Peessuee of the Wind. 



xcv 



4:th, 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. 



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 ; it is to be 

 observed, however, that the morning maximum occurs nearest noon in spring and farthest ffom 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 



p. M. Maximum to 



Whole Oscillations. 



Period 





A. M. Maximum. 



P. M. Minimum. 



P. M. Maximum. 



A. M. Minimum. 



























Total. 



Per Hour. 



Total. 



Per Hour. 



Total. 



Per Hour. 



Total. 



Per Hour. 



Sum. 



Per Hour. 







in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



Nov. Dec. 



Jan. 



0-0308 



0-0060 



00160 



0-0044 



0-0126 



0-0018 



0-0275 



0-0033 



00869 



0-0036 



Feb. Mar. 



Apr. 



•0210 



•0030 



•0140 



•0033 



•0180 



•0035 



•0250 



•0033 



•0780 



-0033 



May June 



July 



•0139 



•0018 



•0230 



•0026 



•0191 



•0031 



-0100 



-0024 



•0660 



-0027 



Aug. Sept. 



Oct. 



•0166 



•0035 



•0208 



•0030 



•0188 



•0031 



-0146 



•0023 



•0708 



•0030 



Year 





•0180 



■0029 



•0150 



•0027 



•0145 



•0026 



•0173 



•0026 



•0648 



•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 it is 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. 



3f?, The whole diurnal oscillation is greatest in winter, and it is least in summer. 



Pressuke of the Wind. 



198, In the volumes for the years 1843 and 1844, both the maximum pressures of the wind occurring 



I betwixt the hours of observations and the observed pressures within 7"* to 10™ at the hours of observation were 



i 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°i at the hours of obser- 



Ivation Avill 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 marlced barometric oscillation at Makerstoun occurs while the temperature and pressure 

 of aqueous vapour are nearly constant, namely, in winter between &^ P.M. and Q"" a.m. 



MAG. AND MET. OBS. 1845 AND 1846. 2 a 



