Whatever the original cause may be, and however the direction, abstractedly, may be regulated by the winds and the forms of the streets, or by local and fixed circumstances, it is plain that the annual extension is the consequence of desertion, and that as the inhabitants retire from before it, it acquires the means of making a new step and a further progress; because thus they withdraw those fires and smoke, or whatever else it be, dependent on human crowds, which decomposes and destroys this substance. And hence it must follow, that as Rome shall become still further abandoned and depopulated, from want of industry, or from political feebleness [p053] added to this cause, the effects must be expected to increase in a sort of geometrical ratio; almost leading to the fear that the whole city itself may, in time, fall a victim to it, or become abandoned to the wolves and mosquitoes. If I dare not inquire more minutely into the remaining circumstances connected with the propagation of malaria, lest I should extend this article to an inconvenient length, it is necessary now to offer some remarks on prevention, and especially as it relates to this circumstance—the propagation of the poison; since the rules for prevention, as far as this relates to production, may be deduced from what was said in a former paper on this subject, and relate chiefly to the drainage of lands, and to other practices, more or less obvious, which a little reflection will, without much difficulty, deduce from what was there said. It is plain, in the first place, that as far as the winds are concerned, it is by opposing obstacles to their course that we must attempt to counteract or divert their influence; and that, in this case, it is through the use of trees alone that we possess any power. Thus reversely, as in the case just stated, the cutting down of trees and forests has often been a serious cause of diseases in certain countries, by admitting a malaria to particular spots; though it is easy to see that where any given spot suffers from malaria, through condensation or confinement, the clearing away of these would be the remedy, by attaining a free ventilation. To detail the particular modes in which remedies may be applied through this species of aid, is obviously unnecessary, and not easy, as it must depend on local circumstances, differing for each place; but I may remark, as an example in illustration of my meaning, that where, as in many of the narrow and prolonged valleys of Greece, the sea shore is a marsh, the remedy would be to plant a screen of trees beyond it, and thus to prevent the sea winds from passing into the interior. And thus did the ancient Romans compel the planting of trees on the shores of Latium, to check the current from the Pontine marshes; rendering groves sacred, under heavy penalties, and enacting other laws with the same intentions.