162 MR. ALFRED FRYER : SUGOESTIONS 



by vessels is less to this construction than to pile light- 

 houses or ordinary lightships, as less surface is presented 

 for collision; but if actually struck, the convex surface 

 and extra-strong plates would most likely cause the tube 

 to glide aside uninjured. 



Objection will not be taken to this plan on the ground 

 of expense. Two thousand five hundred pounds would 

 probably cover the cost, including ballast, but excluding 

 lanterns and illuminating apparatus. 



As the oscillations of this vessel will be slow and small, 

 it is possible that the dioptric method of illumination could 

 be used, especially if the apparatus were judiciously sus- 

 pended, and furnished with a contrivance for retarding the 

 oscillations. Experiment only, however, can determine this. 



Accidents not unfrequently occur from the mariner 

 mistaking his distance from a light. Estimating it to be 

 five miles distant he may find, when upon the rock or 

 shoal he wished to avoid, that a slight haze had partially 

 obscured the light and that he was within a mile of it. 

 Double lights, or lights exposed frotn dificrent elevations in 

 the same tower, are now used in some cases, but solely I 

 believe with a view to prevent confusion, by giving the 

 light a distinctive character. I would propose, in all 

 cases where it is important that the distance from a light 

 should be ascertained, that double lights should be em- 

 ployed. Sailors would learn to estimate their position 

 approximately by the apparent separation of the lights, 

 and the distance could be determined with accuracy by 

 the use of the sextant. The lights here represented are 

 fifty feet apart, and as the height of the lower is thirty feet 

 above the surface of the water, they would be visible at 

 eight miles distance, and the combination would then be 

 clearly discerned as a double light. 



