PRINCIPLES OF VENTILATION. 253 



suspended, as is the case in the artificial atmosphere of a hot- 

 house. And as by artificial means the balance is therein 

 destroyed, so, also, by artificial means must the elements of the 

 atmosphere be adjusted, and the balance maintained. 



It is impossible for us to contemplate so special an adjustment 

 of opposite effects, without admiring this beautiful dispensation 

 of Providence, extending over so vast a scale of being, and 

 demonstrating the unity of the plan upon which the whole sys- 

 tem of the organized creation is designed. And yet man, in 

 his ignorance, has done his utmost to destroy this beautiful and 

 harmonious plan. It was evidently the intention of the Creator, 

 that animal and vegetable life should everywhere exist together, 

 so that the baneful influence which the former is constantly 

 exercising upon the air should be counteracted by the latter. 

 Nothing, therefore, can be more prejudicial to the health of a 

 large population, than the close packing of houses together, as 

 presented in large cities. Hundreds of thousands of men, with 

 manufactories of all kinds, the smoke and vapors of which are 

 still more injurious than the foul air produced by human respi- 

 ration, being crowded together in the smallest possible com- 

 pass, with scarcely the intervention of an open space on which the 

 light and air of heaven may freely play, and without any oppor- 

 tunity for the growth of any kind of vegetation sufficiently luxu- 

 riant to give pleasure to the eye, or sufficiently energetic to 

 answer its natural purpose ; for the close confined atmosphere 

 of crowded cities is almost as injurious to vegetation as to ani- 

 mals; the smoke, which is constantly hovering above them 

 prevents their enjoyment of the clear bright sunshine which 

 they require for their health, and the dust, which is constantly 

 floating in the atmosphere, covers the surface of their leaves, 

 clogs up the pores, and prevents respiration. 



This is the reason why plants thrive so badly in dwelling- 

 houses in large cities, and also in the external air in the streets 

 and squares. But lofty trees are so beneficial in such situations 

 that they have with truth been called the lungs of large cities, 

 so important is the effect produced by them in purifying the air. 

 It is true, they may occasion some degree of dampness in the 

 immediate neighborhood, but this evil is more than counterbal- 



