CHAP. IX.] TIDES 245 



High water at a standard port, or on the neighbouring 

 shore, should always be used as a reference for the direction 

 and rate of the tidal streams at the different hours. Low 

 water should not be referred to, for the reason that in many 

 places it does not take place midway between the two high 

 waters, and mistakes may arise. Thus the period of the stream 

 should be referred to as so many hours before or so many hours 

 after high water. The flood-stream is usually that which 

 continues to run after high water, but it should not be spoken 

 of as such, in order to avoid confusion. 



The direction of the tidal stream will frequently change Time of 

 after high or low water, and when this occurs, we must en- Di'r^ctfon^ 

 deavour to find out whether the change of stream occurs at of Tidal 

 a regular time of the tide, as this is an important point in the "^™^ 

 navigation of channels. 



The streams inshore ma^y also turn at a different time to the 

 streams in the offing. 



It is sometimes convenient to plot on squared paper the 

 interval between high water and the turn of the stream as one 

 ordinate, and the time of high water as the other ; joining the 

 points thus found, the irregularities that occur are sho^^■n 

 graphically. 



If the time of high water is not known, the interval between 

 moon's transit and turn of the stream may be substituted for 

 the first ordinate and the time of moon's transit for the second. 



In channels connecting two open areas of sea, the general 

 law is that the stream will run for some hours, often for tlu-ee 

 hours, after the tide, as indicated by the rise or fall on the 

 shore, has turned. This makes it very confusing to speak of 

 a stream as the flood or ebb stream, and the term east-going 

 or south-going, or whatever the main direction of the st^^eam 

 may be, should always be used in preference ; for in such a 

 case the direction of a stream may be the same for the last 

 three hours of the flood, and the first three hours of the ebb. 



The Theory of the Tides is one of the most complicated Theory of 

 subjects that can be considered. the Tides. 



Recent investigations by Sir W. Thomson and Professor 

 G. H. Darwin have shown that the tidal movement may be 

 considered to be the resultant of as many as thirty-three 

 separate tidal waves. Some are dependent upon the moon. 



