at the Cape of Good Hope. 369 



Let the observed height of the tides about new or full, be 

 arranged in these parcels: 



Those which happen on the half-day of new or full moon. 



Those which happen on the next half-day after new or full. 



Those which happen on the second half-day after new or full. 



Those which h:ippen on the third half-day after new or full. 



Let the height in each parcel be added together, and you 

 will see which sum is greatest. This will show on which half- 

 day the greatest tide happens. 



7. But this only gives the result to half a day. We want it 

 more nearly. 



Secure method of finding how long after new or full moon 

 the greatest tide happens. 



The tides must be referred to the hour and minute of the 

 preceding transit of the moon : and the moon's transit must be 

 obtained both for the east and west sides of the meridian of 

 London, by interpolating between the transits in the Almanack. 



For each spring tide, the time of the moon's preceding transit, 

 and the heights at each tide, must be laid down by lines pro- 

 portional to them, the times being measured along one edge of 

 the paper, and the heights perpendicular to these, from the 

 point at the edge. This may be done easily and rapidly by 

 means of paper ruled into small squares. 



The points at the ends of the perpendiculars being marked 

 with dots, it will generally be found that the line joining these 

 dots will be an irregular or broken line. But we must "draw, 

 by the mere judgment of the eye, and with a free but careful 

 hand, not through but among them, a curve presenting as few 

 and as slight departures from them as possible,"* consistently 

 with the character of large shallow convexity from the edge of 

 the paper. 



In this curve the greatest perpendicular is to be drawn : and 

 the point of the edge of the paper which we thus find will 

 give the time of the moon's transit, corresponding to the 

 greatest tide as given by the spring tide thus taken. 



The time of the moon's transit, corresponding to the greatest 

 tide, is to be taken in the same way for other spring tides. 



The mean of all the times of transit thus found is to be 

 taken. 



The time of the moon's transit, corresponding to the time of 

 highest high water at London is about Ih. 40m.; at the Cape it 

 will, perhaps, be about 1 hour. 



8. It may perhaps be found, in laying down the heights of 

 high water by means of perpendiculars, or ordinates, as directed 

 above, that there is a zigzag form given to the curve, because 



• Sir J. Herschell's Paper on the Orbits of double Stars. 



3a 



