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THE POPULAR SCIENCE MONTHLY.

than before. This high sea continues for a considerable time, sometimes for several hours; and it has been known to last twenty-four hours.

The billows, heavy seas, and tidal waves possess a considerable inertia, and keep up the swell after the tempest has subsided. The real waves fall, while the billows still subsist, but flattened. It is then easy to estimate the distance between them. On the 26th of March, 1882, I counted eight in a space of four thousand metres.

The most serious event that can take place at sea is a change of wind, such as nearly always occurs in cyclones. The phenomena we have described being well established and sure to continue for a considerable time by virtue of their inertia, when the wind veers around so as to reproduce them in another direction, the new waves cross the old ones, and a chopped sea, dangerous to navigation, is the result.

If any one interested in scientific matters comes to the shore to study the formation of waves, he will experience some disappointment, for the configuration of the coasts, the eddies, and the currents, modify the phenomena in a thousand ways. They are, however, always apparent, even in the calmest weather, and vary only in their amplitude. To observe them it is enough to take notice of the level of the water against a post, a jetty, or other structure. Changes of level are produced there quite similar to the pulsations of the sea; and the extent of these pulsations gives quite exact data respecting everything that we have mentioned.

Waves may be classified as direct waves and waves of transmission. The former, with which the surface of the sea is frequently agitated, are those which the wind raises directly. Waves of the second class may be produced in the calmest weather; their origin is frequently quite distant from the places where they are observed; and they reach those places by transmission. A well-known physical experiment will suggest an explanation of the phenomenon of an agitated and raging sea when there is no wind. If we have a long line of billiard-balls arranged in contact one with another, and give a quick blow to the first one, the last one will roll away. The shock is transmitted from the first ball to the last one, without the intermediate ones suffering any appreciable motion. Marine disturbances caused by direct waves, tides, earthquakes, etc., may in the same way be transmitted through molecules of still water without agitating them. If the liquid space is free, the vibrations are gradually extinguished; if they meet an obstacle, there is a shock. If the obstacle is a shore, they form a tidal-wave and raise large billows, while a few miles away from the shore the sea is quiet. When the obstacle is a shoal or a contrary vibration, heavy waves are raised on the surface of the sea; they seem to start from the bottom, and put ships in great danger. The waves produced in both cases are waves of transmission, as also are those which beat on reefs in pleasant weather, and those which prolong the swell after