Mr. W. R. Birt 07i a remarkable Barometiic Depression. 21 



Table VI. — Showing the highest and lowest readings of the 

 Maxima for the respective days and years in which they 

 occurred. 



In this table the highest readings of the maxima show the 

 absolute altitude which the mercurial column attained on the 

 respective days in the years named in the third column. 



The lowest readings of the maxima show that the barometer 

 attained a greater elevation than 29-25 on each day, from the 

 23rd to the 27th of December inclusive, in each year from 

 1806 to IS.'JO also inclusive, 1821 and 1810 only excepted ; 

 the exception of the year 1810 extends only to the 25th. 



The reply which this examination furnishes to the question 

 proposed is this— that the depressions of 1810 and 1821 were 

 accidental, that i.s, they did not form portions of a periodical 

 series of depressions, such as the depression of the latter end 

 of November (see table on p. 359 in the last volume). Mr. 

 Brayley however suggests that they may be instances of a 

 lojiger series. May they not result from annual movements, 

 which from some cause or other are displaced'^ The usual 

 epoch of the crest of the great wave is about the middle of 

 November ; on some occasions it occurs as early as the latter 

 end of October, and on others as late as the latter end of No- 

 vember, having a range of about a month. 



This result conducts us to another of a very interesting 

 character: we have just noticed that the great symmetrical 

 wave is to a certain extent erratic as to the period of its 

 transit. The depression of the latter end of November is to 

 a certain extent constant, the period of its transit being con- 

 fined to about four days: this has been determined from 

 a consideration of the barometric curves of the last four days 

 of November. Mr. Howard's observations do not furnish 

 sufficient data for projecting the curves ; still the readings 

 lead to the conclusion, that some time during the five day.s, 

 from the 23rd to the 27th of December inclusive, the baro- 

 meter passes a maximum. We have an elevation of the mer- 

 curial column about this time. 



From this it appears highly probable that we have two 



