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, 16,000 feet; a drum, 10,560 feet; music, a strong brass band, 15,840 feet; very heavy cannonading, 575,000 feet, or 90 miles.  In the Arctic regions conversation has been maintained over water a distance of 6,766 feet.  In gases the velocity of sound increases with the temperature; in air this increase is about two feet per second for each degree centigrade.  The velocity of sound in oxygen gas at zero C. is 1,040 feet; in carbonic acid, 858 feet; in hydrogen, 4,164 feet.  In 1827 Colladon and Sturm determined experimentally the velocity of sound in fresh water; the experiment was made in the Lake of Geneva, and it was found to be 4,174 feet per second at a temperature of 15 degrees C. The velocity of sound in alcohol at 20 degrees C. is 4,218 feet; in ether at zero, 3,801; in sea water at 20 degrees C., 4,768.  By direct measurements, carefully made, by observing at night the interval which elapses between the flash and report of a cannon at a known distance, the velocity of sound has been about 1,090 per second at the temperature of freezing water.