considered, as it is felt to be, a balmy and refreshing sequel to a hot day. Let this be enjoyed where it can with safety, and as it often may; but such evening walks will not be safe in any of those situations which I need not repeat here; after having detailed them as I have done in the former paper. And lest I should be accused of wishing to excite unnecessary alarm, I consider, on the contrary, that it ought to be diminished by these remarks; because, if we take the whole of [p044] England, there is perhaps not one acre in a hundred thousand where there is danger from night air, or from malaria in any mode; so that to distinguish where that lies, is to have relieved from useless fears all those who may learn to make the distinctions under review. To pass from what relates to climate and season, and to proceed to the propagation, simply, of malaria, it is almost superfluous to say, that its influence, as to the production of disease, is much regulated by proximity, which implies a state of concentration or accumulation. Hence the danger arising from vicinity; while, as I formerly remarked, where the generating source is small, this becomes necessary to its effect, since dilution may be expected to destroy the power of the poison. For analogous reasons, its effect in the production of disease is increased by concentration or condensation; and such a state of things takes place in narrow and confined valleys, or in places surrounded by woods, or in woods themselves; in any situation, in short, where the poison is produced, and is so sheltered from winds that ventilation becomes difficult. And if it is probable that this is one chief reason of the peculiarly insalubrious nature of woods and jungles in hot climates, so is it an universal remark in Italy, that the short valleys in which the air cannot circulate are among the most pestilential spots. And if this explains, also, in some measure, the bad effects of calm weather, so does it account for the unusually pestiferous nature of rivers and lakes confined within wood, as are those of the tropical climates, and as there are many also in different parts of Europe. That we ourselves are not exempt from these additional causes of the influence of malaria, would be easily shown by many references, were it not for the reason which has caused me to exclude them.