132 



tiplied, as the interval is decreased. Intervals of half a lunar hour 

 usually are sufficiently small to give acceptable determinations, 



265. The corrected velocities must be such as to satisfy equation 

 (131): 



Let V be the velocity of the primary current, corrected by i, from 

 table X, on a given lunar half hour, 



6, the further correction to v, 

 5o, the correction at the preceding half hour, 

 I, the length of the section. 



266. For purposes of the computations, the surface head, hs, should 

 have a fluctuation which, although not necessarily a simple harmonic, 

 identically repeats itself every 12 lunar hours if the tides and the 

 surface head are wholly semidiurnal, or every 24 lunar hours if the 

 diurnal components of the head are so large as to require consideration. 

 This result may be accomplished by selecting tidal fluctuations at the 

 ends of the section which identically repeat themselves every 12 or 24 

 lunar hours. Under ordinary circumstances it is indeed apparent 

 that the tides on one day have but little effect upon the currents of the 

 next. 



267. The expression for the acceleration head is, from equation (129) 



ha={llg)b{v+b)lbt 



= {l/g){dv/dt+b8/dt) 



Since this relation remains approximately true when s.mall finite incre- 

 ments are substituted for the differentials, it is permissible to place: 



ha={l/g){^v/At+Ad/At) 



= ll/gAt)iAv+A8) (165) 



In which A^ is the selected time interval, in mean solar seconds, and 

 Av and A5 are the increases in v and 8 corresponding thereto at the 

 given half hour. 



268. It will be convenient to place: 



l/gAt=b (166) 



When the time interval is a half lunar hour: 



A#=)^X 1.035X3,600 seconds 

 and: 



6=0.0000167? (167) 



