light at a distance.  A satisfactory street-lighting unit must emit its light chiefly just below the horizontal in those cases where the lamps must be spaced far apart for economical reasons.  On referring to the chapter on the electric arc it will be seen that the upper (positive) carbon of the open-arc emits most of the light.  Thus most of the light tends to be sent downward, but the lower carbon obstructs some of this with a resulting dark spot beneath the lamp.

The gas-mantle followed closely after the arrival of the carbon arc and is responsible for the existence of gas-lighting on the streets at the present time.  It is a large source of light and therefore its light cannot be controlled by modern accessories as well as the light from smaller sources, such as the arc or concentrated-filament lamp.  As a consequence, there is marked unevenness of illumination along the streets unless the gas-mantle units are spaced rather closely.  Even with the open-arc, without special light-controlling equipment there is about a thousand times the intensity near the lamps when placed on the corners of the block as there is midway between them.

In 1879 the incandescent filament lamp was introduced and it began to appear on the streets in a short time.  It was a feeble, inefficient light-source, compared with the arc-lamp, but it had the advantage of being installed on a small bracket.  As a consequence of simplicity of operation, the incandescent lamp was installed to a considerable extent, especially in the suburban districts.

[Illustration:  THE MOORE NITROGEN TUBE

In lobby of Madison Square Garden]

[Illustration:  CARBON-DIOXIDE TUBE FOR ACCURATE COLOR-MATCHING]

[Illustration:  MODERN STREET LIGHTING

Tunnels of light boring through the darkness provide safe channels for modern traffic]

The open-arc lamp possessed the disadvantage of emitting a very unsteady light and of consuming the carbons so rapidly that daily trimming was often necessary.  In 1893 the enclosed arc appeared and although it consumed as much electrical energy as the open-arc and emitted considerably less light, it possessed the great advantage of operating a week without requiring a renewal of carbons.  By surrounding the arc by means of a glass globe, little oxygen could come in contact with the carbons and they were not consumed very rapidly.  The light was fairly steady and these arcs operated satisfactorily on alternating current.  The latter feature simplified the generating and distributing equipment of the central station.

The magnetite or luminous arc-lamp next appeared and met with considerable success.  It was more efficient than the preceding lamps but was handicapped by being solely a direct-current device.  Those familiar with the generation and distribution of electricity will realize this disadvantage.  However, its luminous intensity just below the horizontal was about 700 candles and its general distribution of light was fairly satisfactory.  Later the flame-arcs began to appear and they were installed to some extent.  The arc-lamp has served well in street-lighting from the year 1877, when the open-arc was introduced, until the present time, when the luminous-arc is the chief survivor of all the arc-lamps.