PRESSURE OF THE WIND. xcv 
4th, The afternoon minimum occurs earliest in winter and latest in summer, the difference of the epochs 
being nearly three hours. The epochs of this minimum have some relation to those for sunset as the morning 
minimum epochs had to sunrise, thus :—In winter, the morning minimum occurs about three hours before 
sunrise, in summer the afternoon minimum occurs about three hours before sunset; in winter the afternoon 
minimum occurs about one hour and a half before sunset, in summer the morning minimum occurs about half- 
an-hour before sunrise. : ane ; 
5th, The afternoon maximum occurs latest in summer and earliest in spring ; the difference of the epochs is 
two and a half hours. 
6th, It is not easy to relate the variations of the epochs of the maxima to those of any other facts ; itis to be 
observed, however, that the morning maximum occurs nearest noon in spring and farthest from noon in summer, 
while the afternoon maximum occurs farthest from midnight in spring and nearest midnight in summer. 
TABLE 89.—Whole Amount and Hourly Rate of the Change of Atmospheric Pressure from Epoch 
to Epoch in the Diurnal Variation for each Quarter, and for the Year. 
A,M. Minimum to | A.M. Maximum to | P. M. Minimum to | Pp. M. Maximum to 
é Se 
A.M. Maximum, | P.M, Minimum. | p.m. Maximum. | A.M. Minimum, _ || Wbole Oscillations, 
Period. 
Total. |PerHour.| Total. |Per Hour.| Total. |Per Hour. . |Per Hour, 
\. in, in. in, in. in. in. in. 
Noy. Dec. Jan. . 0-0160 | 0-0044 | 0-0126 | 0-0018 | 0-0275 | 0.0033 0-0036 
Feb, Mar. Apr. || - -0140 } -0033 | -0180 | -0035 | -0250 | -0033 : -0033 
May June July || - : -0230 | -0026 | -0191 | -0031 | -0100 | -0024 || - -0027 
Aug. Sept. Oct. || - oi -0208 | -0030 | -0188 | -0031 | -0146 | -0023 || - -0030 
Year : -002 -0150 | -0027 | -0145 | -0026 | -0173 | -0026 || . -0027 
197. The total oscillations from one turning point to the next are given in Table 89, with the hourly rate 
of change; from these, we find that the change of pressure, from the morning minimum to the morning 
maximum, is greatest in winter and least in summer ; from the afternoon minimum to the evening maximum, 
it is least in winter and greatest in summer ; from the morning maximum to the afternoon minimum, it is least 
in spring and greatest in summer; from the evening maximum to the morning minimum, it is greatest in winter 
and least in summer. On the whole, when we compare the diurnal variations with respect to season, both as 
to the epochs and relative amounts of the oscillations, from turning point to turning point, we arrive at the fol- 
lowing conclusion :—1st, That the law of diurnal variation of atmospheric pressure at Makerstoun, is almost 
precisely the same in winter as itis in summer, if we substitute noon for midnight, and p.m. for a.m, in the 
former.* 2d, As the diurnal variation for spring is analogous to that for winter, and the diurnal variation 
_ for autumn is similar to that for summer, the same law of opposition holds for spring and autumn as for 
summer and winter. See Plate IX. 
3d, The whole diurnal oscillation is greatest in winter, and it is least in summer. 
PRESSURE OF THE WIND. 
198, In the volumes for the years 1843 and 1844, both the maximum pressures of the wind occurring 
betwiat the hours of observations and the observed pressures within 7™ to 10™ at the hours of observation were 
discussed ; as both discussions gave the same results, and as the latter make an approximation to the actual 
continuous mean pressures, only the means of the pressures occurring within 7™ to 10™ at the hours of obser- 
vation will be considered here. 
* This curious fact, it seems to me, is wholly opposed to what may be termed the temperature theory of the regular diurnal 
variation of atmospheric pressure ; the best marked barometric oscillation at Makerstoun occurs while the temperature and pressure 
of aqueous vapour are nearly constant, namely, in winter between 64 P.M. and 9 A.M. 
MAG. AND MET. OBS. 1845 AND 1846. 2a 
