216 



ON TIDES AND TIDAL ACTION IN HAEBORS. 



PREDICTION OF TIDES. 



Self-registering tide-gauges have been kept in operation for a number 

 of years at different points on both coasts of the United States, in order 

 to obtain from them the data for i^redicting the tides ; and as a result, 

 tide-tables liave been published by the Coast Survey for some years 

 past, giving in advance the times and heights of high and low water for 

 all the principal ports in the United States for every day in the year. 

 In addition to this, the differences are given by which to find the same 

 for intermediate ports. 



A very elaborate discussion of the tides observed at Boston during 

 nineteen years, a full lunar cycle, has been made by Mr. William Ferrel, 

 of the Coast Survey, and has resulted in representing the actual tides 

 with unlooked-for precision. By the Introduction of the consideration of 

 friction Mr. Ferrel has also succeeded in deriving a value for the mass 

 of the moon, which appears to compete in exactness with the values 

 obtained by astronomical methods. It is one seventy-seventh i^art of 

 that of the earth. 



EARTHQUAKE-WAVES. 



The tide-gauges being in continuous operation, all other fluctuations 

 of the ocean-level besides those produced by the tides are likewise reg- 

 istered. The tide-curves of the western coast are frequently found in- 

 dented by fluctuations arising from earthquakes. A remarkable instance 

 of this kind is given in the annexed diagram of earthquake-waves, which 



EARTHQUAKE WAVES AT FORT PT, AS RECORDED ON THE SELF-REGISTERING TIDE GAUGE. 



0„ 

 -I .. 



---^-A^^^f\f\nw^^^ 



recorded the earthquake that destroyed the city of Simoda, in Japan, 

 in December, 1854. The upper curve is a reduction from the tide-gauge 

 register, while the lower shows the earthquake-waves separated from 



