Any natural process having a tendency to cool the air, so that its temperature falls to or below the dew-point, without originating any lateral motion to clear away the extraneous atmospheric impregnations which ensue, will contribute very much to concentrate them in the lower portions of the air.
Now this is the case with terrestrial radiation, which, with a clear and calm atmosphere, rapidly diminishes the temperature of certain parts of the earth's surface, and the circumambient air, condensing the vapour it contains, and creating a local, cold, damp, and very often malarious, atmosphere.
The mists and fogs which frequently form during the night, over valleys, on the banks of rivers, and in all low situations, are, therefore, objects peculiarly claiming the notice of the meteorologist; they are caused by the terrestrial radiation of caloric cooling the air and condensing the aqueous vapour mingled with it; and if this vapour happens to have been the carrying agent to any injurious exhalation, the development of the latter, in a deleterious form, at the same period, commences. This is amply substantiated by the well-known unhealthy character of the night season in tropical climates, where the sun has great power during the day, and where the dews are in an extraordinary degree copious; and is still further confirmed by many instances in which severe epidemic diseases have made their attack during the night.
From a careful consideration of the subject, I cannot but conclude that Terrestrial Radiation is one of the most important of those meteorological phenomena usually considered as influencing the health
