50 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
Reference to Plate I. 
A. The steel axis carried round by the clockwork in twenty-four hours. 
B. The hollow brass cylinder open at its lower end to admit the upper portion of 
the wooden cylinder I. 
C. The slit to admit a sunbeam, covered with clear plate-glass. 
D. The mandril revolving on the hinge E, and having, when in its working position, 
its axis coincident with that of the steel rod A and cylinder B. 
E. The hinge in east and west azimuth. 
F. The obstacle forming part of the bed G H of the apparatus destined to sustain 
the mandril D at its correct elevation. 
G. A similar obstacle to keep it from tilting inconveniently far, when thrown back 
to take off the wooden cylinder. 
H. The bed of the apparatus. 
I. The wooden cylinder sliding on the mandril D, and shaped off for lightness at 
its lower part. 
K. The band of sensitive paper rolled round it. 
L. An exterior cylindrical guard to cut off light when the cylinder is slided up to 
its place on the mandril and clamped there, and to prevent the entry of rain 
below. 
M. The weather-tight case, containing a spring clock to drive the axis A. 
N. The disc covered with sensitive paper. 
O. A close-fitting cover to defend the clockwork from rain, glazed with clear plate- 
glass at its upper surface P, well cemented. The cylindrical rim acts as a 
guard to keep the sun’s direct rays from striking on the sensitive paper. 
Q. A brass diaphragm in the cover O, in which is cut the sector-shaped slit for ad- 
mission of light, having its direction from the centre of the dial-plate to the 
hour-line of noon. 
R. The pillar supporting the upper works. 
