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X. "On the Effect of the Pressure of the Atmosphere on the 

 Mean Level of the Ocean." By Captain Sir JAMES CLARK 

 Ross, R.N., F.R.S. Received June 15, 1854. 



The author states that, in September 1848, Her Majesty's ships 

 Enterprize and Investigator having anchored in the harbour of Port 

 Leopold in lat. 74 N. and long. 91W., a heavy pack of ice was 

 driven down upon and completely closed the harbour's mouth, thus 

 effectually preventing their egress, and compelling them there to 

 pass the winter of 1848-49. It was during that period that the 

 series of observations here presented to the Royal Society was ob- 

 tained ; and, as the observations were made under peculiarly favor- 

 able circumstances, the author considers they will throw some light 

 on the movements of the tides, and on some of the causes of their 

 apparent irregularities. 



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 bay, 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. Under these circum- 

 stances the movements of the tides became to the author an object 

 of great anxiety, and consequently of careful observation, in order 

 to ascertain the amount of irregularities to which they were liable 

 in that particular locality. 



The first few days' observations evidenced much larger differences 

 in the elevation or depression of successive high or low-waters than 

 could be accounted for by any of the generally received causes of 

 disturbance ; and the author was at once led to connect them with 

 changes of the pressure of the atmosphere, from perceiving that on 

 the days of great atmospheric pressure high- water was not so high 

 as it ought to have been, and low-water was lower than its proper 

 height; and that the reverse took place on the days of smaller 

 pressure. 



As it was found that the usual method of determining the mean 

 level of the sea, by taking the mean of successive high- and low- 

 waters, was inadequate to the detection of small quantities arising 

 from a change in the pressure, a system of observation was adopted 



