at the Cape of Good Hope, 371 



long intervals, may be destroyed by setting up a spout with a 

 float in it, the spout communicating with the sea by small 

 holes only. 



12. If it be found easy to observe the greatest height, and 

 difficult to observe the time constantly, try the following 

 method. 



Observe the height carefully at some time within an hour and 

 a half of the high water, (before or after) and compare this 

 height with the greatest height, and also with the rise and 

 fall of the water between high and low water. 



Allow for the interval of time between the observed height 

 and high water. 



This may be done by observing two or three tides every five 

 or ten minutes, (for a few hours near high water,) and thus 

 making a table of the depth of the water from high water, for 

 every five or ten minutes. 



Till such a table is made, the following may be an approxi- 

 mate one: — 



5^ the time to high water is 32 minutes. 



SoS 



46 do. 



73 do. 

 80 do. 

 86 do. 



sfe a - - 56 do. 



,fe - - - 65 do. 



555 • 



sfe - 



If the defect of height be of the whole rise of the water. 



In this method of observing, the times at which the height is 

 taken, ought to be either all before or all after the time of high 

 water. Also the top of the wave ought to be observed, in order 

 to compare with the greatest registered height, (which will, I 

 suppose, be the top of the highest wave.) 



13. By the methods above described, we might make out 

 the age of the tide at the Cape of Good Hope : that is, by 

 finding how old the moon is when the tide is highest ; and also 

 by finding how old the moon is when the interval of the tide and 

 moon's transit is equal to the mean of such intervals. 



But supposing we were to find in this way, that at the Cape 

 the tide is a day, or a day and a half old, the question occurs 

 still— where does it come from ? What is it doing durmg this 

 day and a half? 



To this the answer is, that the tide is not produced at a 

 single point and at a single moment, and then transferred to 

 other points ; but that it is produced by the action of the sun 

 and moon on all parts of the ocean, and that it takes time for 

 this action to perceive its full effect, which makes the tidal 

 state of the ocean correspond, not to the actual position of the 



