TIDES AND CURRENTS. 667 



tide wave nearly 2,300 miles, with a height between trough and 

 crest of about 7^ feet. 



[By comparison of these results with other similar results 

 obtained by the Expedition and with soundings, some important 

 conclusions may be arrived at as to the theory of the tides. 



A table is given comparing the different tidal observations at 

 points on the East Coast of Greenland by M. Koldewey, with 

 those on the West Coast by Drs. Kane and Hayes, showing the 

 progress of the tides from the south along those two coasts. The 

 average height of high water at spring tides on the east side is 

 only about 4 feet, and at neap tides is about 2 feet. It will be 

 seen that the tides in "Polaris" Bay, \Thich are 5 feet at spring 

 tides and 2 feet at neap tides, are nearly of the same amount as 

 those at Sabine Island. At some places M. Koldewey had great 

 difficulty in determining within half-an-hour the time of high 

 water, even by his apparatus, which was more perfect than that 

 used by Dr. Hayes. 



From the account of the observations of tides made by Dr. 

 Kane and Dr. Hayes, it is seen that there are great defects in the 

 apparatus, and even with the improvement adopted by Capt. 

 Koldewey, viz., with a straight scale fixed to the vessel, and rising 

 and falling with it, and an index fixed to the rope, the observations 

 were liable to serious and unknown errors, either (1), from the 

 dragging of the weight ; (2), from the motion of the ice-field ; 

 (3), and especially from the freezing up of the hole through which 

 the rope passes. This is a serious objection to observations 

 made by cutting holes in the ice, for the rope will be frozen to 

 the ice and be lifted or dragged with it, and so the zero of the 

 scale may be altered either by the motion of the ice or even in 

 breaking the ice before making the next observations. 



From Admiral Belcher's observations it is clear that the 

 objections to a tidal pole are even greater than the objections to 

 the above method. 



There is one method by which these difiiculties may be dimi- 

 nished or entirely got rid of, but the ships will require to be 

 specially adapted for the purpose. 



An opening may be constructed in the bottom of the ships, 

 through which direct communication may be had with the water 

 below the ice when the ships are frozen up in winter quarters. 

 Through these openings not only the observations on tides could 

 be made, but also on the temperature and on the specific gravity 

 of sea water at various depths, as well as observations on currents. 



Some of the evident advantages of such a plan are, — 



(1.) Observations made within the ship will be far more 

 accurate, because the observer is not exposed to cold and the 

 weather. 



(2.) The rope does not get frozen to the ice, and therefore will 

 not be subject to a sudden pull, as it would in breaking the ice 

 round it. 



(3.) Valuable observations could be made on currents, and proper 

 apparatus could be fitted in the ships for the purpose. 



