Page:On the various forces of nature and their relations to each other.djvu/187

Rh distance of half-a-mile or more; the other wedge has an angle of 6°, which is the beautiful angle of Fresnel. When the angle is less than 6°, the mariner is not quite sure that he will see the light—he may be beneath or above it; and, in practice, it is found that we cannot have a larger angle than 15°, or a less one than 6°. In order, therefore, to get more light, we must have more combustion, more cotton, more oil; but already there are in that lamp four wicks, put in concentric rings, one within the other; and we cannot increase them much more, owing to the divergence which would be caused by an increase in the size of the light—the more the divergence, the more the light is diffused and lost. We are therefore restrained, by the condition of the light and the apparatus, to a certain sized lamp. At Teignmouth, some of the revolving lights have ten lamps and reflectors, all throwing their light forward at once. But even with ten lamps and reflectors, we do not get sufficient light; and we want, therefore, a means of getting a light more intense than a candle in the space of a candle—not merely