ATMOSPHERE ON THE MEAN LEVEL OF THE OCEAN. 
295 
mean level of the ocean, and may be encouraged to pursue the investigation through 
a more extended series of observations, in order that we may at length arrive at the 
most accurate conclusion that the observed facts may justify. 
For all practical purposes, it may in the mean time be well to assume, what the 
preceding observations seem to indicate, that the ocean is a water barometer on a 
vast scale of magnificence, and that the level of its surface is disturbed by every 
variation of atmospheric pressure, inversely as the mercury in the barometer, and 
exactly in the ratio of the relative specific gravities of the water and the mercury. 
And as all observations of the tides, before they can be safely employed to investigate 
the laws by which they are governed, should in the first place be corrected for the 
large and hitherto mysterious irregularities which the variations in the pressure of 
the atmosphere produce, the following formula may be used to determine the correc- 
tion z to be applied to all observations of the height of the tide, or the mean level of 
the ocean deduced from them, to reduce them to the mean pressure of the atmosphere. 
(1) 2 =(B — /3)D, or (2) L=X+(B— /3)D, 
positive when (3 is greater than B, and negative when less. 
In which B denotes the mean pressure of the atmosphere. 
L the correct height of the tide or mean level of the ocean. 
D the relative specific gravity of sea-water and mercury. 
X the observed height of the tide or observed level of the ocean. 
(3 the corresponding height of the barometer. 
And if we assume B = 29*874 inches, the mean of the preceding observations; 
L=21 ft. 0*21 inch, the mean of the preceding observations; and D=13*224, we 
can readily compute the correction 2 to be applied to any observed tide, having the 
corresponding height of the barometer. For example, on the 3rd November the 
mean barometer for the day was (3 29*351 inches, and the corresponding mean level 
of the sea A=21 ft. S*3 inches; then B — j3 x D = — 6*92, which applied to X= 21 ft. 
1*38 inch ; and on the 13th November the mean barometer was /3 30*225 inches, and 
the corresponding observed level >.=20 ft. 6*4 inches ; again, B— /3xD=+4*64 added 
to >.=20 ft. 11*04 inches. 
On these two days the observed level of the ocean differed no less than 14 inches ; 
but by the application of the correction found by the above formula, the observed 
level in each day is brought to agree with the true mean level to little more than 
1 inch. I may further observe, that much greater irregularities are to be hereafter 
noticed in the more extensive series of observations which followed this, and by the 
same formula are capable of being reduced to an equally near accordance with the 
mean level, as deduced from the whole of the observations. 
Thus it is evident that one of the many causes of the apparent irregularities of 
the tides (and at Port Leopold certainly the greatest of all) is clearly traceable to a 
well-established and invariable law. 
