256 



W, H, Finlay^ 31. A, — Approximate Tide- [Aug. 31, 



A 'mean' tide-table, sufficient for purposes of approximately 

 predicting the time of high water, can easily be formed by a graphical 

 process from the above results for the M. and S. tides. 



The local mean time of the Moon's transit is to be converted into 

 angle at the rate of 30^ to the hour and from it the angle (Kg — ff 

 Km) subtracted : call the result x. Then on any convenient scale 

 take a straight line SO equal to Hg and produce it to any point A. 

 With as centre and H^ as radius describe a circle : and from the 



point O draw a straight line 

 OM making with AO an angle 

 equal to x and cutting the circle 

 in M. 



Then SM is the height of the 

 tide, and the angle OMS con- 

 verted to time at the rate of 29^ 

 to the hour is the correction to 

 be applied to the mean establish- 

 ment to find the interval by 

 which the time of high water follows the moon's transit over the 

 meridian. 



The correction is subtractive if x be less than 180^, additive if 

 greater. 



Table II is such a table ; it gives for Table Bay and Algoa Bay 

 the time of high water aad the height of the tide above the datum- 

 line corresponding to each half hour of the Moon's time of passing 

 the meridian. The hours of the Moon's transit are reckoned from^- 

 0^ to 24^, commencing at noon : and it is to be understood that wbeib. 

 the time of transit is greater than 12^ the correspoading time of high^ 

 water is to be|increased by 12^. 



