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 accompani