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FIG. 6 PLAN AT LEVEL OF THE OPENINGS.

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Before quitting the subject of house ventilation, I will notice one fruitful source of annoyance, which is often closely connected with defective ventilation, I mean smoky chimneys. Want of air causes perhaps more chimneys 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 oftener 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 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 Too of sulphuretted hydrogen in the air will kill a bird, To a dog, and horse.

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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.

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