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nected with defective ventilation, I mean smokey chimnies. Want of air causes perhaps more chimnies to smoke than any thing else. And before any one sets to work to alter the chimney in any respect, it is better to see whether it is not more air that is wanted. This may be easily done by trying the fire with a door or window open, and with the same shut. If in the latter case the fire smokes, while in the former it does not, then want of air is the cause. Care must however be taken, as to what window is open, as gusts of wind or a cross draught will often cause the fire to smoke.

For churches, schools and factories, thermo-ventilation may be used, but probably the fan, or blowing wheel will, in many cases, be more efficient and economical.

I believe it is in use in some buildings in London with very satisfactory results.

Its useful effects will vary with the size of the wheel and the speed at which it is driven, and in designing a fan it should be borne in mind, that a large fan and low speed is generally better than a small one and high speed, chiefly on account of the vibration caused by gearing running at great velocities. The motive power may be either steam, water, or a weight which can be wound up as occasion may require.

It is impossible here to even refer to many noxious manufactures, chemical and otherwise. But of this we may be

sure, that if they are prevented by law from polluting the atmosphere, the science of chemistry is quite equal to finding a remedy, and even to converting the noxious fumes into useful products.

There is an objection to thermo-ventilation, which does not apply to the fan, viz., that as it usually depends for action upon the difference in temperature between the inner and outer airs, so in summer it may come to a stop, or be even reversed.

This objection is not of much weight as people are far oftner out in the air in summer, and at night when gas or candles are lighted ventilation would commence again.

A special shaft would preserve the action constant all the year round though most active in winter.

The fan is probably the leading instrument in mechanical ventilation.

Box states that a fan 5 feet in diameter running at a speed of 50 revolutions per minute will discharge 2,250 cubic feet of air in that time, while a fan of 10 feet diameter at 25 revolutions per minute will discharge 9000 cubic feet.

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 1500 of sulphuretted hydrogen in the air will kill a bird, joo a dog, and 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 chimnies. 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 favourable 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 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 six feet in diameter and five miles long will contain 746,000 cubic feet of air, but after deducting one-third for space occupied by sewage water, 497,550 cubic feet would be the quantity left to be dealt with. As mentioned above, a fan 10 feet in diameter will remove 9,000 cubic feet 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,

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especially in regulating the admission of air into the sewers, the position of the fan, the construction of side entrances, gullies, &c., 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 shews 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 odour 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 vapour

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 a certain quantity of this foul air is thrust into the station by the train as it enters the further end of the tunnel. This foul air, being heated, slowly ascends, filling the streets and neighbouring houses.

The stations are nothing more than dwarf ventilating shafts in which the passengers are partially stifled previous to the completion of the process in the tunnel.

Having stated the conditions of the structure, let us consider what is required to be done.

First we have a certain quantity of foul air to get rid of without harm or annoyance to the public. Secondly the stations must be kept so clear as to allow of passengers using them with comfort. And thirdly, circulation of air should be maintained in the tunnels, which should be kept as sweet as possible, not only for the sake of the passengers, but for the workmen also, and for the greater security of the public against accidents.

It seems to me that these objects will be best accomplished by reversing the present system, turning the stations into downcast shafts, and drawing the foul air away at the middle of the tunnels.

A little consideration will show how a series of tunnels might be connected together by what might be called air sewers, and the foul gases led to one or more pumping stations, there to be forced up high chimnies.

The pumps might be either air pumps, such as are I believe used in some Belgian collieries, or machinery similar to Nixons' ventilator, or one of the many kinds of fan.

Some of these machines are very powerful. At the navigation pit, near Aberdare, Mr. Smyth gives the theoretical quantity of air expelled per minute, by one of Nixon's machines, at 166,000 cubic feet. In Belgium many fans of enormous power are at work ventilating coal mines.

The chief difficulty is the expense, but I submit this ought not reasonably to stand in the way.

I believe, however, that a plan could be found, which would accomplish the object at no very extravagant outlay.

I have said nothing about ventilating ordinary railway stations, first, because it is difficult, if not impossible, to apply any

system to a building which sometimes consists of but one wall, and apart of a roof, and secondly, when the station is of greater pretention, it is almost always open at both ends and has usually a great deal too much air in it already.

Free escape for steam must be provided, and I think a good example of roof for this purpose was shewn, to those of the members who visited the Great Eastern Railway extension works, on Friday last, at the low level station at Shoreditch.

In conclusion, I would again insist on the deep importance of the subject especially to Engineers and Architects. The limits of my space, not to mention other causes, have, I am well aware, prevented justice being done to it; but the object of a paper like this, is more to put forward sound principles and throw out ideas, which may when opportunity requires be worked out to good purpose, than to lay down schemes to meet every or any particular case.

Since the foregoing paper was read before the Society of Civil and Mechanical Engineers it has been suggested that, with some additions, it would form a pamphlet, useful to more general readers than those for whom it was originally intended. All scientific men acknowledge the necessity of ventilating dwellinghouses and buildings such as are mentioned in the paper, but with the general public it is unfortunately far otherwise, and one of the greatest difficulties to be encountered in the progress towards a complete and perfect sanitary condition is this inertia of those most interested. Difficulties are, however, only made to be overcome, and it is in the hope of doing something, however little, towards overcoming this particular difficulty that I venture to trespass further on the patience of the reader, and I propose to confine these remarks to the sanitary condition of our houses as being the subject of most immediate importance to the general public, for ventilation of such structures as factories, railway tunnels, and sewers, though of great moment, must be left in the hands of the Engineer who designs them, each case being treated upon its own merits and with due regard to its peculiarities. All the public can do in such instances is to insist that there shall be ventilation of some sort.

All persons readily admit, as a truism, that they cannot live without air, but, unfortunately, they seldom get much beyond the bare admission. Air is not treated by them as a substance requiring space as solids or liquids do, and means of passage into, or out of, rooms, churches, etc., just as much as water

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