Motions op Clouds. 301 



current is positive of the lower. If we take the mean difference from all the observations we arrive at the fol- 

 lowing results : — 



The direction of the scuds or current immediately above the surface wind is always plus, reckoning from 

 N- to E., S., and W., that of the surface wind, the average difference is greatest in the quadrant between S. 

 and W. ; in this quadrant the scud is, on the average of 197 comparisons, 19° more westerly than the surface 

 current. The difference is least in the quadrant W. to N., where the scud, on an average of 62 comparisons, 

 is only 3°-7 more northerly than the surface current. 



The direction of the cirro-strati, or current immediately above the scuds, differs most from the motion of 

 the surface wind in the quadrant from S. to W., being, on the average of 99 comparisons, 39°-4 more westerly 

 than the surface wind, or nearly twice the deviation of the scud. The difference in the quadrant E. to S. is also 

 positive, being 36°-9 on an average of 14 comparisons. In the other two quadrants the difference is very small. 



The direction of the cirri, or current immediately above the cirro-strati, differs most from the motion of 

 the surface wind in the quadrant E. to S., but there are only 3 comparisons. The difference in the quadrant 

 S. to W. is nearly the same as for the cirro-strati ; there is reason to believe, indeed, that, on many occasions, 

 the two species are confounded, as the cirri frequently pass into cirro-strati. 



The mean differences of the mean motion of the three currents, scud, cirro-strati, and cirri, from the motion 

 of the surface wind, are as follow : — 



o 



N. to E., 147 comparisons ; mean upper current minus surface current = -f- 6-2 



E. to S., 46 = -f- 23-2 



S. toW.,345 = +27-6 



W.toN., 112 =+ 2-5 



The difference of the directions of motion of the surface and mean upper current is therefore a minimum 

 in the quadrant from W. to N. ; the mean upper current is half-a-point S. of the surface current in the quadrant 

 N. to E. ; two points S. of the surface current in the quadrant E. to S. ; and about two points and a half N. 

 of the surface current in the quadrant S. to W. 



If all the cases be taken in which the motions of the upper currents of cirro-strati and cirri were observed 

 simultaneonsly with the current of scud, we obtain the following differences of motion of the mean upper cur- 

 rent (cirro-strati and cirri), and lower current of clouds : — 



N. to E., 32 comparisons ; cirro-stratus current minus scud cui-rent = + ll-I 



E. to S., 11 =- 5-8 



S. toW., 98 = +25-4 



W.toN., 71 = - 12-2 



The numbers of comparisons in the first two quadrants are too few for a good mean ; from the other it 

 appears that the current of cirro-stratus in the quadrant S. to W. is more than two points north of the current 

 of scud ; but, in the quadrant W. to N., it is about one point south of the scud. 



If the mean differences of motion, scud minus wind, and cirro-stratus minus wind, were well determined, 

 it is evident that, in subtracting the former from the latter, we would obtain the differences, cirro-stratus minus 

 scud; and similarly for the differences cirri mmws scud. 



The means for these two differences, thus obtained, are — 



o 



N. to E., cirro-stratus and cirrus current minus scud current = — 2-0 



E. to S., = +54-7 



S. toW., =+19-7 



W. toK, =; - 3-3 



As before, the difference for the quadrant E. to S. depends on too few observations, which is indeed the 

 case with all the quadrants excepting that of S. to W. 



MAG. AND MET. OBS. 1843. 4 g 



