Not only is smoke related to the frequency of fogs, but it also increases both the duration and the density of fogs. The water globules become coated with a film of black, sticky, tarry soot which retards evaporation. These soot fogs, or “pea-soup” fogs as they have often been called because of their grimy opacity, may at times become practically impervious to the luminous rays of the sun. Even in one of the lighter varieties of fogs the soot products may increase seven fold. Many average soot-fogs reduce the amount of light one-half. On an exceptionally dark day in Berlin the light was only 1/500 of what it was on an ordinary overcast day and only 1/3000 to 1/4000 of what is was on an entirely clear day (H. H. Kimball). The smoke in the center of Leeds on an ordinary day absorbs as much as 25% of the total daylight as compared with the suburban sections of Leeds, or 40% as compared with the suburb of Garforth, which is 7 miles distant (4). That smoke must materially decrease the limits of visibility is patent from what has been said. The limits of visibility vary with the number of dust particles in the air: 1,000 particles per cu. cm. render large objects like mountains invisible at a distance of 100 miles; 100,000 particles render them invisible 1 mile away, and 1,000,000 particles 1/10 of a mile distant (Aitken). Smoke not only fills the atmosphere with countless numbers, but with extremely opaque particles. These particles may unite with globules of water and this makes the atmosphere still more opaque. As was stated above, in large cities even in fine weather there may be over 300,000 particles per cu. cm., as against a few hundred in the country. Accordingly, we find in London that the limit of visibility in the winter time, even during the clear part of the day, does not exceed one-half mile.