The bee-hive ovens are usually located near the mines where the cost of coal is low, with small expense for transporting it. On the other hand, the by-product ovens are established near the larger cities in order to dispose of their gas and other by-products. Here the cost of transportation must be added to that of the coal, but the products are marketed near by instead of at a distance, as in the case of the bee-hive ovens. The most improved by-product ovens produce not only coke and gas, but coal-tar, pitch, ammonia, and creosoting oils, all extremely valuable and adding greatly to the value of the output of the ovens. Electricity is another form of light and power which involves a large waste of the energy of coal; only one-fifth of one per cent., that is, one-five hundredth of the value of the coal is used in electricity, and there is at present no known remedy for this. There are methods, however, of lessening even this waste, and these are constantly receiving more attention. One is for the electric plants located in cities to sell their exhaust steam or water heated by the coal as it is converted into electric power, as a by-product. The electric power-house thus becomes a central heating plant to supply stores, offices, and residences. Another system being tried abroad, though scarcely past the experimental stage in this country, establishes great electric power-houses at the coal mines to use the culm, low-grade slack, and lignites, the lowest form of coal, in short, all the waste of the mines. Still another plan is the manufacturing of electricity by water-power, as we have seen in a previous chapter. The manufacturing industries of the country waste a large amount of fuel annually, but here the waste is mostly due to expensive methods of producing power, and to careless stoking, and is largely preventable. As we have shown, gas-engines are a far more economical form of producing power than are steam-engines. Steam uses from five to ten per cent. of the heat-units of coal, gas-producer engines use fifty per cent. and burn a lower grade of coal. One of the great problems of cities is the heavy volume of bituminous or soft coal smoke that hangs over the entire surrounding region, levying a heavy tax in cleaning and laundry work, making the air difficult to breathe, and shutting out the daylight itself. Every residence adds its mite, but the factories and public buildings are the worst offenders. There are several good smoke-consuming devices on the market that have been thoroughly tested by the government, which will furnish their names on application.