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XII. On the Effect of the Pressure of the Atmosphere on the Mean Level of the Ocean. 
By Captain Sir James Clark Ross, R.N., D.C.L . , F.R.S. 8fc. 
Received June 15, — Read June 15, 1854. 
In September 1848 Her Majesty’s ships Enterprize and Investigator entered the 
harbour of Port Leopold, in latitude 74° N. and longitude 91 • W., for the purpose of 
establishing there a depot of provisions, and of extending, in boats, the examination 
of the north, south, and west coasts of North Somerset, in search of the missing expe- 
dition under the command of Sir John Franklin. No sooner, however, were the 
ships anchored, than a heavy pack of ice was driven down upon, and completely closed 
the harbour’s mouth, and this effectually preventing their egress, they were compelled 
there to pass the winter of 1848-49. 
It was during that period that the series of observations, which I have now the 
honour of submitting to the consideration of the Royal Society, on the effect of the 
pressure of the atmosphere on the level of the ocean, was obtained, and as it was 
made under peculiarly favourable circumstances, which I shall presently point out, 
I have no doubt it will contribute to throw some light on the movements of the tides, 
and on some of the causes of their apparent irregularities, not only in the Polar 
regions, but also along our own coasts, which have not hitherto been detected, or 
have not received that attention their importance demands. 
It originated in the following circumstances: — Soon after the harbour had been 
completely frozen over, a very heavy pressure from the main pack forced the newly- 
formed sheet of ice, which covered the hay, far up towards its head, carrying the 
ships with it into such shallow water that at low spring-tides their keels sometimes 
rested on the ground. 
It is well known that from many causes great differences in the rise or fall of the 
tides occur in nearly all latitudes, and as any extraordinary depression, in our case, 
might seriously injure the ships with so great a weight of ice attached to their sides, 
the movements of the tides necessarily became to me an object of great anxiety and 
of careful observation, in order, if possible, to ascertain the amount of irregularities 
to which they were liable in this particular locality, and to investigate the cause. 
The first few days’ observations evidenced much larger differences in the elevation 
or depression of successive high or low waters than could he accounted for by any of 
the generally received causes of disturbance; and I was at once led to connect them 
with the changes of pressure of the atmosphere, from perceiving that high water was 
not so high, and that low water was lower, on the days that the pressure of the atmo- 
sphere was greater, and that high water was too high, and low water not so low as 
it ought to have been, on the days of smaller atmospheric pressure. 
The observations during September and October were limited to the register of 
