A small instrument has been constructed which can bring about results sufficiently accurate for ordinary purposes. It is so constructed that, when the different parts are unscrewed, they fit into a case 4-1/2 inches by 2-1/2 by 1-1/4 deep--about the size of an ordinary cigar-case. After knowing this, we are apt to wonder why our lungs do not get clogged up with the enormous number of dust-particles. In ordinary breathing, 30 cubic inches of air pass in and out at every breath, and adults breathe about fifteen times every minute. But the warm lung-surface repels the colder dust-particles, and the continuous evaporation of moisture from the surface of the air-tubes prevents the dust from alighting or clinging to the surface at all. CHAPTER VII DUST AND ATMOSPHERIC PHENOMENA Dr. Aitken has devoted a vast amount of attention to the enumeration of dust-particles in the air, on the Continent as well as in Scotland, to determine the effects of their variation in number. On his first visit to Hyeres, in 1890, he counted with the instrument 12,000 dust-particles in a cubic inch of the air: whereas in the following year he counted 250,000. He observed, however, that where there was least dust, the air was very clear; whereas with the maximum of dust, there was a very thick haze. At Mentone, the corresponding number was 13,000, when the wind was blowing from the mountains; but increased to 430,000, when the wind was blowing from the populous town. On his first visit to the Rigi Kulm, in Switzerland, the air was remarkably clear and brilliant, and the corresponding number never exceeded 33,000; but, on his second visit, he counted no less than 166,000. This was accounted for by a thick haze, which rendered the lower Alps scarcely visible. The upper limit of the haze was well defined; and though the sky was cloudless, the sun looked like a harvest moon, and required no eagle's eye to keep fixed on it. Next day there was a violent thunder-storm. At 6 P.M. the storm commenced, and 60,000 dust-particles to the cubic inch of air were registered; but in the middle of the storm he counted only 13,000. There was a heavy fall of hail at this time, and he accounts for the diminution of dust-particles by the down-rush of purer upper air, which displaced the contaminated lower air. At the Lake of Lucerne there was an exceptional diminution of the number in the course of an hour, viz. from 171,000 to 28,000 in a cubic inch. On looking about, he found that the direction of the wind had changed, bringing down the purer upper air to the place of observation. The bending downwards of the trees by the strong wind showed that it was coming from the upper air.