1877.] 



the Lines of equal Barometric Pressure. 



529 



that the directions o£ the isobars varied most in the same months, May 

 being the month of greatest variability*. 



I have added the perpendicular distances (I) between the tenth-inch 

 isobars in geographical miles (deduced from the values of /3, second 

 column of Table A^II.). These intervals show a remarkable constancy of 

 ratio to the Aariability of the wind. In the mean for the four years at 

 Makerstoun, 



M 0-52 



-g-=^=2-60; 1=278 miles. 



The intervals between the tenth-inch isobars increase with the variability of 

 the wind. These, it will be remembered, are mean results. The mean 

 pressure of the wind has its mean value in the two months of April and 

 May, when the direction is most variable ; but in September, the next 

 epoch of maximum variability, the mean pressure is a minimum. 



It appears also from the fourth column of Table YIII. that the direc- 

 tion of the cirrus current was most variable in April and Ma}'' ; the next 

 epoch of maximum variability was, however, in August instead of Sep- 

 tember, as for the surface current. 



3^ Atmosplieric Currents. 



In the preceding investigations we have employed the observations of 

 the direction of the surface current only. We require greatly to know 

 to what extent the direction would vary in ascending from the earth. 

 The late Mr. Johnson, Director of the Oxford Observatory, found that 

 an anemometer at a height of 110 feet from the ground showed a wind- 

 velocity two and a quarter times that indicated by a similar instrument 

 22 feet from the soil ; part of this difference was probably due to build- 

 ings and trees in the neighbourhood ; but there can be little doubt that 

 proximity to the soil is a cause of diminished velocity. I am acquainted 

 with no observations, made at points -free from ail obstructions, which 

 can answer the question, whether the direction of the wind varies con- * 

 siderably within a few hundred feet of the ground. The only way, then, 

 of investigating this subject must be by comparisons of the direction of 

 the T^'ind near the surface, in well-exposed localities, with that of the 

 motions of the clouds. 



Such observations require in general much time ; to be of any use to f 

 science they should be made frequently and systematically. This can be 



* I may remark here that the variability of the direction of the -wind and of the 

 intervals and direction of the isobars have no relation whatever to the diurnal varia- 

 tion of temperatiu'e, nor to the variation of temperature from day to day. The 

 diurnal range of temperature vras greatest in the mean of the four years in June and 

 August. The difference of the daily mean temperature from the monthly mean was 

 least in August, May, and July ; less in these months than half the mean difference 

 for January (5°). ("Eesults of Makerstoun Observations," Trans. Roy. Soc. Edinb. 

 vol. xix. pt. 2, p. Ixxxv.) 



