Royal Society. 319 



mean level and in the mean pressure of the atmosphere followed 

 each other in a remarkable manner, so that a rise in the former cor- 

 responded to a diminution in the latter. Subsequently however 

 hourly observations were adopted. 



The peculiar advantages of the position of the ships at Port Leo- 

 pold for making tidal observations are stated to have consisted in : — 



1. The great width of the entrance of the harbour admitting 

 the free ingress and egress of the water, combined with the large 

 field of ice which covered the whole of the bay, completely subduing 

 every undulation of the water. 



2. The steady movement of the immense platform of ice, rising 

 and falling with such singular regularity and precision as to admit 

 the reading off the marks of the tide-pole with the greatest exactness, 

 even to the tenth of an inch. 



3. The shallowness of the water and the evenness and solidity 

 of the clay bottom admitting the fixture of the tide-pole with im- 

 moveable firmness. 



4. The whole surface of the sea in the neighbourhood being, for 

 the greater part of the time, covered by a sheet of ice, preventing 

 those irregularities which occur in other localities from the violence 

 of the wind raising or depressing the sea in as many different de- 

 grees as it varied in strength or duration. 



For fixing the tide-pole for the "Enterprize" a hole 2 feet square 

 was cut through the icy platform, and a strong pole, nearly 40 feet 

 long, was passed through it and driven firmly down several feet into 

 the clay, being fixed by heavy iron weights, which also rested on 

 the clay and prevented any movement of the pole. It was placed in 

 about 21 feet depth of water at the time of mean level of the sea. 

 Another such tide-pole was, in a like manner, fixed through a hole 

 in the ice close to the " Investigator," for the sake of reference and 

 comparison. 



Hourly observations of the height of the tide and of the barometer 

 were commenced on the 1st of November, and were continued by 

 the officers of each ship throughout the whole of the nine following 

 months to the end of July. After forty- seven days of observation 

 an interruption in one of the series occurred in consequence of the 

 tide-pole of the "Enterprize" having been drawn up by the ice, to 

 the under part of which it had become frozen. The amount of dis- 

 placement of the pole was easily determined by a comparison with 

 that of the ** Investigator," but several days elapsed before it could 

 be satisfactorily fixed at the same point in which it had been origi- 

 nally. The observations of these forty-seven days are those which 

 are given in the paper, and their discussion is the immediate object 

 of the communication. 



It is stated that subsequent observations seem to show that, from 

 the time of the interruption to the middle of July, there was a pro- 

 gressive elevation of the mean level of the sea, which, although of 

 small amount, was sufficiently evident from month to month to 

 render the subdivision of the series desirable, in order that the indi- 

 vidual observations of each separate division should be strictly com- 

 parable. 



