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This would seem at first sight a rude method and a dangerous one. Experience, however, teaches us that the evils naturally to be expected from such a system are not so serious in practice as the nature of sewer gases would lead us to think. No doubt the diffusive property of gases has much to do with this favorable result. By the proper use of charcoal it is stated that the possible evil is reduced to a minimum.

I do not think any inseparable difficulty would be encountered in applying mechanical ventilation to sewers. Certainly there is the cost, but this should not stand in the way for a moment where the public health is concerned. Let those in authority say it must be done, and I feel sure the way would soon be found.

A sewer 6 ft. in diameter and five miles long will contain 746,000 cubic ft. of air, but after deducting one-third for space occupied by sewage water, 497,550 cubic ft. would be the quantity left to be dealt with. As mentioned above, a fan 10 ft. in diameter will remove 9,000 cubic ft. of air per minute, so that the entire quantity in the

sewer might be changed once every 56 minutes. Certainly nothing like this would be required.

Practical difficulties would, no doubt, be encountered, especially in regulating the admission of air into the sewers, the position of the fan, the construction of side entrances, gullies, etc. ; but these difficulties must be encountered and overcome if we wish to apply mechanical ventilation to the sewers of London. I am aware that there is nothing new in this proposed method of ventilating sewers, as it was suggested by Mr. Gibbs, I believe in 1871, when it was met by objections that unless every gully or air hole were stopped up when the fan was at work, the air would enter the sewers at the nearest inlet with great velocity, which would diminish the further the distance was from the fan.

I think, however, it is not beyond the engineering talent of the country to devise inlets which shall admit the air to sewers in such proportions and at such places as may be required.

The difficulty would be much diminished

if a system of sewers had but one or two outlets; and if the sewers were used simply for sewage and not for surface and subsoil. water, the difficulty would be very small indeed.

The same objections will, moreover, apply to any system of artificial ventilation for sewers, whether it be by the furnace or by simple ventilating shafts.

The following table, compiled by Mr. Haywood, C. E., showing the temperature of the City sewers, will not, I think, be out of place :

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The table shows us we cannot at all seasons rely on the superior temperature of the sewers for ventilation.

The last part of my subject presents fewer difficulties, I think, than the sewers.

No description of the way in which underground railways are now ventilated need be. entered into.

We are all acquainted with the stifling atmosphere underground, and the choking, burning-matches odor which pervades the stations and streets near them. The insertion of iron grids in the roofs of the tunnels and the discharge of the noxious vapor and steam into the streets can hardly be considered a satisfactory solution of the question.

I believe we must here call mechanical ventilation to our aid. But first, let us consider the condition of the structure we are going to treat.

Taking one of the metropolitan railways as a type, we see it consists of a series of tunnels, and open or semi-open stations. The foul air is generated chiefly during the passage of trains through the tunnels, and

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