304 CHANGES OF BAROMETRIC PRESSURE 



observations by more than 22J° on eacb side of one of tbe eight prin- 

 cipal points ; and as such comparative constancy in direction will 

 usually occur only when the interval is short, it was found convenient 

 to employ only the differences between 6 a.m. and 8 a.m., between 

 2 p.m. and 4 p.m., and betwean 10 p.m. and midnight. 



The total change in the reading between two consecutive observa- 

 tions being first diminished by the change due to diurnal variation, the 

 remainders were then classed according to the direction of the wind in 

 the interval, and their averages in each class taken, for the year collec- 

 tively as well as separately for the two half-years. 



The average changes of barometric pressure which take place in two 

 hours, and found in the manner just described, are given below for 

 each of the principal eight point of the wind's direction. 



APEIL'TO SEPTEMBER. 



N. N.E. E. S.E. S. S.W. W. N.W. 



+ •0085 — -0043 —-0113 —-0057 —•0084 —•0041 -f-0132 4-^0150 



"W. N.W. 



4.-0087 —^0160 —•0334 — •OSIS —^0222 — -OOST -f^OlGS +'0209 



vr. N.w. 



+ •0086 -^0103 -^0215 — "0164 -^0129 — -0039 +'0156 +^0180 



The most probable values of the changes corresponding to interme- 

 diate directions of the wind are given by the following formulse, 

 where Wi W^ ¥^ represent the changes for the two half years and year, 

 and ^ the angular distance of the point from which the wind blew, 

 measured from the North towards the East, end expressed in degrees. 



APRIL TO SEPTEMBER. 



•5^1 = + -0004 + -0125 sin (0 + 141° 29^ + "0044 sin (2 0+ 186° 29') 

 + -0025 sin (3 0+ 14° 2^ 



OCTOBER TO MARCH. 



•»?2 =• — •0075 + -0281 sin (0 + 148° 14^ + -0024 sin (2 0+ 160° 49) 

 + -0014 sin (3 + 30° 15') 



THE YEAR. 



•^3 = — -0028 + -0195 sin (0 + 148° 2') + ^0040 sin (2 + 174° 17') 

 + ^0021 sin (3 0+ 10° 47) 



PEESSUEE OF DRY AIR. 

 The average changes in the pressure of dry air in two hours with 

 different winds, and the corresponding formulse of interpolation, are as 

 follows. 



