370 On the prohahle importance of Tide Observations 



there is a regular difference of morning and evening tide. I 

 have found this to occur all along the south coast of England. 

 It would be very interesting to examine this matter at the Cape 

 in the same way. The neap tides may be examined in the same 

 way as the spring tides, and it may thus be found how long after 

 the' first or third quarter of the moon the lowest tide is ; or 

 with what hour (greater than 6th) of her transit the lowest 

 tide happens. 



9. I noAV proceed to point out another method of determining 

 the age of the tide at the Cape. 



The time of high water must be observed (every tide) care- 

 fully for a long period, and the interval at which it follows the 

 moon's transit must be written down opposite to the time of the 

 moon's transit for each tide. 



A complete number of serailunations being: taken, the mean of 

 all the intervals must be found. This is the mean interval or 

 corrected establishment of the Cape. 



It then requires to be determined how old the moon is, or 

 rather what the time of moon's transit is, when the actual interval 

 of the tide after moon's transit is equal to the corrected 

 establishment. If the tide were produced by the moon and sun 

 instantly, the actual interval at new and full moon would be 

 equal to the mean interval. But as the tide takes time in being 

 produced and propagated, the age of the moon when this 

 equality occurs is the age of the tide. 



When many observations of the time of high water about the 

 springs have been collected, we may find the mean of them as 

 directed for the heights: that is, by laying dowa the time of 

 moon's transit along the edge of the paper, setting off the 

 interval of tide after transit as an ordinate, drawing a free 

 curve through the points, taking the point of the curve when 

 the ordinate is equal to that which represents the connected 

 establishment, and noting the time of moon's transit, which 

 corresponds to this, as given on the edge of the paper. This 

 gives the time of moon's transit for one spring, and taking this 

 time for many springs, and taking the mean, we have the time 

 of moon's transit corresponding to the age of the tide ; which 

 ought to agree with that found before, from the heights of 

 spring tides. 



10. There may be some difficulty in observing the times of 

 high water at the Cape, arising from the smallness of the tide 

 and the Atlantic swell ; but these are difficulties which may be 

 got over ; and without a good series of times, no great pro- 

 gress can be made in general views. 



I will mention one or two suggestions which may perhaps be 

 of use in diminishing these difficulties. 



11. The effect of the swell, except its impulses be at very. 



