1818.] the Imperfection of the Barometer. 201 



satisfactory comparison. Mr. Howard and Mr. Forster fortu- 

 nately both registered their observations upon the barometer on 

 the same plan, not noticing it at particular hours, but giving the 

 maximum and minimum of each diurnal period. I have arranged 

 their observations in the tabular form. No. 2, contains a view of 

 the maxima of Mr. Howard's and Mr. Forster's barometers, 

 for each day during the months of September and October, 1811, 

 and No. 3, the minima for the same period. The inspection 

 of the tables shows us that the barometers at Plaistow and at 

 Clapton were nearly as different from each other as those at 

 Lyme and at Liverpool ; and of course would lead us to con- 

 clude, that a considerable part at least of the difference that was 

 observed between them in the former case depended upon the 

 instruments, and not upon the state of the atmosphere. 



If we descend to particulars, we shall observe, in the first 

 place, that the absolute height of the Clapton barometer is 

 greater than that of the Plaistow barometer, although Plaistovr 

 is on a lower level than Clapton. In the second place, the pro- 

 portional heights are not the same ; the difference between them 

 is perpetually varying, and sometimes the Clapton barometer is 

 even lower than that at Plaistow. In the third place, when any 

 considerable elevation or depression has occurred, the amount 

 of it has been very different in the two instruments ; and the 

 greatest elevation or depression has sometimes been a day later 

 at one place than at the other. On Sept. 20, for example, the 

 barometers were both low, the one at Plaistow -18 lower than that 

 at Clapton : they both rose on that day, but in very different 

 degrees ; the Plaistow barometer rose -27, while the Clapton 

 barometer rose only -02; so that the barometer at Plaistow 

 became -07 higher than the one at Clapton. But from the 21st 

 to the 22d, the Plaistow barometer was stationary ; while, during 

 the same period, the Clapton barometer rose -21, so thatit became 

 •14 higher than the other. From Sept. 30, to Oct. 1, one 

 barometer was rising while the other was falling ; and the same 

 circumstance occurred from the 2d to the 3d, and from the 7th 

 to the 8th, of the same month ; also from the 14th to the loth; 

 and most remarkably from the 29th to the 30th, when the 

 Plaistow barometer rose '50, while the one at Clapton fell -04. 

 But it will be unnecessary to enter into more particulars on this 

 point, as we have, I conceive, the most unequivocal evidence 

 that, even in the hands of those the best qualified to employ the 

 barometer, it is not to be depended upon as an accurate method 

 of ascertaining the weight of the atmosphere. 



As the barometer is an instrument which is very liable to be 

 deranged by being moved from place to place, I thought it 

 desirable to institute a comparison between the action of two 

 instruments that had not been subject to any injury from this 

 cause, and which were, at the same time, so near each other, as 

 that it was scarcely possible to conceive there could be any 



