BAROMETRIC PRESSURES ON THE GREAT LAKES 37 



For the definitions of the meanings of b u -o, b w \ b nZ , b nS , consult page 

 24. In the above formulae the unit in which these quantities are to be 

 expressed is 0.01 inch. In these formulae the unit in which the barometric 

 effect is expressed is in units of 0.001 foot. 



The method of computation actually used to secure the barometric effects 

 for each d&y consists of five steps, as follows: 



(1) The hourly barometric effects were computed by the method set 

 forth on pages 32-36 for the first, the last, and a few intermediate days of 

 the long series of days under consideration. The daily barometric effect 

 was obtained for each of these selected days by taking the mean of the 24 

 hourly effects for that day the most accurate method. 



(2) The change in barometric effect from day to day throughout the 

 whole series was computed from the appropriate one of formulae (46) 

 to (50). 



(3) The changes from day to day computed in step (2) were applied one 

 by one to the computed barometric effect for the first day of the series ob- 

 tained in step (1) to secure the barometric effect for each day in turn up to 

 and including the next selected day on which the daily barometric effect had 

 been computed in step (1). 



(4) At this point, the second selected day, a discrepancy appeared between 

 the barometric effect as computed for this day by step (1) and as computed 

 by step (3). This discrepancy was distributed proportionally to time be- 

 tween the first and the second selected day, and all intermediate values 

 corrected, so as to make the discrepancy disappear at the second selected 

 day. The values so corrected were adopted as sufficiently accurate. 



(5) Steps (3) and (4) were taken from the second to the third selected day, 

 from the third to the fourth selected day, and so on to the end of the series. 



The computation for August 26 to September 24, 1910, at Milwaukee, 

 made in accordance with the above statement, is shown on the following 

 page. The unit is one foot. 



The values in the column headed "Change in barometric correction" 

 were computed from formula (48). Each value is the change in barometric 

 correction from the day before to the day indicated on the line on which the 

 value is placed. 



Of the values in the column headed "Barometric correction based directly 

 on computed values," those included in parentheses for the selected days 

 were computed from hourly barometric effects as indicated in step (1), and 

 the others, without parentheses, were computed as indicated in step (3) 

 by applying from day to day the changes shown in the preceding column. 



At the second selected day, September 4, there are two values in the third 

 column, namely, +0.045, computed by step (3), and ( 0.070), computed 

 by step (1). The discrepancy is 0. 115 foot. This discrepancy was dis- 

 tributed over the interval August 26-September 4 at = 0.0128 foot 



y 



per day, as shown in the fourth column. 



