220 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



CHAPTER IT. 



LIGHTHOUSES. 



I MARINE SIGNALS THE FIRST CATOPTRIC OR REFLECTING 



LIGHTHOUSES. 



T IGHTHOUSES were not unknown to the ancients ; for example, 

 J-J the beacon lighted on a high tower in front of the port of 

 Alexandria, and which it. appears still existed in the twelfth century. 

 The island on which this tower was built gave its name to the 

 building, which has passed on to all the lights on the coasts for the 

 protection of shipping. Lighthouses were still very few in the 

 middle ages, but increased in numbers in proportion as navigation 

 extended itself, and in the present day, they light up with their 

 various fires, all the coasts frequented by shipping of all nations. 



It is only since the last century that people have endeavoured to 

 profit by the laws of the reflection and refraction of light, to increase 

 the power of the lights in lighthouses, and therefore the distance 

 at which they can be seen. In former times they were but simple 

 fires lighted on the top of a tower, and exposed to every change of 

 weather. Gradually, lamps protected by panes of glass were sub- 

 stituted, then came the idea to send the light to a distance, by using 

 reflectors of polished metal. In this way, reflecting or catoptric light- 

 houses were established. At first they were not a great success ; the 

 lamps were defective, and the reflectors being of a spherical form, 

 only received a small fraction of the rays of light, or did not project 

 them in the required direction. " In 1782, this kind of light was 

 established at Cordouan ; but, although no fewer than twenty-four 

 lamps, each possessing a reflector, were used, it shed such a feeble 



