226 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in 



Light-houses are divided into lights of the first, second, and third 

 order, according to the intensity and the range of their light. In 

 li<*ht-houses of the first order the lamps have four concentric wicks, 

 three in those of the second, and two in the third. The brilliancy 

 varies in the ratio of the numbers 4, 2, and 1, and equals twenty, 

 ten, and five Carcel lamps. 



FIG. 150. Lenticular apparatus and lamp of a first-class revolving light. 



So much as to the intensity. Secondly, they vary in their 

 colour. Thirdly, the lights are distinguished by being either fixed or 

 revolving. In the latter a variation is introduced by the length of 

 the intervals which separate the flashes. Thus we have fixed lights 

 produced by a lenticular apparatus of cylindrical form; then 



