RECORD AND DISCUSSION OF ATMOSPHERIC PRESSURE. 1()7 



January, 1855. 20th, 21st, 22d, 23d, and 24th. The temperature readings for these days have bein 



taken from page 422 of the second volume of the Narrative. 

 25th, 26th, 27th, 28th, 29th, 30th, and 31st. The barometer and temperature readings have been 



taken from the same page of the Narrative. 



The last horizontal column of the reduced barometer readings is derived from the preceding column 



by subtracting 0.024 inch, so as to allow for the introduction of the means of the last seven days. 



February, March, and April, 1855. For these months the original record could not be found. The 



daily and monthly means of the atmospheric pressure (and corresponding temperature), will 



be found in Appendix No. XII. of the second volume of the Narrative. 



Diurnal Variation of the Aimosplteric Pressure. The following table exhibits the 

 diurnal change of the barometric pressure, for each month of the year and for the 

 whole year, as made out from the preceding abstract. For the months between 

 September and January (inclusive), the mean from the two sets is given. 



Owing to the small amplitude, the comparatively short period of observation, 

 and the magnitude of the occasional disturbances, the law of the diurnal variation 

 is apparently .subject to considerable fluctuations; and it has, therefore, only been 

 attempted to express the figures in the last vertical column, or the mean variation, 

 analytically. Using Bessel s formula, the variation can be expressed by the for 

 mula: 



Inches. Inch. Inch. 



20.705 + 0.0034 sin (6 + 200) + 0.0022 sin (20 + 204); 



the terms containing 3 0, etc., becoming too small to have any real value. The 

 angle counts from neon, and is expressed in degrees at the rate of 10 an hour. 



