LIGHTS FOR A LIGHT-HOUSE. 175 



CHAPTER XX. 



THE CORRELATION OF FORCE. 



THE work of establishing a light-house upon the sea- 

 coast 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 be- 

 sides the actual brightness of it. The concentration of 

 the radiant point is very important, inasmuch as light is- 

 suing 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 



