470 Lient.-O( 11- r.il Strachcy and Mr. Wliij.ple. [Apr. 



he was directed to read both his circles, and to enter the readings, 

 with the times of the two exposures and the numbers of the plate- 

 holders in his book, A doing the same for his own instrument. 



Having deposited the plate-holders in the light-tight carrying boi 

 another charged pair were taken, and a fresh cloud in another part 

 the sky selected, and the operations already detailed were repeat 

 until the stock of charged holders was exhausted. 



The observers ' then, by means of the telephones, again comi 

 their watches, and noting their differences, if any, sighted tl 

 cameras on each other, and read their mutual bearings and altituc 

 This was done in order to be sure no displacement had taken place 

 either the orientation or level of the instruments. They tl 

 unlocked the stands, dismounted the cameras, and put them away 

 the lockers of the pedestals, ready for use on another occasion, 

 veying the plates to the photographic laboratory for development 

 subsequent treatment. 



From time to time, the empty plate-holders were taken out, 

 lenses directed to each other, and settings made and circles 

 with the view of determining the true bearings of the fiducial 

 before described, from which the angular position of the cloud-j 

 dealt with were obtained. 



On removal of the exposed plates from the holders, the dates 

 the observation having been written on each of the films in pencil, 

 well as a register number, development proceeded. This was 

 ducted in a wooden tray with a glass bottom specially adapted 

 hold four plates. The two A's and two B's forming one set 

 pictures were usually selected for simultaneous development, in or 

 that the negatives obtained might possess the same degree of 

 tensity. Before hydrokinone became an article of commerce, 

 solution of pyrogallic acid or sulphate of iron was employed as 

 developing agent, but, since 1889, Edwards's hydrokinone develc 

 has been employed by preference, as being less liable to prodi 

 fogged plates. 



Owing to the efforts of the Kew observers being chiefly directed 

 photographing high cirrus clouds, very careful and slow developmc 

 was required, to produce satisfactory negatives, and it has 

 generally necessary to continue the operation for about forty minnt 

 to bring out a successful result. In some cases of very thin fil 

 cirrus, the so-called mare's tail clouds, the development occuj 

 l hours, before the picture appeared. 



For discussion of the photographs, in most cases prints were 

 of the negatives by the ordinary albuminised paper process. 



Various methods of obtaining the heights and velocity of motion 

 the clouds from the photographs thus made have been attempt 

 The computation by the ordinary trigonometrical formulae from 



