1877.] the Lines of equal Barometric Pressure, 531 



Table YIII. — Eesultant Direction of Cirri and Surface-wind, 

 1843-1846. 







Cirri 

 resultant. 



Wind 





IVTonfli anrl Vpnv 



1^0. of 

 days. 



Dir. 



]yo. 



res. 

 direc. 









^. 



p. 100. 



<p. 









o 





o 



o 





57 



279 



62-6 



241 



+38 



February 



41 



303 



62-7 



287 



+ 16 



March 



42 



301 



62-7 



244 



-J- ,57 



-A.pril 



50 



242 



39-4 



274 



— 32 



May 



44 



291 



39-9 



24 



-94 





51 



263 



56-9 



233 



+30 





42 



269 



58-0 



230 



+39 





56 



267 



48-0 



247 



-(-20 





34 



274 



60-4 



244 



+30 





35 



276 



56-0 



237 



+39 





38 



235 



56-9 



211 



+25 



December 



44 



308 



69-3 



249 



+59 



1843 



119 



276 



48-0 



249 



+27 



1844 



155 



288 



56-4 



249 



+39 



1845 



150 



280 



50-9 



247 



+33 



1846 



110 



259 



59-1 



229 



+30 



4 years 



534 



277 



.520 



243 



+ S4 



I have entered for comparison the resultant directions {(f) of the sur- 

 face current at Makerstoun. It T\-ill be seen that in ten months of the 

 year the A^alues oi — <p are positiv e ; in two months, April and May, 

 they are negative* : the greatest positive difference is that for December. 



When we determine the resultant directions from all the observations 

 in each of the four years (see Table VIII.), we find that the difference of 

 directions of the cirrus and surface currents may be given as 3-1° + 2°. 



It has to be pointed out, however, that these directions are not strictly 

 comparable ; the direction of the wind is observed at all hours when the 

 wind blows ; those of the currents carrying clouds can be observed only 

 when clouds exist in them. The motion of the ciiTi \\'as observed at 

 Makerstoun only in tw o days out of five, whereas the direction of the 

 wind was observed on most days in each year. This fact may affect the 

 exactness of the differences for limited periods ; but it will be shown 

 that it affects little the final results. There is, however, a correction re- 

 quired that should not be neglected. The observations of cloud-motions 

 were generally during the day, that is, on the mean of the year, from 



* This difference is, I believe, chiefly due to the north-east winds which blow so 

 frequently in these months. The [scud carried by this current is the lowest cloud 

 observed. This and the cirrus stratum appear generally to be set in motion by 

 different causes ; the cirri move rarely from an easterly point, and the maximum of 

 frequency shown for north-east winds in Great Britain does not appear in the results 

 for the cirrrus stratum (Comptes Eendus, t. Ixxxi. p. 34, 1875). 



TOL. XXY. 2 Q 



