All know that thick haze is usual in very sultry weather. The wavy, will-o'-the-wisp ripples near the horizon indicate its presence very plainly. During the intense heat there is generally much dust in the atmosphere; this dust, by the high temperature, attracts moisture from the apparently dry air, though above the saturation point. In all circumstances, then, the haze can be accounted for by the condensing power of the dust-particles in the atmosphere, at a higher temperature than that required for the formation of fogs, or mists, or rain. CHAPTER XIII HAZING EFFECTS OF ATMOSPHERIC DUST The transparency of the atmosphere is very much destroyed by the impurities communicated to it while passing over the inhabited areas of the country. Dr. Aitken devoted eighteen months to compare the amount of dusty impurities in different masses of air, or of different airs brought in by winds from different directions. He took Falkirk for his centre of observations. This town lies a little to the north of a line drawn between Edinburgh and Glasgow, and is nearly midway between them. If we draw a line due west from it, and another due north, we find that, in the north-west quadrant so enclosed, the population of that part of Scotland is extremely thin, the country over that area being chiefly mountainous. In all other directions, the conditions are quite different. In the north-east quadrant are the fairly well-populated areas of Aberdeenshire, Forfarshire, and the thickly populated county of Fife. In the south-east quadrant are situated Edinburgh and the well-populated districts of the south-east of Scotland. And in the south-west quadrant are Glasgow and the large manufacturing towns which surround it. The winds from three of these quadrants bring air polluted in its passage over populated areas, whereas the winds from the north-west come comparatively pure. The general plan of estimating the amount of haze is to note the most distant hill that can be seen through the haze. The distance in miles of the farthest away hill visible is then called "the limit of visibility" of the air at the time. For the observations made at Falkirk, only three hills are available, one about four miles distant, the Ochils about fifteen miles distant, and Ben Ledi about twenty-five miles distant--all in the north-west quadrant. When the air is thick, only the near hill can be seen; then the Ochils become visible as the air clears; and at last Ben Ledi is seen, when the haze becomes still less. After Ben Ledi is visible, it then becomes necessary to estimate the amount of haze on it, in order to get the limit of visibility of the air at the time. Thus, if Ben Ledi be half-hazed, then the limit of visibility will be fifty miles. In this way all the estimates of haze have been reduced to one scale for comparison.