494 THE POPULAR SCIENCE MONTHLY. 



than before. This high sea continues for a considerable time, some- 

 times for several hours ; and it has been known to last twenty-four 

 hours. 



The billows, heavy seas, and tidal waves possess a considerable in- 

 ertia, 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 2Gth 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 con- 

 siderable 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 disappoint- 

 ment, for the configuration of the coasts, the eddies, and the currents, 

 modify the phenomena in a thousand ways. They are, however, al- 

 ways 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 every- 

 thing 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 obsta- 

 cle, 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 



