They are now usually of special design, and may be divided into three classes - indirect radiators, direct radiators and direct ventilating radiators.
In devising a system of ventilation it is customary to subdivide the workings so that the resistance to the ventilating current in each split shall be nearly equal, or so that the desired amount of air shall be circulated in each without undue use of regulating appliances which add to the friction and increase the cost of removing the air.
Ventilation may be produced by heating the air of the mine, as for example, by constructing a ventilating furnace at the bottom of an air shaft.
Underground boilers placed near the up-cast pit so that the smoke and gases help the ventilating furnace have been largely used but are now less favourably regarded than formerly.
The best form of open fireplace is the ventilating stove, in which fresh air is passed around the back and sides of the stove before being admitted through convenient openings into the room.
The illustrations show two forms of ventilating fireplace, one (fig.
Ventilate but little, and with care; raise the ventilating sash only high enough to let the heated air from the greenhouse drive back the outer air so as not to chill the plants.
In cases where the direction of the air motion is always the same, as in the ventilating shafts of mines and buildings for instance, these anemometers, known, however, as air meters, are employed, and give most satisfactory results.
The efficiency of such ventilating furnaces is low, and they cannot safely be used in mines producing fire-damp. They are sometimes the cause of underground fires, and they are always a source of danger when by any chance the ventilating current becomes reversed, in which case the products of combustion, containing large quantities of carbon dioxide, will be drawn into the mine to the serious danger of the men.
Its lumber and flour mills are its largest industries, but the following are found: aerated waters and breweries, tent makers, baking-powder manufactories, box manufacturers, brick makers, broom, brushes and carriage makers, cement blocks, manufacturing chemists, chocolate and cigar manufacturers, confectionery, copper plate, cornice makers, engine builders, gas fitters, ink manufacturers, jewelry makers, lime makers, milliners, opticians, paint makers, paper-box makers, photographers, pickle makers, planing mills, pork packers, publishers, pump makers, rubber-stamp makers, sash, door and blind factories, upholsterers, ventilating manufactory, vinegar factories, foundries, wire and fence manufactories.