ore rapidly in the slates than in the hard limestone, which,
therefore, overhangs like the projecting leaf of a table, and the
collected volumes of water hurl themselves over it. But when the
limestone is so far undermined that it is no longer able to bear the
weight of the water, fragments break off from time to time from its edge
and fall into the abyss with a deafening noise. Thus in time the fall
wears away the barrier and Niagara is moving back in the direction of
Lake Erie."

"Moving, do you say? The movement can surely not be rapid."

"Oh no; Niagara needs about seventeen thousand years to move half a mile
nearer to Lake Erie."

"That's all right, for now I can be sure it will be there when I visit
it at some future opportunity."

"Yes, and you would find it even if a crowd of railway lines did not run
to it. You hear the roar of the 'thunder water' forty miles away, and
when you come closer you see dense clouds of foam and spray rising from
the ravine 150 feet below the threshold of the Fall. Yes, Niagara is the
most wonderful thing I have seen. In all the world it is surpassed only
by the Victoria Falls of the Zambesi, discovered by Livingstone. One
feels small and overawed when one ventures on the bridges above and
below the Fall, and sees its 280,000 cubic feet of water gliding one
moment smooth as oil over the barrier, and the next dashing into foam
and spray below with a thundering noise."

"It would not be pleasant to be sucked over the edge."

"And yet a reckless fellow once made the journey. For safety he crept
into a large, stout barrel, well padded inside with cushions. Packed in
this way, he let the barrel drift with the stream, tip over the edge of
the barrier, and fall perpendicularly into the pool below. As long as he
floated in the quiet drift, and even when he fell with the column of
water, he ran no danger. It was when he plumped down on to the water
below and span round in the whirlpools, bumped against rocks rising up
from the bottom, and was carried at a furious pace down under the watery
vault. But the traveller got through and was picked up in quiet water."

"I suppose that there are bridges over the Niagara River as over all the
others in the country?"

"Certainly. Among them is an arched bridge of steel below the Falls
which has a single span of 270 yards, and is the most rigid bridge in
the world."

"Tell me, where does all this water go to below Niagara?"

"Well, it flows ou