1851.] OF THE ROYAL INSTITUTION. 25 
68 feet high, and the base 26 feet in diameter, (being barely less 
than the surface of the rock on which it stands). It is built of 
stone; the stones are dovetailed together, and “ joggled” as it is 
termed, so as to prevent the courses of stones from sliding on each 
other. It is situated in the midst of the sea, nine or ten miles distant 
from Plymouth. 
The Bell-rock Lighthouse stands on a rock of the same name on 
the east coast of Scotland. It is surrounded by the sea, and is 100 
feet high, and 42 feet in diameter at the base. It was built by 
Robert Stevenson, and finished in 1810. Its construction is similar 
to that of the Eddystone. 
The Skerrevore Lighthouse was built by Alan Stevenson, son of 
the architect of the Bell Rock Lighthouse. The mass of stone in this 
structure is more than double that used in the Bell Rock, and 
five times that contained in the Eddystone. The tower is 138 feet 
high, and the diameter at the base is 42 feet. It stands on a gneiss 
rock, the area of which is just large enough for the foundation. In 
constructing this lighthouse, the architect appears to have chiefly 
relied on the weight, rather than on the extension of the materials, 
for efficient resistance to the impact of the waves. The stones were 
not dovetailed or joggled, but tree-nails were used merely to keep 
the work together during its erection. 
Several lighthouses have of late years been constructed of cast iron. 
One designed by Mr. Alexander Gordon, and made by Messrs. 
Cottam and Hallen, has been erected at Bermuda; it is 130 feet 
high. 
Messrs Walker and Burgess have recently constructed efficient 
lighthouses on iron piles, which are fixed in the sand by means of a 
screw, invented by Mitchel. The Maplin and Chapman lights, at the 
mouth of the Thames, and those at Fleetwood and Belfast, are on 
this principle. Professor Cowper invited attention to the mode in 
which these structures are rendered compact by means of cast-iron 
braces. 
The Sources of Light and mode of diffusing it were next adverted 
to. 
Common fires, first of wood, and then of coals, were originally 
used to furnish light. A coal fire was employed for this purpose in 
the Isle of May for 180 years (as late as the year 1816). Tallow candles 
succeeded ;— candles fastened on wooden rods (as they are sometimes 
seen arranged before booths in fairs,) were burnt in the Eddystone 
Lighthouse for 40 years after it was completed by Smeaton. Then 
came lamps with twisted-cotton wicks, and then common Argand 
lamps : all these however are now superseded by—(A.) Argand lamps 
and reflectors ; (B) one Argand lamp, with lenses and reflecting mir- 
rors ; and (C) one Argand lamp with lenses and reflecting prisms. 
Mr. Cowper here illustrated the laws of reflection by several 
models, diagrams, and familiar examples. As instances of refraction, 
he alluded to the line of light produced on rippling water by the 
