108 RECORD AND DISCUSSION OF ATMOSPHERIC PRESSURE. 



The annexed diagram exhibits the observed (by dots) and the computed (by a fine 

 line) diurnal variation. Its principal feature is the afternoon inflection, with a 



DICKXAL VARIATION. 



Inches. 



29.776 

 .774 



.772 

 .770 

 768 

 .766 

 .764 

 .762 

 .760 

 .758 

 29.756 



maximum pressure at about 10 P. M. The principal minimum is reached at 1 

 P. M. The 10 P. M. maximum is in strict conformity with the general law deduced 

 from observations in the northern hemisphere. The 1 P. M. minimum seems to 

 occur about three hours earlier than indicated by more southern stations. Kaemtz 

 takes as amplitude of the diurnal variation, the difference between the mean of 

 the maxima and the mean of the minima. Other meteorologists take the maxi 

 mum difference. According to this latter view, we have the diurnal amplitude 

 29.7701 29.7604 0.0097 inches. The diurnal fluctuation here considered is 

 the change in the gross pressure of the atmosphere, there being no means on hand 

 for separating the pressure of dry air from the pressure of aqueous vapor. 1 



In connection with this subject, it may be stated that the latitude of Van Rens- 

 selaer Harbor is about 14 higher north, than the latitude (64) in which the mean 

 height of the barometer is a minimum (at the level of the sea). 



Annual fluctuation of (he Atmospheric Pressure. The following is an abstract of 

 the monthly means of the barometer readings (reduced to 32). The values for 

 February, March, and April, 1855, were taken from the second volume of the 

 Narrative. 



According to the resulting values, the height of the barometer is above the mean 

 in the months of January, February, March, April, and May, and descends below 

 the mean in the remaining summer and autumn months. The maximum pressure 



1 Although a long series of hygrometric observations were made, yet, owing to the peculiar delicacy 

 which such observations require in a latitude of such low temperature, they proved, on close examina 

 tion, too uncertain to be relied on in their results. 



