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.