184 Things not generally Known. 



a foot in height, and lasting for about half an hour, followed by 

 seven lesser waves, at intervals of half an hour each. 



The velocity with which a wave travels depends on the depth 

 of the ocean. The latest calculations for the Pacific Ocean give 

 a depth of from 14,000 to 18,000 fathoms. It is remarkable 

 how the estimates of the ocean's depth have grown less. La- 

 place assumed it at ten miles, Whewell at 3*5, while the above 

 estimate brings it down to two miles. 



Mr. Findlay states, that the dynamic force exerted by Sea- 

 Waves is greatest at the crest of the wave before it breaks ; and 

 its power in raising itself is measured by various facts. At 

 Wasburg, in Norway, in 1820, it rose 400 feet; and on the 

 coast of Cornwall, in 1843, 300 feet. The author shows that 

 waves have sometimes raised a column of water equivalent to 

 a pressure of from three to five tons the square foot. He also 

 proves that the velocity of the waves depends on their length, 

 and that waves of from 300 to 400 feet in length from crest to 

 crest travel from twenty to twenty-seven and a half miles an 

 hour. Waves travel great distances, and are often raised by 

 distant hurricanes, having been felt simultaneously at St. He- 

 lena and Ascension, though 600 miles apart ; and it is probable 

 that ground-swells often originate at the Cape of Good Hope, 

 3000 miles distant. Dr. Scoresby found the travelling rate of 

 the Atlantic waves to be 32 67 English statute miles per hour. 



In the winter of 1856, a heavy ground-swell, brought on by 

 five hours' gale, scoured away in fourteen hours 3,900,000 tons 

 of pebbles from the coast near Dover ; but in three days, with- 

 out any shift of wind, upwards of 3,000,000 tons were thrown 

 back again. These figures are to a certain extent conjectural; 

 but the quantities have been derived from careful measurement 

 of the profile of the beach. 



OCEAN-HIGHWAYS I HOW SEA-EOUTES HAVE BEEN 

 SHORTENED. 



When one looks seaward from the shore, and sees a ship 

 disappear in the horizon as she gains an offing on a voyage to 

 India, or the Antipodes perhaps, the common idea is that she 

 is bound over a trackless waste ; and the chances of another 

 ship sailing with the same destination the next day, or the 

 next week, coming up and speaking with her on the "path- 

 less ocean," would to most minds seem slender indeed. Yet 

 the truth is, the winds and the currents are now becoming so 

 well understood, that the navigator, like the backwoodsman 

 in the wilderness, is enabled literally to " blaze his way" across 

 the ocean ; not, indeed, upon trees, as in the wilderness, but 

 upon the wings of the wind. The results of scientific inquiry 



