There are other ways of utilising the burning-glass to give a signal
than to adjust it so that the rays, at noon, shall fire off a cannon, as
at the Palais Royal, Paris. For instance, let the action of the focussed
light and heat operate on a delicate spring retaining the hammer of a
bell, the valve of a chamber of compressed air, with its outlet forming
a whistle, &c., &c.