WicHam—Llluminated Buoys and Beacons for Harbours, &c. 521 
The occultations are occasioned by causing the lamp itself to be 
‘the motive power; the heat of the lamp acting upon the mica 
blades of a small fan which is caused continuously to revolve by 
the upward current of air resulting from the combustion of the 
lamp. Slight arms of steel which support the two screens of 
opaque, or coloured, or translucent material, such as talc, are so 
arranged that they balance each other and keep up, as they pass in 
front of the flame, a continual occultation and re-exhibition of the 
light. If it be desired to have a quicker repetition of the occul- 
tation than can be produced by two shades, three or four can be 
substituted for two, and thus the flashes will be rendered more 
frequent. an 
I exhibited on a former occasion, for the illustration of one of 
my Papers, a pulsating light, consisting of eight first order 
annular lenses, by which it was demonstrated that by the plan of 
rotation which I had devised the powerful light transmitted by 
these lenses was caused to remain in the eye as a continuous light, 
even when the lenses were continually revolving, and would other- 
wise cause intervals of darkness. I refer to this here merely 
because the same device is employed in this case. The effect of 
the continuous light which I have described was produced by the 
method by which the lenses were caused to revolve. They were 
mounted on a steel pivot, working on a piece of agate, causing so 
little friction that the great group of lenses (weighing about one 
ton) could be made to revolve by an exceedingly slight pressure, 
and hence great rapidity of revolution was practicable. 
The same contrivance is adopted in the case of the little lamp 
here described. ‘The pivot system is adopted also in this case, and 
so little friction is created that an exceedingly feeble upward current 
of air is sufficient to put the apparatus into motion. The sim- 
plicity of this device is its chief recommendation, for clockwork 
complications or other mechanical appliances would be quite 
unsuitable for the exposed positions in which such lights are 
placed. The balancing of the fan is so exact that, although the 
light may be fixed to a buoy which is subjected to the motion of 
the sea, the rotation is maintained with practical uniformity. Of 
course, if the lamp be used in a beacon which is perfectly steady, 
the rotation of the flashes is absolutely periodic; but even in the 
