In general, there is a large increase in the various items during the decade represented.  The output of the central stations doubled in the five years from 1907 to 1912, and doubled again in the next five years from 1912 to 1917.  Street lamps were not reported in 1907, but in 1912 there were 348,643 arc-lamps served by the public companies.  The number of arc-lamps decreased to 256,838 in 1917.  On the other hand, there were 681,957 electric filament street lamps served in 1912, which doubled in number to 1,389,382 in 1917.  The cost of construction and equipment of these central stations totaled more than $3,000,000,000 in 1917.

Although there is no immediate prospect of the failure of the coal and oil supplies, exhaustion is surely approaching.  And as the supplies of fuel for the production of gas and electricity diminish, the cost of lighting may advance.  The total amount of oil available in the known oil-fields of this country at the present time has been estimated by various experts between 5,000,000,000 and 20,000,000,000 barrels, the best estimate being about 7,000,000,000.  The annual consumption is now about 400,000,000 barrels.  These figures do not take into account the oil which may be distilled from the rich shale deposits.  Apparently this source will yield a hundred billion barrels of oil.  In a similar manner the coal-supply is diminishing and the consumption is increasing.  In 1918 more than a half-billion tons of coal were shipped from the mines.  The production of natural gas perhaps has reached its peak, and, owing to its relation to the coal and oil deposits, its supply is limited.

Although only a fraction of the total production of gas, oil, and coal is used in lighting, the limited supply of these products emphasizes the desirability of developing the enormous water-power resources of this country.  The present generation will not be hard pressed by the diminution of the supply of gas, oil, and coal, but it can profit by encouraging and even demanding the development of water-power.  Furthermore, it is an obligation to succeeding generations to harness the rivers and even the tides and waves in order that the other resources will be conserved as long as possible.  Science will continue to produce more efficient light-sources, but the cost of light finally is dependent upon the cost of the energy supplied to these lamps.  At the present time water-power is the anchor to the windward.

XVII

LIGHT AND SAFETY

It is established that outdoors life and property are at night safer under adequate lighting than they are under inadequate lighting.  Police departments in the large cities will testify that street-lighting is a powerful ally and that crime is fostered by darkness.  But in reckoning the cost of street-lighting to-day how many take into account the value of safety to life and property and the saving occasioned by the reduction in the police-force necessary to patrol the cities and towns?  Owing to the necessity of darkening the streets in order to reduce the hazards of air-raids, London experienced a great increase in accidents on the streets, which demonstrated the practical value of street-lighting from the standpoint of accident prevention.