ON TIDES AND TIDAL ACTION IN HARBORS. 217 



the tidal wave. The time required for the transmission of the sea-waves 

 from Simoda to San Francisco was twelve hours aud thirty-sis minutes. 

 The distance being 4,500 miles, the transmission of the wave was at an aver- 

 age rate of 3G0 miles per hour. The theory of wave-motion teaches us that 

 this velocity will be attained by a free-moving wave in a depth of 1,440 

 fathoms, which may be taken as the average depth of the Pacific Ocean 

 between Japan and California. It will be observed that the crests of 

 the waves occur at intervals of about twenty-three minutes, correspond- 

 ing to a length, from crest to crest, of 150 miles. The height when 

 the waves arrived at San Francisco was about eighteen inches from 

 hollow to crest, the high waves caused by the original impulse having 

 gradually flattened out to that form in their transmission across the 

 ocean. 



The great earthquake which occurred in Peru, in August, 18SS, was 

 likewise recorded on the tide-gauges at San Diego, San Francisco, and 

 Astoria. The fluctuation of the ocean was so great in this instance as 

 to be very sensible to casual observation, and was noted in Australia, 

 at the Sandwich Islands, and at Kodiak, in Alaska. The data obtained 

 from these observations, combined with the result before mentioned, 

 indicate that the average depth of the Pacific Ocean is about 1,800 

 fathoms. 



MOVEMENT OF TIDAL WAVES. 



The waves above described, originating with an impulse at one definite 

 point, and propagated freely through the ocean in every direction, with 

 a velocity depending upon the square root of the depth of the sea, may 

 serve as good illustrations of the manner in which tides are propagated 

 through sounds, bays, and rivers. The following table gives the rate of 

 motion for difl'erent depths : 



Depth in feet 10 Miles per hour 12. 2 



" GO " " .'^0. 



" " 100 " " 38.7 



" " 1, 000 " " 122. 3 



" " G,000 " " 299.5 



That movement of the ocean, however, which we have designated by 

 the name of tide- wave, does not partake of the nature of a wave in the 

 common acceptation of the term, but it is rather to be conceived as a 

 general movement of the water toward a point under the attracting body, 

 and again away from it. Its periodicity is strictly dependent upon that 

 of the attracting body. The velocity of the movement is about 1,000 

 miles per hour on the equator; it extends to the bottom of the ocean, 

 the depth of which is inconsiderable compared with the radius of the 

 earth. It is not attended by a sensible elevation of the water in mid- 

 ocean ; and in this respect the characteristic of what we call a wave is 

 absent. The movement may be likened to that of an impulse given to 

 a very long rigid bar, as of iron. In this case, a sensible time will be 



