Just two hundred and forty years ago, Mr. John Evelyn, F.R.S., a well-known writer on meteorology, sent a curious tract to King Charles II., which was ordered to be printed by his Majesty. It was entitled "Fumifugium," and dealt with the great smoke nuisance in London. I find from the thesis that he had a very hazy idea of the connection between fog and smoke; and no wonder, for it is only lately that the connection has been fully explained. We know that without dust-particles there can be no fog, and that smoke supplies a vast amount of such particles. Therefore, in certain states of the atmosphere, the more smoke the more fog. In Mr. Evelyn's day the fog, which he called "presumptuous smoake," was at times so dense that men could hardly discern each other for the "clowd." His Majesty's only sister had complained of the damage done to her lungs by the contamination, and Mr. Evelyn was disgusted at the apathy of the people to do anything to remedy the nuisance. He deplored that that glorious and ancient city of London should wrap her stately head in "clowds of smoake, so full of stink and darknesse." He was of opinion that a method of charring coal so as to divest it of its smoke, while leaving it serviceable for many purposes, should be made the object of a very strict inquiry. And he was right. For it is now known that fog in a town is intensified by much smoke. In a city like London or Glasgow, where a great river, fed by warm streams of water from gigantic works, passes through its centre, fogs can never be entirely obliterated, for the dust-particles in the air (often four millions and upwards in the cubic inch) will seize with terrible avidity the warm vapour rising from the river. That is the main reason why fogs cannot there be put down. Smoke is being consumed to a great extent; yet we find particles of sulphur remaining, which seize the warm vapour and form fogs dense enough to check all traffic. The worst form of city fogs seems to be produced when the air, after first flowing slowly in one direction, then turns on its tracks and flows back over the city, bringing with it a black pall, the accumulated products of previous days, to which gets added the smoke and other impurities produced at the time. What irritated Mr. Evelyn was that, outside of London, the air was clear when passengers could not walk in safety within the city. So vexed was he about the contamination, that he made it the occasion of all the "cathars, phthisicks, coughs, and consumption in the city." He appealed to common sense to testify that those who repair to London soon take some serious illness. "I know a man," he said, "who came up to London and took a great cold, which he could never afterwards claw off again."