258 MR. J. BAXENDELL ON PERIODIC DISTURBANCES 



which it might be expected would be useful for deter- 

 mining readily the monthly and annual sums of the 

 oscillations ; but the systems on which these tables are 

 formed at the two observatories are so widely different 

 that while the sums of the oscillations for the years 1856 

 and 1857 by the Oxford tables amount to 70*95 inches 

 and 70'62 inches, by the Greenwich tables they are only 

 58*22 inches and 56*22 inches respectively, the difference 

 in the one year being 12*73 inches, and in the other i4'40 

 inches. In the Oxford volumes it appears from the head- 

 ing of the tables that all oscillations above one-tenth of an 

 inch are included ; but in the Greenwich volumes it is not 

 stated what limit has been adopted, and as the Greenwich 

 barometric observations are not published in detail, it is 

 impossible to ascertain with accuracy the maximum extent 

 of the oscillations which have been excluded; and the 

 results derived from the tables are therefore not available 

 for comparison with those derived from observations made 

 at other stations = 



In the present state of practical meteorology accurate 

 values of the barometric dynamical element can be ob- 

 tained for those places only where hourly or bi-hourly 

 observations are made. In by far the greater number of 

 cases observations are made only twice, or at most four 

 times a day; and the hours of observation are generally 

 such as to render it difficult to separate readily the regular 

 diurnal oscillations from the irregular and larger fluctua- 

 tions which we are now considering. Under these circum- 

 stances I have thought it desirable to confine my attention, 

 in the first instance at least, to oscillations derived from 

 observations made once a day only. By this plan the 

 regular diurnal oscillations are completely eliminated, the 

 results obtained are more uniform, and better adapted for 

 comparison with each other ; and a greater number of sets 

 of observations become available for the purposes of our 



