526 Scientific Proceedings, Royal Dublin Society. 



these extra lights ; for during the greater portion of the year the 

 weather is clear, and one burner is then sufficient ; showing to the 

 horizon a good well-individualized light. In such weather the 

 sailor needs nothing more ; it is only when the weather thickens 

 that the superposed lights are put into operation, being added one 

 by one or all together according to the density of the mist or fog 

 which may require to be illuminated. While the fog lasts, the 

 extra lights are kept burning; but the moment the fog passes 

 away they are extinguished, and then of course the cost of main- 

 taining them ceases. This system I named biform, triform, and 

 quadriform, according to the number of lights superposed. It has 

 hitherto been employed solely for revolving lights with annular 

 lenses, and of course without any top or bottom prisms. These 

 lenses, because they parallelize the light of the burner vertically as 

 well as horizontally, must of necessity continually revolve, in order 

 that all portions of the sea may receive the powerful beams which 

 they transmit. The complete apparatus at which we were first 

 looking is a fixed light apparatus, the illumination from which is 

 constantly visible from all parts of the horizon, and is therefore 

 much less powerful than the concentrated beam of the revolving 

 light. 



It is necessary here to interpose a few words respecting group 

 flashing lights. Groups of flashes are caused by breaking up the 

 beam of the annular lenses by continually extinguishing and re- 

 igniting the gas lights which are in their foci. This is done by 

 the same clock-work which causes the lenses to revolve, and the 

 result is that the sailor sees, instead of a long flash of light recur- 

 ring at certain intervals, a group of shorter flashes, say about 

 three, four, or five in number, as might be determined upon. It 

 is found that these groups of flashes are better calculated to arrest 

 the attention of the sailor than an unbroken beam, and that their 

 characteristic appearance is also useful in enabling him to identify 

 the lighthouse. Hence it is that this system of group flashing is 

 now employed in the greatest lighthouses of Ireland. 



The difficulty of increasing the height of lighthouse lanterns and 

 the tendency of lighthouse engineers to increase the height of 

 lighthouse lenses, seemed to indicate that it was not expedient to 

 go much further in a vertical direction in the multiplication of 

 lights ; therefore it occurred to me that the great advantage which 



