1892.] facilitating the Reduction of Tidal Observations. 347 



them to show how the strips are to be set. The guide sheet is laid on 

 a drawing board, and the pins on the strips pierce the paper and fix 

 them in their proper positions. 



The shifting of the strips from one arrangement to the next is cer- 

 tainly slower than when they slid into their plaoes automatically, 

 but I find that even without practice it only takes about 7 or 8 

 minutes to shift 74 of them from any one arrangement to a new 

 one. 



The strip belonging to each mean solar day is divided by black 

 lines into 24 equal spaces, intended for the entry of the hourly 

 heights of water. The strip is 9 in. long by \ in. wide and the 

 divisions (|- by ^) are of convenient size for the entries. There was 

 much difficulty in discovering a good material, but after various 

 trials artificial ivory, or xylonite, was found to serve the purpose. 

 Xylonite is white, will take writing with Indian ink or pencil, and 

 can easily be cleaned with a damp cloth. It is just as easy to write 

 with liquid [ndian ink as with ordinary ink, which must not be used, 

 because it stains the surface. 



The strips have a great tendency to warp, and I have two methods 

 of overcoming this. A veneer of xylonite on hard wood serves well, 

 or solid xylonite may be stiffened by sheet brass let into a slot on the 

 under side. In the first plan the pins are fixed in the wood, and in 

 the second the brass is filed to a spike at each end. Whichever plan is 

 adopted, the strips are expensive, costing about 7 for a set, and I do 

 not at present see any way of making them cheaper. 



The observations are to be treated in groups of two and a half 

 lunations or 74 days. A set of strips, therefore, consists of 74, num- 

 bered from to 73 in small figures on their flat ends. 



If a set be pinned horizontally on a drawing board in vertical 

 column, we have a form consisting of rows for each mean solar day 

 and columns for each hour. The observed heights of the water are 

 then written on the strips. 



When the 24 columns are summed and divided by the number of 

 entries we obtain the mean solar hourly mean heights, The har- 

 monic analysis of these means gives the mean solar tides. But for 

 evaluating the other tides the strips must be rearranged, and to this 

 point we turn our attention. 



Let us consider a special case, that of mean lunar time. A mean 

 lunar hour is about l h 2 m m.s. time ; hence the 12 h of each m.s. day 

 must lie within 31 m m.s. time of a mean lunar hour. The following 

 sample gives the incidence to the nearest lunar hour of the first few 

 days in a year : 



