Barrett — On the Double Quadriform Lighthouse Lamp. 77 



only by the aid of an opera glass, the Bailey light was occasionally 

 found. 



After another interval the Double Quadriform was again 

 lighted, and this time made to revolve. As before, it was a striking 

 object to the naked eye, arresting the attention at once, and the 

 period of its revolution was easily noted by the unaided eye. 



Evening of November 23. — This night the experiments were 

 repeated under still more crucial conditions. A muoh thicker fog, 

 with drizzling rain, hid all lights from view except those near at 

 hand. Even the two fine lights at the " Poolbeg," one of them a 

 first- order revolving oil light, were completely obscured, though 

 these lights were less than half the distance of the experimental 

 light. The Bailey light itself was entirely cut off, and could not be 

 picked up even with a powerful glass. It was then burning, as I 

 learnt next day, its maximum light of 108 jets. Not the faintest 

 trace of the fog siren at the Bailey could be heard, though, as after- 

 wards ascertained, it was sounding with its full power during these 

 experiments. Precisely at the time appointed for the lighting up 

 of the Double Quadriform a sudden glare was seen on the horizon. 

 With the opera glass the shape of the light was easily defined, but 

 no trace of the adjacent Bailey light could be found, even after the 

 most careful search through the glass. There were in all 632 gas-jets 

 burning, and as these, by prearrangement, were raised and lowered, 

 a flashing light was produced readily seen by the naked eye. The 

 double triform arrangement was next tried : this could be seen 

 faintly with the naked eye. The biform was now tried, but this 

 was invisible even with the glass. This is important as showing 

 the advantage of the multiple lights ; for, in this experiment, the 

 character of the lights and lenses were the same as the double 

 quadriform, only of one-fourth the total power. The double 

 quadriform was again put on, and, as before, its glare was at once 

 seen and its position determined with the naked eye, the exact 

 quadrangular shape of the light being easily made out with the 

 glass. All the other lights remained utterly invisible, even with 

 the aid of a good glass and a knowledge of their exact position. 



I cannot but think that the facts here recorded are worthy of 

 attention. They demonstrate that the double quadriform arrests 

 the attention, as a conspicuous glare to the naked eye, and as 

 a clearly-defined object in an opera glass, through a fog of suf- 



