384 Prof. G. H. Darwin. On an Apparatus for [Dec. 15, 



Whatever unit of length be taken it is important, for the saving of 

 work, and it is sufficient, that all or nearly all the heights should be 

 expressed by two digits. 



Completion of record. 



If there is an accidental break in the record, it is very important 

 that it should be completed according to the method shown in 12, 

 or by some other equivalent plan. 



The computation forms are drawn up on the supposition that the 

 year of observation is complete, but with proper alterations, which 

 will now be indicated, they may be used in other cases. 



In 4 it is shown how to treat the tides of the S group when the 

 observations have been subject to a long stoppage in the course of the 

 year, and also when thfe observations extend over any period from six 

 months to a year. 



In 5 it is shown how to treat the S group for a short period of 

 observation. 



If the stoppage be a long one, the method explained in 12 must 

 be adopted for all the other tides. The same section also shows the 

 treatment for observations extending over any period, long or short, 

 less than a year. 



Entries and summations. 



The computing strips are intended to take writing in pencil or 

 liquid Indian ink, but not in common ink. 



They are to be cleaned with a damp cloth, and a little soda may be 

 put in the water if they become greasy. 



Lay the red S sheet on one drawing board and set up the strips 

 with their ends abutting against the corresponding numbers. The 

 strip numbered 60 is also to be put on the board. 



Write the hourly heights for each day on the strip bearing the 

 corresponding number, strip for day 0, strip 1 for day 1, and so on 

 up to strip 73 for day 73. The 24 hourly heights are to be written in 

 the 24 divisions of each strip, beginning on the left with O h and end- 

 ing on the right with 23 h . 



Remove strip 60. 



Sum the 24 columns formed by the divisional marks on consecutive 

 sets of 30 strips. Thus, days to 29 afford 24 sums ; days 30 to 59 

 afford the second set of 24 sums ; days 61 to 73 afford 24 sums, which 

 are the beginning of a third 30, to be completed when the second set 

 of 74 days shall have been written on the strips. 



The numbers 0, 1, 2, , 23, 0, 1, , 23 at the head and foot of 



the guide sheet indicate the hours corresponding to the columns. 



The sums of the columns on the board are to be entered in the 



