ON DISCUSSIONS OF BRISTOL TIDES. 81 



" gave nearly the same result, only a trifle less in the depression of high water 

 " for an inch rise of mercury. The specific gravities of mercury and water 

 " being not far (if I recollect right) from this ratio of 14 or 15 to 1, it would 

 " seem that the total weight of the compound column of air and water raised 

 " by the force which produces the tide, remains nearly unaifected by the 

 " changes of atmospheric pressure. By introducing this new correction, there- 

 " fore, a very considerable portion of our Residual Error is accounted for." 



The barometric observations which Mr. Bunt used for finding the effects 

 of atmospheric pressure on the heights of high water at Bristol, were those 

 contained in the register kept at the Bristol Institution, which extended back 

 to a period earlier than the commencement of the tide observations. 



As it had appeared that all the other eff"ects of external forces upon the 

 height and time of high water corresponded not to the forces at the moment 

 of observation, but to a state of the forces at an anterior period, it occurred 

 as possible that this might also be the case with the eff'ect of the atmospheric 

 pressure upon the height of the tide ; and that the correction corresponding 

 to this effect might be most accurately obtained by taking the state of the 

 barometer at some period anterior to the time of high water ; for instance, 

 twelve hours or twenty-four hours. If this were the case, we should be able to 

 predict the effect of atmospheric pressure upon the tide, a day or half a day 

 previous to the event. As this prospect gave an additional interest to the 

 inquiry, I begged Mr. Bunt to try the comparative results of contempora- 

 neous and anterior epochs of the barometric observation. This he proceeded 

 to do, by arranging various portions of our observations according to the 

 heights of the barometer. The following is the account of the result. 



"Bristol, Feb. 18, 1841, 



" I send you diagrams of the effects of atmospheric pressure on the heights 

 of high water for every tenth of an inch height of the barometer, from 

 29'2 or '3 inches to 30*4 or '5 inches, for the years 1834, 35, 39 ; barometer 

 and tide contemporaneous. Also for the year 1834, barometer heights being 

 twenty-four hours anterior to high water. Also for 1839, barometer 29*2, 

 •3, '4, '5 inches to 30'2, '3, "45 inches, twelve hours anterior to high water. 

 Also the mean of the three years, giving about 14 inches depression of tide 

 to one inch rise of barometer. 



" I have also taken the sums of the'residues left after introducing the baro- 

 meter correction, first, contemporaneously with high water, and secondly, 

 at twenty-four hours anterior to high water, for the first six months of the 

 year 1 834, measuring the residue at about every high water. The total re- 

 sidues, in the two cases, were so nearly alike, as to leave it doubtful which 

 epoch should be preferred. The diagram for 1834, made from observations 

 of the barometer twenty-four hours anterior to high water, appears about as 

 good as the one from contemporaneous observations of barometer and tide. 



The extreme groups, however, for 29*2, '3, "4 inches, 30*4, "5, '6 inches 



barometer, approximate slightly towards the mean line : the same tendency 



appears in the double groups 29*2, '3, "4, "5 inches, 30*2, '3, '4, "5 Inches, 



for 1839, barometer observed contemporaneously with, and at twelve hours 

 anterior to, high water. Hence I should be disposed to infer, that we do not 

 improve the result by going back to an anterior epoch ; for I take it for granted 

 that the true epoch is that which shows the greatest amount of elevation and 

 depression of tide corresponding with the least and greatest heights of the 

 barometer ; or that which makes the greatest angle of inclination between 

 the line connecting the several points or groups, and the axis. 



"There is one peculiarity which I have noticed in these barometrical results, 

 and in others which I obtained in my earlier trials, namely, that the effect 



