X = 15100 + 15100 = ·30
In this country, the rule has generally been adopted that public street lamps burning 5 cube feet per hour of 15 candle gas should not be placed at a greater distance than 60 yards apart, the average distance in most English towns being about 40 yards.
On this question, the following interesting particulars by Monsieur Servier will be of special interest.[115]
It appears to M. Servier that up to the present there has been too much straining after intensity, with insufficient care for the object of obtaining a proper quantity of light uniformly spread over the surface of the ground. The paper in question is therefore intended in the first place to elucidate this latter subject, so as to determine beforehand the necessary intensity for luminous centres, gas or electric, and also their height from the ground and distance from each other required to produce a certain effect. With this purpose M. Servier proposes to determine for any point of the road-surface, by the law of the squares of the distances, the intensity of light, in terms of the Carcel standard, which is spread at that point by one or more lights of given power. Representing these intensities by proportional ordinates, the extremities of these ordinates form an irregular surface, and the volume contained between this surface and that of the roadway represents a specific value equivalent to the total luminous intensity distributed over the soil. In default of a better term, M. Servier calls this a volume of cubic Carcels, a cubic Carcel being the intensity of a Carcel (9·5 standard candles) multiplied by a square mètre of surface. The different cases capable of being valued in this manner are as follows:
1. A burner consuming 140 litres (5 cubic feet nearly), and of 1·1 Carcels (10·45 candles) illuminating power, placed at the height of 3 mètres (9 feet 6 inches). This burner gives at the foot of the lamp-pillar a maximum intensity of 0·122 Carcel (1·159 candles), and at 10 mètres (32·8 feet) away the illuminating power is reduced to 0·01 Carcel (0·095 candle). The distance of 20 to 30 mètres kept between the street lamps, even in the best-lighted towns, is therefore excessive, for these should not be more than 13 mètres (14 yards) apart in order to obtain between them the minimum illuminating power of 0·05 Carcel (0·475 candle), sufficient for enabling passengers to read.
2. The second case is that of a burner consuming 1400 litres (50 cubic feet nearly) of gas, with an illuminating intensity of 14 Carcels (133 candles), placed at the height of 3·20 mètres; this being the class of burner fixed in the Rue du Quatre Septembre. The intensity of light at the foot of the lamp-pillar is 1·367 Carcels (13 candles nearly), and to obtain the light of 0·05 Carcel (0·475 candle) already mentioned as the least intensity enabling one to read, a point must be fixed in a circle of 16 mètres radius from the lamp as a centre. Taking now a group of six lamp-columns, three on each side of the street, and overlapping, as in the Rue du Quatre Septembre, it will be found that the distribution of light is defective. The most brilliantly lighted point at the foot of the column has an intensity of 1·367 Carcels (13 candles), or more than triple that of the darkest point, which has an intensity of 0·5 Carcel (4·75 candles) at 4·58 mètres distance.
3. A lamp of 50-Carcel (475-candle) power, gas or electric, fixed at the height of 8 mètres (26·24 feet). The illuminating intensity at the point vertically under the light is reduced to 0·7 Carcel (6·65 candles); but the light of 0·5 Carcel (4·75 candles) is to be found in a circle of 6 mètres radius from this point. It will therefore be observed that the distribution of light over the ground is better in proportion as the luminous centre is higher; but conversely also, the amount of light thrown on the ground is greater as the luminous centre is lower. It consequently results that the power of the light and its height should be determined in every case with reference to the effect desired. The method shortly described shows that, in the case of the lighting of the Rue du Quatre Septembre, the mean amount of light per square mètre of the roadway is 855 décicarcel-cubes, the best lighted parts having an intensity of 1·62 cubic Carcels, and the darkest portions an intensity of 0·50 cubic Carcel.
M. Servier has examined the question of lighting a street 20 mètres wide and one or more kilomètres long, with the condition that the illumination of the ground shall present a mean determinate quantity of light per square mètre, or a given intensity at the darkest points. Some interesting results are thus obtained. Thus, by substituting for the 14-Carcel (133-candle) lamps in the Rue du Quatre Septembre, burners of 50-Carcel (475-candle) power, with the condition of giving the same intensity of 0·5 Carcel (4·75 candles) to the darkest points, a quantity of light more considerable than before will be required. That is, a greater number of Carcels (3000 as against 1848 per kilomètre in length) will be necessary in the larger burners than were required in the original smaller lamps. It is therefore imperative, in order that the lighting shall be equally economical, that the unit of intensity—the Carcel or candle power—shall be less costly in a lamp of 50 Carcel (or 475-candle power) than in the smaller lamps. By fixing lamps of 50-Carcel (475-candle) power in the centre of the street, instead of along the side walks, maintaining the condition of giving the light of 0·5 Carcel (4·75 candles) in the darkest parts of the thoroughfares, it is found that the pillars must be 8 mètres high and 20 mètres apart. The best-lighted part of the road would then have the intensity of 1 Carcel (9·5 candles), and would therefore be only twice as brilliantly lighted as the darkest corner; the mean quantity light per square mètre would be 755 décicarcel-cubes.
Lastly, the same method of lighting has been applied to the “ordinary,” as distinguished from the “luxurious” lighting of the public thoroughfares, assumed to be 20 mètres wide, giving a light of 0·05 Carcel (0·475 candle) at the darkest points. With ordinary street burners consuming 200 litres (7 cubic feet) of gas per hour, and giving 1·72-Carcel (16·34-candle) power, it is found that the lamps should be 18 mètres (20 yards nearly) apart, the burners being 3 mètres (9 feet 10 inches) high. With burners of 14-Carcel (133-candle) power placed at the height of 3·20 mètres (10 feet 6 inches), the lamp-pillars would be 106 mètres (115 yards) apart. Or with lamps of 50-Carcel (475-candle) power placed at a height of 8 mètres (26·24 feet), the distance between the pillars may be increased to 270 mètres (494 yards).
In the case of electric lighting M. Servier has studied two examples—the Jablochkoff candle, and an arc light (system not stated). The former is credited with the illuminating power of 16 Carcels (152 candles), and is fixed at the height of 5 mètres (16 feet 3 inches), on pillars 110 mètres (120 yards) apart. This would give a light of 0·65 Carcel (6·27 candles) at the foot of the pillar, and a minimum intensity of 0·05 Carcel (0·475 candle) midway between the lights. The arc light is purposely made exactly equal in computed efficiency to the larger Siemens burner of 50 Carcels (475 candles). In the matter of expense, however, using the data applicable to Paris, with 12-candle gas at 6s. 6d. per 1000 cubic feet, M. Servier makes a striking comparison. The cost of lighting a kilomètre of road in the “ordinary” manner last described varies very little for the three classes of gas lamps—small, large, and very powerful—included in the calculation, and ranges from 3·33 frs. to 3·96 frs. per hour. The cost of the same work done by the Jablochkoff candle is estimated at about double, or 6·91 frs. per hour; and with the arc light the cost would be 4 frs., or still higher than with the most costly system of gas lighting, although less than the expense of the Jablochkoff electric light.