It is the usual practice to adjust the intensity of street lighting to the real estate and traffic conditions of streets, lighting the important streets more brilliantly than the secondary streets. Appropriations for street lighting purposes are often somewhat inadequate with the result that all classes of streets are not as well lighted as the engineers in charge would like to have them. In recent years the great growth of automobile traffic has complicated the street lighting situation in a number of ways-for one thing the use on the automobile of too brilliant lamps at night has introduced a serious problem. Where street lighting is so inadequate as to make their use important, automobile lamps for lighting the roadway safeguard the automobilist but introduce a large measure of annoyance and some danger for other travelers. Where street lighting is reasonably adequate within municipal limits, it is entirely feasible to abolish the use of head-lamps, as is demonstrated by the experience of some years in New York City. If instead of seeking to eliminate the difficulty solely by limiting the light from head-lamps municipal authorities would see to it that the street lighting is adequate, the whole problem could be dismissed by prohibiting the use of head-lamps within the city limits. The advances of the last ten years in efficiency of light production. have led to considerable improvement in street lighting. Not all of these advances have been put into increased lighting; part have been realized by municipalities in reduced lighting costs. the great opportunities offered by the new illuminants for better street lighting have not been grasped in their entirety, though very large improvement has resulted. In the last two or three years marked impetus has been given to the lighting of village roads and of rural highways.2 Counties, towns and villages are awaking to the value of such highway lighting. Street lighting growth is reactive. If a street is lighted because there is traffic requirement for lighting, the result is increased traffic, which in turn brings the demand for better street lighting. The tendency of municipalities to appropriate inadequate sums for street lighting has offered an opportunity which the American business man has not been slow to grasp, with the result that there has entered into American practice the merchants' display lighting system, the so-called "white way" lighting, whereby a given street through the activity of a merchants' association is much more. brilliantly lighted than adjacent streets. This is considered good advertising in that it attracts people to the locality. Installations. of this sort are generally of distinctive design; many of the earlier ones were of the incandescent lamp cluster type;3 some consisted of arches across the street; others of festoons of lamps mounted over the curbs. The more recent installations have consisted of a post and a single ornate lighting unit enclosing either an arc4 or an incandescent lamp. When well organized and operated under the strict supervision of a strong merchants' association or of the municipality, this sporadic street lighting has been successful in setting a new and higher intensity for street lighting, tending to advance the standard everywhere. When not controlled, it has sometimes led to isolated and ill-considered instances of merchants' lighting which has resulted in a hodge-podge, often three or four different lighting units being installed on a single block, some lighted at night and others unlighted. In a few instances municipalities have installed special high intensity street lighting systems at a greater expense than would ordinarily have been incurred for the street selected, the city having in mind something of the same considerations as have actuated merchants' associations, namely the advertising of the locality in order to bring traffic to it. Prevailing intensities in street lighting practice in a measure are dependent upon the extent and intensity of private lighting with which they are brought into frequent comparison. In a city where the private lighting is highly developed, the general level of street lighting intensities is likely to be higher than in other cities. In passing it may be noted that the converse is also true, and that the introduction of higher intensities in street lighting tends to increase the intensities of private lighting with which it is contrasted. The general practice is to light street lamps shortly before dark and to allow them to continue in service until after daybreak. The 4000-hour year in this latitude is standard. No self-respecting city continues the archaic moonlight schedule whereby street lamps are not lighted when the almanac states that the moon should be shining. A more reasonable modification of lighting practice, applicable to merchants' lighting rather than to civic lighting, is the reduction of the amount of street lighting after the midnight hour, as for example, the extinguishing of a certain percentage of the lamps. Like many other municipal enterprises, street lighting serves the entire populace and is a matter for community interest. More than most such municipal activities, its status is apparent to the citizen and to the visitor. A city is likely to be judged by its municipal undertakings, and the most casual observer takes cognizance of the condition of the street lighting. "A city is judged by impressions. It may have the finest climate in the world; it may be fortunately situated near rivers and railways; it may have every natural advantage that a business man may desire. Yet if it be unattractive, dirty and gloomy, its development will be slow. When it does develop, the first impetus will be given by changing its appearance for the better; and in that change street lighting will play an important part.' 995 Something of this view is manifesting itself in many cities of the country, and the newer installations are reflecting in their enhanced attractiveness and effectiveness the municipal pride which underlies design. The cost of civic street lighting per capita generally ranges from 60 cents to $1.20. It amounts to perhaps, from 2 to 3 per cent. of the total municipal expenditure. Its value must be taken to include not alone the safety features which are its primary purpose, but as well a part of the city growth and the promotion of the industry and the welfare of its citizens. It may be claimed also that the large expenditures for highway construction are rendered of greater utility by street lighting, and that to a degree the street lighting must be credited with the promotion of the enormous traffic which these highways bear. To quote a recent expression: "It is earnestly believed that when the economic value of a system of street lighting is compared with other public works, such as schools, bridges, police force, fire department, etc., the price required to be paid for the protection of the life and limb of the entire citizenry of the inhabitants; the protection of our wives and daughters from crime and annoyance; the protection of our property from burglary by supplementing the police force by adequately lighted streets; the various conveniences to the public, secured by sufficient illumination on the streets, and the advertising and æsthetic values of adequate street lighting to the city at large, and to the individuals-the cost of an adequate street-lighting system will be found to be insignificant compared to the value received, and the expenditure well worth while." ILLUMINANTS Recent History.-The earliest development of importance to this discussion was that of the open carbon arc lamp and series direct current lighting systems. The early records of street lighting in this country show a number of competing companies manufacturing equipment for such systems. They became recognized standard street lighting systems and continued as such until the development of the enclosed carbon arc lamp in 1894.7 The open carbon arc lamp in this country, generally speaking, did not attain to the development which it received abroad. It is understood that the carbon electrodes produced in America were inferior to those later employed in Europe. The superiority of the electrodes available and the lower prevailing cost of labor in Europe led to the continuation of the open carbon arc lighting system for years after its decline began in this country. The enclosed carbon arc lamp by reason of its relatively long electrode life and low maintenance cost received in this country a measure of development which it could not experience abroad. It possessed other advantages over the open carbon arc lamp which aided its success, including a better light distribution characteristic and a greater steadiness of light. In the course of the decade succeeding its invention, the enclosed carbon arc lamp became the standard street lighting lamp in this country. As subsidiary to the arc lamp, small illuminants were used in the lighting of streets of minor importance. These were the gas mantle lamp, the gasoline lamp and the series carbon incandescent lamp. The period of modern street lighting illuminants was ushered in by the development of the metallic electrode lamp in 1904.8 This lamp, known as the magnetite lamp, and the metallic flame arc lamp and also as the luminous arc lamp, attained great eminence in street lighting practice in this country during the decade following its invention. Within this same decade the flaming arc lamp was the subject of much experimental and development work on the part both of arc lamp manufacturers and of electrode makers. The same difficulty of high maintenance costs in this country placed it beyond practicability to utilize the short-life flaming arc lamps of European development. As in the case of the carbon arc lamp the difficulty was reduced by enclosing the arc and securing a much longer electrode life. For a time the enclosed flame arc lamp with long life electrodes bade fair to become a real factor in the street lighting situation in this country. Several important extensive installations were made with more or less satisfactory results. The development of the gas-filled tungsten lamp known as the "Mazda C" lamp, in 9 1914, however, introduced competition which the flame arc lamps of present types have not been able to meet except in special cases. The Mazda C lamp superseded a number of illuminants in the street lighting field. It hastened the displacement of open and enclosed carbon arc lamps which were still in the field. Its availability in small sizes resulted in its substitution very generally for the gas and gasoline mantle lamps which had very largely claimed the secondary streets for their own, and of course it displaced the inefficient carbon series lamps wherever they were in service. The development of street lighting practice is so largely involved with the development of street illuminants that this brief account of the development of the latter serves to recall the history of street lighting as a whole. The development of other phases of street lighting practice has perhaps lacked the definite steps of advance which are apparent in the record of the illuminants employed, but the progress has been none the less real on that account. Modern Lamps.—As electric and gas illuminants are the subjects of other lectures, their qualities need not be discussed in detail in this connection. Street lighting lamps in this country are as follows: Gas filled tungsten lamps (Mazda C). The incandescent lamps are usually of the series type, though in some cities, notably New York City, multiple lamps are employed. American selection of illuminants differs somewhat from that in European countries due to the higher labor costs and greater distances prevailing here. Thus it may prove economical for us to sacrifice something of efficiency in order to secure lower maintenance cost, while in Europe the maintenance cost is not so large a factor in the total. This in part accounts for the fact that flaming arc lamps and pressed gas lamps have not come into use largely in this country as abroad. On the contrary, the magnetite lamp has been used largely here and hardly at all abroad. Through the courtesy of the Lamp Committee of the Association of Edison Illuminating Companies, the accompanying table of data on electric street lamps is available. In this connection Fig. I will be of interest. This shows the lumens produced, the watts, and by reference to the diagonal lines, the efficiency of the principal electric street lamps. The lumens per watt output of street lamps is by no means a final measure of |