286 
SIR JAMES C. ROSS ON THE EFFECT OF THE PRESSURE OF THE 
high and low water, but various causes of disturbance so frequently masked the 
effects of the pressure of the atmosphere, that the four observations on each day were 
not sufficient to determine the amount of effect of its variations: and also, owing to 
the semidiurnal inequality and other causes of derangement, the usual mode of deter- 
mining the level, by taking the mean between successive high and low waters, was 
found inadequate to the detection of small quantities arising from variation of pres- 
sure. I therefore adopted a different system of observation from any that has here- 
tofore been practised, in order to determine the mean level of the ocean on each day. 
I began by instituting simultaneous observations of the height of the tide and of 
the mercury in the barometer at every quarter of an hour throughout the day and 
night, and from these I found that the mean level of the ocean for each day could be 
determined with a great degree of accuracy, and that the variation in the daily 
mean level and in the mean pressure of the atmosphere, as indicated by the baro- 
meter, followed each other in a very remarkable manner, but in an inverse ratio, 
which could only be accurately obtained by a much more extended series of obser- 
vations. 
But the fatigue of making a long-continued series of observations of this nature, at 
every quarter of an hour, during the inclemency of an arctic winter, was greater than 
I could expect the officers to endure who had thenceforward to continue the observa- 
tions which I had begun. Hourly observations were at length determined on. Those 
which I had previously made having been several times interrupted by the necessity of 
moving the ships into deeper water, could not be brought into strict comparison 
with each other without such a complication of corrections to be applied to each 
set as would have greatly and uselessly extended this communication; they have, 
therefore, not been employed, and the conclusions which have been arrived at are 
entirely derived from the observations contained in the following tables. 
The peculiar advantages of our position at Port Leopold, to which I have before 
alluded, for making tidal observations were, — 
1st. In the great width of the entrance of the harbour admitting the free ingress 
and egress of the waters, combined with the large field of ice which covered the 
whole extent of the bay, containing more than ten square miles of surface, and com- 
pletely subduing those undulations of the water, which in other places render tidal 
observations uncertain. 
2nd. In the steady movement of this 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 ; although such 
minuteness was not always attempted, the nearest quarter of an inch being generally 
deemed sufficient. 
3rd. The shallowness of the water, and the evenness and solidity of the clay bottom 
admitting the fixture of the tide-pole with immoveable firmness. 
4th. The whole surface of the ocean 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 ocean in 
as many different degrees as it varies either in strength or direction. 
