the continents. The more detailed distribution of these currents, however, can be better gathered by a little study of the current chart found in any good atlas, than from any description, however elaborate.