Returning to Scotland, he continued his observations at Ben Nevis and at Kingairloch, opposite Appin, Mr. Rankin using the instrument at the top of the mountain. These observations showed in general that on the mountain southerly, south-easterly, and easterly winds were more impregnated with dust-particles, sometimes containing 133,000 per cubic inch. Northerly winds brought pure air. The observations at sea-level showed a certain parallelism to those on the summit of the mountain. With a north-westerly wind the dust-particles reached the low number of 300 per cubic inch, the lowest recorded at any low-level station. The general deductions which he made from his numerous observations during these two years are that (1) air coming from inhabited districts is always impure; (2) dust is carried by the wind to enormous distances; (3) dust rises to the tops of mountains during the day; (4) with much dust there is much haze; (5) high humidity causes great thickness of the atmosphere, if accompanied by a great amount of dust, whereas there is no evidence that humidity alone has any effect in producing thickness; (6) and there is generally a high amount of dust with high temperature, and a low amount of dust with low temperature. CHAPTER VIII A FOG-COUNTER Next to the enumeration of the dust-particles in the atmosphere is the marvellous accuracy of counting the number of particles in a fog. The same ingenious inventor has constructed a fog-counter for the purpose; and the number of fog-particles in a cubic inch can be ascertained. This instrument consists of a glass micrometer divided into squares of a known size, and a strong microscope for observing the drops on the stage. The space between the micrometer and the microscope is open, so that the air passes freely over the stage; and the drops that fall on its surface are easily seen. These drops are very small; many of them when spread on the glass are no more than the five-hundredth of an inch in diameter. In observing these drops, the attention requires to be confined to a limited area of the stage, as many of the drops rapidly evaporate, some almost as soon as they touch the glass, whilst the large ones remain a few seconds. In one set of Dr. Aitken's observations, in February 1891, the fog was so thick that objects beyond a hundred yards were quite invisible. The number of drops falling per second varied greatly from time to time. The greatest number was 323 drops per square inch in one second. The high number never lasted for long, and in the intervals the number fell as low as 32, or to one-tenth. If we knew the size of these drops, we might be able to calculate the velocity of their fall, and from that obtain the number in a cubic inch.