22 Mr. W. R. Birt on a remarkaUe Barometric Depresion. 



points approximately fixed in our annual barometric curve, 

 the depression on or about the 28th of November, and the 

 elevation on or about the 25th of December. It also appears 

 that we have two kinds of atmospheric movements that affect 

 the barometer ; those of an erratic character as to their 

 })eriod— the great symmetrical wave being an instance — and 

 those of a more constant character, as the depression and 

 elevation just noticed. 



We now pass on to the evidence which the Greenwich ob- 

 servations afford as to the elevation of the 25th of December. 



Table VIL— Barometric Readings, Royal Observatory, 

 Greenwich. 



From this table we find that the barometric readings on 

 the 25th are strictly in accordance with the foregoing deduc- 

 tions, being considerably above 29*500 ; and we further learn 

 that the maximum of this elevation occurred on the 26th at 

 10 at night; the readings of the 25th were therefore those of 

 the anterior slope of the elevation. The maximum of the 

 26th was also the maximum of the month. 



Table VIII. — Barometric Readings, Royal Observatory, 

 Greenwich. 



