Light (Volume 2) (Paperback)

Book may have numerous typos, missing text, images, or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1871. Excerpt: ... LIGHTS FOR A LIGHT-HOUSE. 175 CHAPTER XX. THE CORRELATION OF FORCE. The work of establishing a light-bouse upon the seacoast for the guidance of mariners naturally divides itself into two portions, or, rather, there are two distinct ends to be secured, each of which is essential to success. The first is to devise some method of making a very bright light, and the second the means of gathering the beams that would naturally radiate backward over the land, or upward into the sky, and throwing them all forward over the sea, so that they may be brought to combine their luminous effect in the direction where the light is required. In respect to the former point--that is, the source of light itself--there are many things to be considered besides the actual brightness of it. The concentration of the radiant point is very important, inasmuch as light issuing from a point is much more manageable by lenses and reflectors than that which comes from a large surface, which is, in effect, the same thing as coming from a great many different points at a greater or less distance from each other. In former times a compound Argand burner was generally employed, and is still in very extensive use. This kind of burner consists of several concentric wicks-- that is, circular wicks one within another--the outer one being three or four inches in diameter. Such a light, of course, consists of quite a large flame, and is not so easy to be controlled by reflectors or by lenses as the same amount of light from a single point would be; so that when, at length, the means of producing very bright lights from a single radiant point--or at least from a surface of very limited extent, such as the oxyhydrogen and the electric light--were discovered, it was at once seen that some verygreat advantages would result fro...