ON TIDES AND TIDAL ACTION IN HARBOKS. 



213 



of their orbits. The efficacy of a heaveuly body in raising tides is shown 

 by theory to be inversely proportional to the cube of the distance. 

 Hence the efficacy of the sun will fluctuate between the extremes nine- 

 teen and twenty-one, taking twenty for its mean value, and that of the 

 moon between forty-three and fifty-nine. Taking into account this 

 cause of difference, the highest spring-tide will be to the lowest neap as 

 59+21 to 43 — 19, or as eighty to twenty-four, or ten to three ; leaving 

 out of consideration the local circumstances of access and depth, which, 

 as we have stated, modify those proportions in a marked degree. 



TYPE CURVES. 



The three principal forms of tides are illustrated in the annexed dia- 

 gram, which exhibits the tides at New York, San Francisco, and Galves- 

 ton for two days from actual observation. Of these, that for San Fran- 

 cisco may be taken as the normal type, showing the diurnal inequality, 

 while that at New York, as at other ports on the Atlantic coast, is not 

 sensibly affected by it. The explanation of this feature is iirobably to 

 be found in the supposition that the tide-wave which advances up into 

 the Atlantic Ocean from the continuous tide in the Southern Ocean ar- 

 rives on our shores twelve hours later than the direct tide-wave which 

 crosses the Atlantic from east to west. In this way the diurnal inequal- 

 ity will be eliminated by the superposition of the two tides, the greater 

 high water of the former coinciding with the lesser of the latter, and 

 vice versa, leaving the semi-diurnal tides of equal height. 



TIDE REGISTERS 



50VERN0RSI. 

 N.Y. A 



The tide at Galveston, on the other hand, furnishes a case of the elim- 

 ination of the semi-diurnal tide, leaving as a residual only the diurnal 



