XLII 



INTRODUCTION. 



From the preceding table we learn that the mean daily motion of the mercurial column com- 

 prises two maxima and two minima. These are of'unequal values. Of the former, the greater 

 takes place ahout 8 A. M. local time during the summer months, (December, January, and Feb- 

 ruary ;) hut through the autumn months, not until two hours later, or near 10 A. M. In 

 winter the normal oscillations are very frequently masked by extraordinary fluctuations ; whilst 

 in spring, the hours are 8, 10, and 9 respectively for September, October, and November. In 

 the average of the whole year, 10 A. M. coincides most nearly with the greater elevation of the 

 column. The lesser maximum takes place in summer and autumn at 9 p. M., in winter at 10 

 p. M., and in spring at 8 P. M. During the winter months, the greater and lesser maxima are 

 very often inverted ; but the mean hour of the lesser maximum for the year is 9 P. M. 



Of the minima, whilst the monthly results show that in seven of the twelve periods the 

 greater of the two takes place at 5 P. M., the lowest mean depression for the whole year is found 

 to be at 4 P. M. Examining the oscillation by seasons in spring, summer, and autumn there 

 is uniformly least pressure at 5 P. M. ; whilst the abnormal fluctuations of winter already men- 

 tioned throw forward the hour to 2 p. M. Between midnight and 6 A. M., there were term-day 

 records only on nine of the twenty-seven days of observation not a sufficient number to deter- 

 mine the period of the lesser minimum definitively; but most probably it does not differ much 

 from 3 A. M. 



These observations also point out an annual tide in the atmosphere whose maximum is in 

 August, and minimum in February a fact more positively demonstrated in the results of all 

 the tri-hourly records, where the change of pressure amounts to j}^ part of the whole. 



Comparing the numbers in the preceding columns with the daily means for each correspond- 

 ing month deduced from all the tri-hourly observations, the following table is formed : 



TABLE IX. 



Excess of the atmospheric pressure at every hour above the mean daily pressure for the month, 



shown by the tri-hourly observations. 



