above the screw-blade was always ruffled. Liquid lenses were thus
formed, by which the coloured light was withdrawn from some places and
concentrated upon others, the water flashing with metallic lustre. The
screw-blades in this case played the part of the dinner-plate in the
former case, and there were other instances of a similar kind. The
white bellies of porpoises showed the green hue, varying in intensity
as the creatures swung to and fro between the surface and the deeper
water. Foam, at a certain depth below the surface, was also green.
In a rough sea the light which penetrated the summit of a wave
sometimes reached the eye, a beautiful green cap being thus placed
upon the wave, even in indigo water.

But how is this colour to be connected with the suspended particles?
Thus. Take the dinner-plate which showed so brilliant a green when
thrown into indigo water. Suppose it to diminish in size, until it
reaches an almost microscopic magnitude. It would still behave
substantially as the larger plate, sending to the eye its modicum of
green light. If the plate, instead of being a large coherent mass,
were ground to a powder sufficiently fine, and in this condition
diffused through the clear sea-water, it would also send green light
to the eye. In fact, the suspended particles which the home
examination reveals, act in all essential particulars like the plate,
or like the screw-blades, or like the foam, or like the bellies of the
porpoises. Thus I think the greenness of the sea is physically
connected with the matter which it holds in suspension.

We reached Portsmouth on January 5, 1871. Then ended a voyage which,
though its main object was not realised, has left behind it pleasant
memories, both of the aspects of nature and the kindliness of men.

********************

VII. NIAGARA.

[Footnote: A Discourse delivered at the Royal Institution of Great
Britain, April 4, 1873.]

It is one of the disadvantages of reading books about natural scenery
that they fill the mind with pictures, often exaggerated, often
distorted, often blurred, and, even when well drawn, injurious to the
freshness of first impressions. Such has been the fate of most of us
with regard to the Falls of Niagara. There was little accuracy in the
estimates of the first observers of the cataract. Startled by an
exhibition of power so novel and so grand, emotion leaped beyond the
control of the judgment, and gave currency to notions