es, and
crowned with crenallated battlements, and towers 30 to 40 feet high.
In numerous places the wall climbs such steep declivities that its
top ascends from height to height in flights of granite steps. An army
could march on the top of the wall for weeks and even months, moving
in some places ten men abreast.

Limits of Natural Vision.--This question is too indefinite for a
specific answer. The limits of vision vary with elevation, conditions
of the atmosphere, intensity of illumination, and other modifying
elements in different cases. In a clear day an object one foot above
a level plain may be seen at the distance of 1.31 miles; one ten feet
high, 4.15 miles; one twenty feet high, 5.86 miles; one 100 feet high,
13.1 miles; one a mile high, as the top of a mountain, 95.23 miles.
This allows seven inches (or, to be exact, 6.99 inches) for the
curvature of the earth, and assumes that the size and illumination of
the object are sufficient to produce an image. Five miles may be taken
as the extreme limit at which a man is visible on a flat plain to an
observer on the same level.

THE NIAGARA SUSPENSION BRIDGE.--For seven miles below the falls,
Niagara river flows through a gorge varying in width from 200 to 400
yards. Two miles below the falls the river is but 350 feet wide, and
it is here that the great suspension bridge, constructed in 1855 by
Mr. Roebling, crosses the gorge, 245 feet above the water. The length
of the span, from tower to tower, is 821 feet, and the total length of
the bridge is 2,220 feet. The length of the span, which is capable of
sustaining a strain of 10,000 tons, is 821 feet from tower to tower,
and the total length of the bridge is 2,220 feet. It is used both for
railway and wagon traffic, the wagon-road and foot-way being directly
under the railway bed. There is another suspension bridge across the
Niagara river at a distance of only about fifty rods from the falls,
on the American side. This is only for carriages and foot travel. It
was finished in 1869. It is 1,190 feet long from cliff to cliff, 1,268
feet from tower to tower, and 190 feet above the river, which at this
point is a little over 900 feet in width.

THE SPEED OF SOUND.--It has been ascertained that a full human voice,
speaking in the open air, calm, can be heard at a distance of 400 feet;
in an observable breeze a powerful human voice with the wind is audible
at a distance of 15,840 feet; the report of a musket, 16,00