1877.] 



the Lines oj equal Barometric Pressure, 



533 



Year. (ip). 0. {4')—(p. 



1842 230 221 + 9* 



1843 257 232 +25 



1844 283 251 +32 



1845 251 219 +32 



1846 252 216 +36 



5 years 255 226 +29 



I£ we admit that a correction should be applied, as at Makerstoun, 

 on account of the di:fference of the direction of the wind during the day 

 hours when the cloud-motions are observed, we find for Brussels 



The agreement of the results for the last four years, which are the 

 same as in the preceding discussion, is very remarkable ; thus the direc- 

 'tion of the surface-wind at Brussels became 32° more southerly in 1845 

 than in 1844, and the direction of the cloud-motion changed exactly by 

 the same number of degrees and in the same direction. A similar 

 though less marked result wiU be seen at Makerstoun (Table YIII.), the 

 cirri moving 'from a point 21° more southerly in 1846 than in 1845, 

 while the surface current changed 18° in the same direction. 



It has been remarked (p. 531) that the wind and cloud directions are 

 not exactly comparable, as the latter cannot be observed so frequently as 

 the former ; this fact has less weight in the results for Brussels than in 

 those for Makerstoun, since the former include clouds of all kinds, 

 which are observed much oftener than the cirri alone. There is, however, 

 another method of comparing the directions of motion which was em- 

 ployed by me at first in the discussion of the Makerstoun observations 

 for 1843. The differences of the directions of the surface and cloud 

 currents when observed simultaneously were noted : when several such 

 comparisons made at successive hours gave nearly the same difference, 

 the mean was termed a " result ;" when the differences varied con- 

 siderably two or three results might be obtained in the same day. From 

 these results for the four years 1843 to 1846 the following mean 

 differences were obtained : — 



1. Cirrus current 7ninus surface current. 



Number of results. Mean diflference. 



. T Per 1000. , ^ 



Total. _j_ _ — r~0' 



359 769 181 50 +29°-6 

 The total number of results was 359 ; as the motions of tne cirri were 

 observed on 534 days in the four years, there were several days on which 

 no surface current was blowing when the motions of the cirri were 



* The small value of {\p) — (p for 1842, compared with those for the following years, 

 may perhaps be due to some instrumental cause, as this was the first year in which 

 the anemometer was employed, 



2q 2 



