274 
POPULAK SCIENCE EEVIEW. 
as much room, produces but half the light for the same motive 
power, and for the same brilliancy of light is twice as expen- 
sive.* During the last two months the electric light has been 
exhibited from the Clock Tower, Westminster, in competition 
with Mr. Wigham’s gaslight, the design of the Board of Works 
being to adopt that which will answer better as a signal light 
for the House of Commons. The machine is fitted up in the 
engine-room under the Peers’ lobby, whence the conducting 
wires — *425 in. in diameter — leading from the terminals, are 
carried up to the lantern, a distance of 900 feet. Here the 
current is admitted into a Serrin’s regulator, and the light is 
projected by one of Chance’s holophotes in a beam of parallel 
rays. By a very ingenious contrivance, devised by Mr. 
Conrad W. Cooke — to whom the electrical arrangements on 
the Clock Tower have been entrusted — this pencil may be 
directed into any azimuth or angle of depression lying between 
certain fixed limits. This is effected by a horizontally-placed 
rotating-table, movable on a central pivot by means of a worm 
and worm-wheel. A second table is joined with the outward 
edge of the first, while the other may be raised or lowered by 
a vertical screw of considerable play. On this is adjusted a 
little trolley, on which two regulators are fixed, the whole being 
susceptible of a reciprocating movement, which is, moreover, 
so arranged that when one regulator is out of use, the other is 
brought into the focus of the holophote, and at the same time 
into connection with the machine. This latter is effected by 
providing each regulator with a pair of underlying metallic 
strips, which press upon a pair of copper studs in electrical 
communication with the polar terminals. The object of this 
arrangement is to prevent any sensible interruption in the 
light when, by their consumption, it becomes necessary to 
change the carbons. These are 8 in. long and in. square ; 
they last about four or five hours. At the end of that time, 
a slight rectilinear movement is imparted to the small rolling- 
table, and the second lamp comes into position. As this may be 
done very rapidly, the discontinuance in the light is scarcely 
perceptible. 
On Monday night. May 26, we had, in company with several 
other gentlemen, an opportunity of judging of the intensity of 
this light. The coils of the machine were driven at a speed of 
300 revolutions per minute, and the light emitted was estimated 
equal to that of 8,000 candles. The emergent rays formed a 
bright silvery beam, which reminded one, says a contem- 
porary, of the lustrous appearance of Donati’s comet when in 
perihelion. At Trafalgar Square, a black shadow of the pillar 
* Engineering.” April 2o, 1873. 
