The lamps may take either the “arc” form, or the “incandescent.” The former is produced by the electric current passing between carbon points, and requires considerable electrical pressure; they give a light of from 1500 to 4000 candle power; the mechanism of arc lamps has to be of the most delicate kind to ensure the proper distance of the carbon points being maintained. The lamps should be guarded by globes of frosted glass, not only to prevent incandescent pieces of carbon from falling, but to lessen the glare of the light. “Incandescent” lamps are of small size, giving a light of from 8 to 50-candle power, which is produced by the heating of a filament of carbon in a vacuum owing to the resistance caused to the electric current by this contraction of the conductor. (3.) With regard to the value of the light produced, and its adaptability to the requirements of any town, it will be seen on reference to the opening of this chapter that at present considerable doubt exists as to its adaptability for general public lighting, and as each town varies in the length, straightness, and width of its streets, the number of its large squares or confined courts and alleys, the surveyor must use his own judgment as to the suitability of the light before recommending his corporation to adopt it. As to the value of the electric light, there can be no doubt that a most brilliant and powerful light is produced by the voltaic arc: so brilliant indeed, as to render it necessary to screen it nearly always behind frosted or opalescent glass globes, the former being found to be much the best for many reasons. As to the photometrical value of the light, some considerable difficulty has hitherto been experienced in obtaining accurate observations, principally owing to the peculiar colour of the electric light, and also from its fluctuating character; but these difficulties are being steadily overcome, and with a photometer mounted on a light frame with wheels, some excellent experiments have been made in the public streets upon the comparative values of different lights. (4.) The last and really one of the most important questions remaining to be discussed is that of the cost of the electric light as compared with gas. With reference to the cost of the electric light, the following table may be of use; it is compiled from an excellent paper on electric lighting, by Mr. James N. Shoolbred:[118] TABLE OF COMPARATIVE ESTIMATES OF FIRST OUTLAY AND OF WORKING EXPENSES OF SOME SYSTEMS OF ELECTRIC LIGHTING.