where the water is shallow, and abounds with rocks,—then is his time of danger, when he most wants this light. I am going to shew you how, by means of a little steam, I can completely obscure this glorious sun, this electric light which you see. The cloud now obscuring the light on the screen is only such a cloud as you see when sitting in a train on a fine summer's day. You may observe that the vapour passing out of the funnel casts as deep a shadow on the ground as the black funnel; the very sun itself is extinguished by the steam from the funnel, so that it cannot give any light; and the sun itself, if set in the light-house, would not be able to penetrate such a vapour.
Now, the haze of this cloud of steam is just what we have to overcome, and the electric light is as soon, proportionally, extinguished by an obstruction of this kind as any other light. If we take two lights, one four times the intensity of the other, and we extinguish half of one by a vapour, we extinguish half of the other—and that is a fact which cannot be set aside by any arrangement. But, then, we fall back upon the amount of light which the
