and of the effete matters given off from the lungs in the expired air is much greater than in hot climates, and the body is also cooled by the evaporation of water in the form of aqueous vapor.  Moist air is a better conductor of heat than dry air, which accounts for much of the discomfort felt in winter when a thaw takes place as compared with the feeling of elasticity when the air is dry.  In cold weather, therefore, moist air cools down the skin and lungs more rapidly than dry air, and colds consequently result.  London fogs are injurious, not only on account of the various vapors given off by the combustion of coal, but in consequence of the air being in winter generally saturated with moisture at a low temperature.  The injuriousness of fogs and low temperatures will be presently dwelt upon at greater length.

Variations in the pressure and temperature of the atmosphere exert a considerable influence on the circulation of air contained in the soil, which is called ground air.  As all the interstices of the ground are filled with air or water, the more porous the soil, the greater is the bulk of air.  The quantity of air contained in soil varies very much according to the material of which the soil is composed, as it is evident that in a gravelly or sandy soil it must be greater than when the ground consists of loam or clay.  The estimates vary from 3 to 30 per cent., but the latter is probably too high.  If, therefore, a cesspool leak into the ground, the offensive effluvia, if in large quantities, will escape into the soil, and are given off at the surface of the ground, or are drawn into a house by the fire; but, if small, they are rendered innocuous by oxidation.  The distance to which injurious gases and suspended or dissolved organic matters may travel through a porous soil is sometimes considerable, as I have known it pass for 130 feet along a disused drain, and above 30 feet through loose soil.

Winds exercise a great effect on health both directly and indirectly.  Directly, by promoting evaporation from the skin, and abstracting heat from the body in proportion to their dryness and rapidity of motion.  Their indirect action is more important, as the temperature and pressure of the air depend to a great extent on their direction.  Thus winds from the north in this country are usually concomitant with a high barometer and dry weather; in summer with a pleasant feeling, but in winter with much cold.  Southwest winds are the most frequent here of any, as about 24 per cent. of the winds come from this quarter against 161/2 from the west, 111/2 from the east, and the same from the northeast; 101/2 from the south, 8 from the north, and a smaller number from the other quarters.  Southwest winds are also those which are most frequently accompanied by rain, as about 30 per cent. of the rainy days are coincident with southwest winds.  Another set of observations give precisely the same order, but a considerable difference in their prevalence, viz., southwest 31 per cent., west 141/2, and northeast 111/2 per cent.  Easterly winds are the most unpleasant, as well as the most injurious to man of all that occur in this country.