Effect of the Pressure of the Atmosphere on the Ocean. 53 
As it was found that the usual method of -determining the 
mean level of the sea, by taking the mean of successive high- an 
low-waters, was inadequate to the detection of small quantities 
arising from a change in the pressure, a system of observation 
was adopted different from that heretofore practised, in order to 
determine the mean level of the sea on each day. 
In the first instance, simultaneous observations of the height of 
the tide and of the mercury in the barometer were made every 
quarter of an hour throughout the twenty-four hours. From 
these it was found that the mean level of the sea for each day 
could be determined with great accuracy, and that the variation 
in the daily mean level and in the mean pressure of the atmos- 
phere followed each other in a remarkable manner, so that a rise 
in the former corresponded to a diminution in the latter. Subse- 
quently, however, hourly observations were adopted. 
The peculiar advantages of the position of the ships at Port 
Leopold for making tidal observations are stated to have consisted 
in:— 
1. The great width of the entrance of the harbor admitting 
the free ingress and egress of the water, combined with the large 
field of ice which covered the whole of the bay, completely sub- 
duing 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 exact- 
ness, 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 
immovable firmness. 
4, The whole surface of the sea in the neighborhood being, 
for the greater part of the time, covered by a sheet of ice, pre- 
venting those irregularities which occur in other localities from 
the violence of the wind raising or depressing the sea in as many 
different degrees as it varied in strength or duration. 
For fixing the tide-pole for the “Enterprise” a hole 2 feet 
square was cut through the icy platform, and a strong pole, nearly 
4U feet long, was passed through it and driven firmly down sev- 
eral feet into the clay, being fixed by heavy iron weights, which 
also Tested 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 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 ba- 
tometer were commenced on the Ist of November, and were con- 
tinued by the officers of each ship throughout the whole of the 
hine following months to the end of July. After forty-seven days 
