534 Mr. J. A, Broun on the Directions ^c, of [Feb. 1, 



determined. The numbers of results in 1000 for which ^ — was 

 positive ( + negative ( — ), or the same ( = ) are given. The final value 

 of — is the same to a fraction of a degree as that founds p. 532. This 

 confirmation is of importance, as it shows that the method of calculating 

 the resultant directions does not affect the accuracy of the final values. 



It is probable that this difference between the directions of the cirrus 

 and surface currents will not take place per saltern, and that the obser- 

 vations of cloud-motions at different heights should show some variation 

 in the value of -ip — cp. If we should find that the lowest stratum of 

 clouds, that of scud and cumuli, which may be considered generally at 

 from 2000 to 5000 feet above the ground, has its direction intermediate 

 between that of the cirrus and that of the surface, we shall have a most 

 conclusive proof, were other proof needed;, that the diff'erences of motion 

 found are not due to local causes. 



In the four years 1843 to 1846, 339 results were obtained from 

 simultaneous observations of the cumulus and cirrus motions ; from these 

 and comparisons of the other currents the f oUovring quantities are derived, 

 where xj,, \p', -ip", and ^ are the mean directions of the cirrus, cirro-stratus, 

 cumulus, and surface currents respectively : — 



2. Cirrus current minus scud and cumidus cairrent. 



Number of results. Mean difference. 



Total. ^PerlOOO. ^ 



339 666 251 83 +13°-7 



The difference is somewhat less than half of that between the cirrus 

 and surface currents. As the currents approach each other the differences 

 become smaller, and the relative number of negative results increases. 

 Thus when we examine the 683 results derived from simultaneous ob- 

 servations of the cirro-stratus and cumulus currents which pass into 

 each other, we find : — 



3. Cirro-stratus current minus cumulus current. 



Number of results. Mean difference. 



Total. _^Perl000. ^ 



683 568 297 135 +6°-9 



which is again nearly half the difference between the cirrus and cumulus 

 current. 



Having found, I think conclusively, that the differences of motion 

 observed between the surface and cirrus currents are independent of 

 circumstances of locality, we may with confidence examine the results 

 derived from the more numerous comparisons of the surface current 

 with the cirro-stratus and cumulus currents. Thus we find : — 



