398 POPULAR LECTURES AND ADDRESSES. 



period was the sole distinction between one flashing 

 light and another. Thus, in the " Admiralty List 

 of Lights for the British Islands " for 1875, we find 

 about 100 flashing lights of different periods, from 

 the four-seconds' period of Ardrossan Breakwater 

 Light to the two-minutes' period of the upper light 

 of Lundy Island, of the South Stack, Holyhead, 

 and of one of the lights on Slyne Head, off the 

 west coast of Ireland ; and the distinction of each 

 one of these 100 lights was solely its period as a 

 simple flashing light, except in cases in which the 

 objectionable distinction by colour was put in 

 requisition. When it was determined to choose 

 periods the same, or nearly the same, for neigh- 

 bouring lights, it was found necessary to add 

 distinction by colour, objectionable as this is if not 

 enforced by necessity. Thus, for example, the 

 Gull Stream lightship, in the fairway between the 

 Goodwin Sands and the Kentish Coast, is a 

 revolving light of twenty seconds' period, while the 

 East Goodwin lightship, about six miles from it, is 

 a revolving light of fifteen seconds' period. With- 

 out greater accuracy than is generally to be found 



