1877.] 



the Lines of equal Barometric Pressure, 



535 



4. Cirro-stratus current minus surface current. 



Number of results. Mean difference. 



Total. ^PerlOOO. ^ 



754 788 183 29 -f22°-8 



5. Cumulus current minus surface current. 



Number of results. Mean difference. 



Total. ^i'erlOOO. ^ 



1434 791 171 38 +14°-5 



I£ we subtract the 5th mean difference from the 1st and from the 4th , 

 we find 



^ -•4."= + l5°-l, 



f -;//"=+ 8°'3. 



These two mean differences are slightly greater than were found by the 

 direct comparisons 2 and 3. 



If we give the 1st, 4th, and 5th mean differences weights corresponding 

 to the number of results from which they are obtained, then we find 



+ 190.1 ; 



and if the three mean differences receive equal weights, 



(•W-<^=+22°-3; 



where indicates, as before, the mean direction from the north (through 

 east) of all the species of clouds. The former of these results may repre- 

 sent approximately the observed difference when no attention has been 

 paid to the species of cloud : the latter may be considered an approxima- 

 tion to the mean difference for the whole mass of air in motion. Each 

 of these results is somewhat less than that found for Brussels (24°). 



"We have no knowledge of the mean law according to which the velocity 

 of the air in motion varies with height above the ground. Erom previous 

 observations, while observing the aerial currents at Makerstoun, I had 

 concluded that the maximum velocity occurred rarely at a greater height 

 than 6000 feet, and that the mea7i stratum of maximum velocity was pro- 

 bably not above 5000 feet. Erom this height, then, the velocity diminishes 

 as we ascend and descend, but according to different laws. At the surface 

 of level ground the velocity is, on an average, probably not one third of 

 that at the maximum ; the upper Limit, where the motion ceases, is in all 

 probabilit}'' little above the average height of the cirri. I had frequently 

 observed at Makerstoun that the angular movement of cirri passing through 

 the zenith was generally very much smaller compared with that of the 

 lower clouds than could be explained by the difference of heights had the 

 velocities been the same. 



