Trees are found to give great shelter and salubrity to towns in this way, acting as they do as so many sieves retaining impurities. It is understood that the effluvia arising from putrefying vegetable matters ascend high in the atmosphere under the influence of the solar rays, and spread far and wide, and that at night during the cold they fall with the dew to the ground again, and impart to it and to those exposed to its action, much virulence. The ground is there known to be extremely unwholesome, and those who have been compelled by want, by sickness, while travelling, overtaking them, or by being benighted, to lie down with nothing but the soil for a couch, and with no shelter from the vapours and dew that falls at night, save the sky itself, have felt this pestilential influence: on the morrow they awake distressed, parched, and affected with headach, and the usual symptoms of malignant fever. With the close of day or the setting of the sun, the pestilential vapour falls and envelopes the country and the habitations of men with a deadly mantle—and it is then unsafe to venture into the open air in many of the finest countries of the world. The pestilential effects of exposure to these night dews and vapours have, on many occasions, been experienced by soldiers encamping in the open grounds, and our gallant countrymen on foreign service are wont to yield in fearful numbers to a foe, merciless and unsparing. But it is not in swampy grounds only that these vapours arise, for there is reason to think that in those places where sickness is constant, and where no such dampness of ground is observed, that decomposition of animal and vegetable matter is going on some depth below the surface, and that the extricated gases issue through the soil. This is rendered almost certain, by the fact which has sometimes been observed, that the most dangerous and sickly season is, when the ground is parched and rent with heat, permitting the exhalations generated below to ascend into the atmosphere. Instances of this occurred among our soldiers in the Peninsular war—the season, marked with the greatest prevalence of disease, the common result of vitiated air, being that when the soil was most rent with heat.