for the purposes of Illumination in Lighthouses. 45 



I. On the Imperfection of the present system of Illumination by 



Hammered Reflectors. 



The best constructed lighthouses in Great Britain are fitted 

 up with parabolic reflectors, like that represented in Plate III. 

 Fig. 6. The dimensions of these reflectors are 



Diameter AB, 24 inches. 



Depth CD, 10i 



Centre of wick from apex, or LC, 4 

 Circumference of wick from apex C, Si- 



Circumference of glass-chimney from 

 apex C, 



The reflecting material, before it is hammered, is a flat disc 

 of copper plated with silver, which, by repeated hammering up- 

 on a polished steel anvil, is beaten into the form of a paraboloid, 

 by the assistance of a gauge, which the workman constantly ap- 

 plies to the hammered surface. When the reflector is brought 

 as nearly to the concavity required as the gauge and the eye of 

 the workman can determine, it is then polished with the hand, 

 by rubbing it with a piece of leather and the usual polishing 

 material *. It is then fitted up, as shewn in the figure, with 

 an argand-burner placed in the focus of the paraboloidal sur- 

 face, and supplied with oil by the lamp behind. 



* " The reflectors," says Mr STEVENSON, " consist of a circular sheet of copper, 

 measuring, when, flat 26| inches in diameter ; weighing 11 ft. on an average, and 

 plated with silver in the proportion of 6 oz. to each pound avoirdupois of copper. 

 These plates are formed into the parabolic curve by a very nice process of hammer- 

 ing, and afterwards set into a bezil or ring of brass." Account of the Bell Rock 

 Lighthouse, p. 527. 



