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Otago Daily Times, Issue 18294, 11 July 1921, Page 8

A real genuine London fog is, os most people realise, extremely dirty, as well as unpleasant (comments Everyday Science), but' even the most disgusted housewife may be surprised at the actual amount of grime in lb weight that the fog-atmosphere carries about. Recently a fine sample gave the Meteorological Office an opportunity they had been waiting for for a long time; they promptly put it through the new weighing machine that they, ne.vised. and found that there wore lllh of dirt to every million cubic yards of fog. As 31b of dirt in the same extent of atmosphere will make an appreciable fog, it will bo gathered that the specimen was a good one. The weighing machine is a piece of pure white tissue paper, through which samples of fog are forced. The amount of dirt is calculated from the degree to which the white paper is coloured and darkened. The amount, of dirt borne in the air of a big and smoky city is astonishing. * It is calculated tliat in the course of a year considerably over 11 tons of air-borne dirt are deposited on each square kilometre ol London. The city,_ however, is steadily growing cleaner, for in 1916 the deposit was more than 14£ tons, and it has steadiljr decreased year by year since. The air is dirtiest in London about 10.30 a.m.; it then clears and becomes dirty again between I and 5 p.m. The kindling of morning and evening fires is mainly to blame for tho pollution.