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



done well only at observatories where assistants are continually on the 

 watch, during the day at least; the employment of self-registering in- 

 struments has diminished the number of observers required in perma- 

 nent institutions, and observations of cloud-motions can now scarcely be 

 expected with the requisite completeness. On board ship, these obser- 

 vations are made with even more difficulty than on shore ; rapidly 

 moving upper currents, especially those in which clouds are seen moving 

 from very different or nearly opposite directions, catch the eye at once, 

 and are most frequently recorded. JS'o serious investigations can be 

 founded on observations when the most frequent motions are really not 

 observed. 



Believing that any just conclusions as to the atmospheric currents 

 could be obtained only from long and careful observations of cloud- 

 motion, I began a series of observations in 1842 at Makerstoun, which 

 I continued in the following years with my assistants, the late Mr. John 

 "Welsh, E.Ii.S., and Mr. A. Hogg*. Four currents were distinguished 

 at different heights. 1st, the surface current observed by the wind-vane ; 

 2nd, the current of loose cumulus {scud) and cumulus ; 3rd, the current of 

 cirro-stratus ; 4th, the current of cirro-cumulus and cirrus t. The ob- 

 servations of these currents have been only partially 'discussed ; I shall 

 give here some of the results which bear upon the subject of the present 

 investigation, derived from the observations made in the four years 1843 

 to 1846, for which the directions of the surface-wind and of the isobaric 

 lines have been already obtained. 



Direction of the Cirrus current. — Having found the mean direction in 

 which the cirri and cirro-cumuli moved for each day on which their 

 motion was observed during the four years, I have combined these by 

 Lambert's formula (on the assumption that the velocity was the same on 

 each day) in order to obtain the resultant direction (;//) for each month ; 

 these are given in Table YIII. 



^ The motions of the clouds were determined by seeking till a marked portion was 

 found which seemed to run up or down one of the foiir corners of the observatory, 

 the points of the compass relatively to which were marked on a surrounding paling ; 

 when no portion of cloud could be found that would pass or had passed through the 

 zenith, the vanishing-point of the. motions of different portions of cloud in the same 

 stratum could generally be ascertained very nearly. It is believed that in a great 

 majority of cases the directions were found to within half a point of the compass, 

 though the nearest point was always noted. In the year 1844 alone 4370 observations 

 were made of the directions of motion of the cloud-currents. 



t The same classification of cloud-motions was proposed by M. Poey to the Paris 

 Academy of Science in 1864 (Comptes Eendus, t. Iviii. p. 669), excepting that he sepa- 

 rated the loose cumuli from the cumuli : this separation I had also made, especially 

 in the north-east quadrant, in which the loose cumuli have a much lower position 

 than in the other quadrants ; but in taking the mean directions the two strata were 

 included in one. M. Poey has, I believe, recommended this system of observation of 

 the atmospheric currents to the Meteorological Qommittee of the Eoyal Society, a 

 recommendation in which I entirely concur. 



