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A special shaft would preserve the action constant all the year round, though most active in winter.
The fan is prohably the leading instrument in mechanical ventilation.
Box states that a fan 5 ft. in diameter running at a speed of 50 revolutions per minute will discharge 2,250 cubic ft. of air in that time, while a fan of 10 ft. diameter at 25 revolutions per minute will discharge 9,000 cubic ft.
The third case I mentioned above was that of sewers. It may be considered established that the exhalations from fresh sewage are not injurious, but it is equally sure that those from putrid sewage are highly poisonous. Of all of them sulphuretted hydrogen may be fairly considered the most deadly. This gas is slightly heavier than air, hence it is more difficult to remove than the lighter gases.
According to Thénard, a proportion of Toto of sulphuretted hydrogen in the air will kill a bird, Todo a dog, and to a horse.
Some of the gases generated in sewers
are explosive when mixed with the atmosphere, hence it would be dangerous to introduce them into factory chimneys. The experiment was tried in Southwark with the blowing up of the furnace as the result. With sewers, as in other cases, ventilation must be subject to modifications.
In villages and small towns where pipe sewers are alone employed, small ventilating shafts carried up the nearest building, tree, or other suitable object, will answer the purpose, viz., that of relieving the sewers of any pressure of gas.
Their useful effect will be increased by the Archimedean screw cap as used in Liverpool, and a wire basket containing charcoal may be beneficially placed when the gas must pass through it. · Large brick sewers must not only be relieved of pressure, but must be clear enough for men to work in them.
In London we all know how they are supposed to be kept clear of foul gas, that is by the numerous open gratings which are seen in every street.
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 :