154 REPORT— 1839. 



the critical hours mentioned by M. de Humboldt in his obser- 

 vations in the torrid zone ; which is not a little remarkable. 



These hours are also mentioned by M. Lamanon, engaged 

 in the voyage of La Perouse, 



10. The times of these semi-diurnal variations are therefore 

 equal, but the variations unequal. The ascent = '0204 in the 

 evening, exceeding the ascent = "01 33 in the morning by about 

 '0071 of an inch, or one-third nearly. 



A similar result is observable in the descending branches ; 

 the times are the same, but the fall between 10 a.m. and 4 p.m. 

 = '0166 is somewhat less than that between 10 p.m. and 4 a.m. 

 = '0171 by about '0005. These differences are, however, 

 very small. 



On a further examination we find that the two maxima differ 

 by about '0038 of an inch, that at 10 p.m. being the greatest, and 

 the minima by about '0033 of an inch. These differences, 

 however, are likewise inconsiderable, the values being nearly 

 the same. 



If, therefore, we divide the daily march of the pressure into 

 two complete waves extending through periods of twelve hours 

 each, we observe the wave at night somewhat exceeds the wave 

 by day. 



11. The principal elements of this horary oscillation are as 

 follow : 



Table VI. 



Times of Mean Pressure 7 a.m. 12 noon. 6*30 p.m. 1-30 a.m. 



Critical Hours 4 a.m. 10 a.m. 4 p.m. 10 p.m. 



Mean Oscillation -01685. 



It is not unworthy of remark, that the mean oscillation as 

 above deduced, corresponds nearly with that arrived at by Mr. 

 Daniell from his meteorological register near London, the 

 numbers being '0168 and "0150, a difference not probably 

 greater than might be expected from the nature of the obser- 

 vations and difference of latitude. 



Horary Oscillation, ^c, in Different Periods of the Year. 



12. In Plate VI. will be found delineated the hourly march of 

 the pressure through a mean day of summer and winter, di- 

 vided into periods of six months each. The period of summer 

 being from May to October inclusive — the period of winter from 

 November to April inclusive. The following Table contains 

 the oeneral results as deduced from Table II. : 



