1892.] facilitating the Reduction of Tidal Observations. 385 



corresponding columns of the form " Hourly sums of S series in 

 twelve months." 



Lay the red M guide sheet on the other drawing board, and 

 transfer the strips from the first board to the new arrangement shown 

 by the zigzag lines, strip 60 being now reintroduced. 



There will now be 48 columns (more exactly 47, since one of the 

 columns will be found to have nothing in it), numbered at top and 



bottom 0,1, , 23, 0, 1, . . . ., 23. Each of the 48 columns is to be 



summed from bottom to top (not as for S in groups of 30), and the 

 sums are to be entered in the form " Sums of series M." The 24 

 sums which come from the left half of the board will be entered in 

 the row marked " red left," those from the right in the row marked 

 " red right." 



Lay the red N sheet on the other board, and transfer the strips. 



In accordance with 9, it will now usually suffice to sum only the 

 columns appertaining to the even hours 0, 2, 4, . . . ,, 22 ; as these 

 hours are repeated twice, there will now be 24 columns to sum. 



The sums are then to be entered on the form " Sums of series N," 

 in the alternate columns. The complete form is provided, so that all 

 the 24 hourly values may be used if it be thought desirable, but this 

 labour seems unnecessary, at least in a long series of observations. 



Lay the successive red sheets on the vacant board, transfer, sum, and 

 enter, until all the red sheets are exhausted. 



In the case of S, M, MS the sums of all the columns are necessary, 

 but in the other eight arrangements only the sums of the alternate 

 columns, those of the even hours, are usually necessary. For a short 

 series of observations it may be best to use all the columns, but in this 

 case it will certainly not be worth while to attempt the evaluation of 

 *>, J, Q, ytt, 2SM, which are all small in amount. 



If the tides of long period MSf, Mf, Mm are required, the 24 

 numbers written on each strip must be added together, and the sum 

 entered in the form " Long period tides daily sums." 



Clean the strips. 



In exactly the same way work through the next 74 days, from 74 d 

 to 74 d + 73 d , with yellow guide sheets. Then clean the strips, and 

 take another 74 days with green guide sheets, and so on with the 

 blue and violet. 



In the last (violet) set attention must be paid to the rules as to the 

 places where the analysis is to stop in each arrangement. 



If the year of observation is so incomplete that the hiatus cannot 

 be made good by interpolation, or if the series does not run over the 

 complete year, the series must stop with one of the days specified in 

 the table at the end of 7, and a note must be made of the number of 

 days used in each series. 



The strips marked for omission on the violet sheets,, , or those 



