TIDES AND CURRENTS. 6G1 



Comparing Upernavik and Van Eensselaer, the velocity cor- 

 responds to a depth of 800 fathoms. These are probably extreme 

 values. 



7. Observations by Dr. Hayes at Port Foulke, in 1860-61. 

 " Smithson. Contrib.," vol. xv. Consisting of two series. 



In the first, for 17 days in November and December, half- 

 hourly observations of height of tide were made, and in the 

 second series, in June and July 1861, observations were made at 

 intervals of 10 minutes about the time of high and low water 

 only. 



The readings are regular, and are not affected by agitation of the 

 surface because of the surrounding ice. The graphical method of 

 recording the tides was adopted, and was found to be the best. 

 By means of it the actual times of high and low water can be 

 more exactly determined. 



The tide-gauge consisted of a tripod mounted over a hole in 

 the ice, and supporting a pulley, over which passed a rope ; the 

 rope was fastened to a heavy weight at the bottom, and was kept 

 stretched by a counterpoise. 



The rope was graduated, and the exact readings of the lengths 

 of the portions of it measured. 



The following are some of the difficulties in the way of making 

 accurate observations : — 



(1.) The weight at the bottom may drag along the bottom from 



currents or ice-motion. 

 (2.) The rope may stretch or may contract from becoming 



soaked with water. 

 (3.) The ice-field may have a slow motion, and incline the 



rope. 

 From these causes the zero level may be lost. 

 Note. — With an index on the rope, and a scale fixed vertically- 

 on the tripod, any dragging or inclination of the rope may be 

 detected. 



There is great advantage in referring all tidal observations to 

 half-tide level — 



(1.) For testing for any change of zero. 



(2.) For measuring any secular change of level, i.e., relative 



level of sea and land. 

 (3.) For separating and measuring the changes due to the 



lunar and solar tides. 

 To construct a diagram showing the half- tide level, the mean 

 of two successive high tides is placed opposite the reading of the 

 intermediate low tide, and the mean of these gives one point 

 showing the half-tide level. In the same way the mean of two 

 successive low tides is compared with the intermediate high tide, 

 and their mean gives another point showing the half-tide level. 

 In this way the half-tide level curve is traced out. 



