RECORD AND DISCUSSION OF ATMOSPHERIC PRESSURE. 109 



was observed in May, tlie minimum in September. The range between these two 

 months is 0.284 inch. 



Inches. 



30.000 



29.960 

 .920 

 .880 

 .840 



29.800 

 .760 

 .720 

 .680 

 .640 



29.600 



^ 



OJ 



^ 



to 

 <1 



A fe 



According to the above diagram, in which the dots indicate the mean monthly 

 readings, the law of the annual fluctuation does not, perhaps, as plainly appear as 

 we might expect from a longer-continued series of observations. In the month of 

 January, for instance, we have a difference in the height of the barometer in the 

 two years (1854 and 1855) of not less than 0.631 inch. 



The general law that the height of the barometer is less in summer than in 

 winter, is here prominently brought out. In the following expression, I have 

 attempted to exhibit the course of the annual variation :— 



luches. Inch. Inch. 



5= 29.770 + 0.079 sin {6 + 4°) + 0.044 sin (2 6 + 194°), 

 the angle 9 counting from January 1st, and is expressed in degrees at the rate of 

 30° a month. 



The computed annual range is 29.875 — 29.G68 = 0.207 inch. If we add 0.005 

 as correction to the constant 29.770, to refer it to the level of the sea, we find the 

 mean barometric height, in the latitude 78° 37' N., 29.775 inches. 



Irregular Oscillations of the Pressure ; Montlihj and Annual Extremes.— The irregu- 

 lar changes in the atmospheric pressure are, like those of the temperatures, much 

 greater in winter than in summer, of which an instance has already been given 

 (see the means for the months of January and February of 1854 and 1855).^ If 

 we deduce the average difference, irrespective of sign, in the barometric height 

 between any two consecutive days, we obtain the following table of mean diurnal 

 change, as made out from 17 months of observations: — 



Inch. 



0.10 

 0.11 



August . 

 September 

 October . 



November 

 December 



Mean 



0.15 



0.17 

 0.26 



0.15 



