PREFACE 



The treatment of the tides, and of tidal datum planes, contained 

 in the first four chapters of this text, and of the reduction of measured 

 tidal currents, in chapter X, is drawn from the manuals issued by the 

 United States Coast and Geodetic Survey, and from Harris's Manual 

 of Tides, published in past reports of that Survey, but now out of 

 print. As no engineer outside that Survey may expect the occasion 

 to undertake the laborious harmonic analysis of the tides at a station, 

 the voluminous tables required for the purpose are not included. 



The cubature of a channel, described in chapter VI, is set forth in 

 a number of French texts. The detailed procedure explained is that 

 developed in the United States Engineer office at Philadelphia. 



A method is developed in chapters V and VIII for computing tidal 

 currents from the constants commonly used for steady flow, by a 

 procedure somewhat analogous to that used in ordinary hydraulic 

 computations. Quite obviously, the varying and periodically revers- 

 ing flow in a tidal channel has somewhat the same relation to steady 

 flow that an alternating electric current has to a direct current. As 

 alternating currents depend upon the reactance and capacity of the 

 circuit as well as upon its resistance, so tidal currents depend upon 

 the acceleration head and the storage and release of w^ater in the 

 channel as well as upon frictional resistance. Wlien these factors 

 are included, computations of tidal flow should be as reliable as are 

 those for steady flow. The application of these principles to natural 

 tidal channels is taken up in chapter IX. 



(HI) 



