Harmonie Analysis of Tidal Observations. 11 



they draw will appear on the sheet beside the tidal curve. Gene- 

 rally, the positions of the peu cils are so adjusted that the curve is 

 wholly included between these two lines. The measurement of 

 the marigram is made downwards from the upper line, so that 

 when the number is large the tide is lower, and when it is smaller 

 the tide higher. Now if we wish to know the actual distance of 

 the sea-water from the zero-point of the instrument, at a given 

 time, we must determine the actual position of the line to which 

 the upper datum line on the sheet corresponds. In order to ac- 

 complish this, the following arrangement is provided for the tide 

 observatories belonging to the Land Survey Department. A board 

 is placed at a fixed level between the zero-point of the instrument 

 and the surface of the water. Every few days, the float is slowly 

 drawn up until the bottom of the float is just above the upper surface 

 of the board. Of course, while this process is going on, the re- 

 gistering pencil makes a corresponding vertical motion and finally 

 marks the whole amount of displacement of the bottom of the 

 float. If we measure the distance between the final mark and the 

 curve at a given time, it will be the distance on the reduced scale 

 corresponding to the actual distance between the upper surface of 

 the board and the sea surface plus the sink of the float. Then the 

 distance of the sea surface in the cylinder from the zero-point of 

 the instrument will be determined if we know the distance between 

 the zero-point and the upper surface of the board. We have ac- 

 cordingly gathered the necessary constants below, where D 

 represents the distance between the zero-point and the upper surface 

 of the plank, and S the amount of the sink of the float. 



