And it is found that by employing either “Borradaile’s,” “Sugg’s,” or other regulators the consumption of the gas can be readily adjusted to consume from 3 to 6 cubic feet per hour, according to the requirements of the situation of the lamp.[113]
Lamp posts and lanterns are of innumerable sizes, shapes, and patterns, but the following hints in connection with them may be of some service.
The lamp must not only be ornamental by day, but useful by night.
The light must not be placed either too high or too low.
The post must not be too clumsy so as to interfere with the pedestrian traffic, nor too fragile so as to be easily broken if driven against. Bracket lamps have advantages in these respects, and also in the very important one of throwing no downward shadow,[114] as well as being cheaper.
The lantern should be made with the lightest possible amount of metal frame compatible with sufficient strength, the angle bars should be very narrow to avoid shadow, trap doors of perforated zinc or glass should be provided at the bottom for the admission of the torch, and a good outlet at the top is essential for the escape of the heated air. Flat glass is much cheaper and easier of repair than curved. The top of the lantern should be furnished with a reflector cover, otherwise a large percentage of the light is lost: this is very observable on approaching a large city, by the glare which is thrown upwards. Some hundreds of different patterns of lanterns for street lamps have been designed from time to time, and it is not necessary, nor have I space, to describe them.
The burners should have steatite tips and be of varying size to suit the requirements of the locality, the regulators which I have previously mentioned must be kept in good repair. A lever tap is indispensable with the torch for lighting, as well as the trap door or opening in the bottom of the lantern through which the torch is inserted.
Each public lamp post should be legibly numbered, and the surveyor should keep a register in his office of all the public lamps in his town.
In order to determine the distance apart of the public lamps in a street, it must be remembered that the intensity of light is directly proportional to the illuminating power of the light, and inversely proportional to the square of the distance of the light, if unreflected. For instance, the illumination of any point between lamps may be arrived at by adding all the quotients obtained by dividing the illuminating power in standard sperm candles of each lamp, by the square of its distance in yards from the point.
Thus a point midway between two lamps of 15 candles each, 20 yards apart, would be reckoned thus: