364< REPORT — 1872. 



17. We -will iio-w describe the method of reduction pursued, in the first 

 place confining ourselves to the statement of -what was actually done for 

 the j^ear 186-4 and the harbour of Eamsgate. 



A datum-line 10 feet belo"w the previously supposed mean level was 

 chosen*, and the height of the curves marked by the self- registering tide- 

 gauge ■w'as measured from this datum-line in feet and decimals of a foot for 

 each integral mean solar hour of the year, and entered in the Table. A 

 period of SeO"" 3'', or rather more than a year, -n-as taken as being to the 

 nearest hour t-welve and a half lunations or t-wenty-five periods of spring- 

 and neap-tides, and therefore giving a least possible amount of influence of 

 the mean lunar and solar semidiurnal tides, each on the sets of averages used 

 in the calculation of the other. 



This period has been used for the evaluation of the -u'hole of the remaining 

 short-period tide-components contained in the previous schedule, -with the 

 exception of the elliptic diurnal and semidiurnal tides, for -which the folio-w- 

 ing periods -were chosen for similar reasons : — 



Lunar elliptic semidiurnal tides (L and N) 358* 6*". 

 Lunar erection semidiurnal tides (\ and r) 349* 22''. 

 Lunar elliptic diui-nal tides (J and Q) 370'' 0^. 



18. These averages -were taken according to the folio-wing rule : — First for 



the S tides, twenty-four means of the heights at 0'', l^ 2^, 23'' of S hours 



(or ordinary mean solar time) were taken. Next for the M tides, twenty- 

 four averages were taken of heights grouped similarly according to the M 

 hours. In thus averaging- for the M tides every height which was recorded 

 at a time within half an M hour before or after 0'' M time was taken as if it 

 had been observed at 0'' M time, and so for l*", 2^, 3^, &c. of the M time. 

 The proper correction on this was applied afterwards, as will be described later 



* Tbe true mean level for the year 1864 was found to be 10-192 above this datum-line, 

 or -192 of a foot higher than was supposed. 



