1891.] Cloud Photography. 473 



similar process will be found tbe distance D a and a height of the 

 cloud determined from Z a . 



The position of the point vertically under the selected cloud-point 

 will be determined with sufficient accuracy graphically, by the inter- 

 section of the two distances measured from the ends of a line drawn 

 to represent the base. 



The repetition of this process for the second set of photographs will 

 in like manner give the position of the cloud-point after the interval 

 elapsed between the taking of the two sets of pictures, and the dis- 

 tance travelled being measured on the diagram, the velocity can be 

 found, and the direction of motion will be shown in relation to the 

 direction of the base. 



Irrespective of the laborious nature of this process, it was found 

 that the angles on which it was based were often so small that the 

 results obtained were inconsistent and unreliable. 



In 1890, therefore, it was decided to try another method of 

 observing, which would admit of much simpler treatment. This was 

 to fix the cameras so that the optical axes were directed to the zenith, 

 ind to photograph clouds which passed across the field of view which 

 is comprised within a circle described at an angular distance of about 

 15 round the zeniths of the two stations. The defect of this method 

 18 that it very materially limited the scope of operations, and reduced 

 }he opportunities of taking pictures to a comparatively small number, 

 :or it was found that a large proportion of the clouds which seemed 

 ipparently favourable for photographing when viewed by reflected 

 i jolar light incident upon them at oblique angles became almost 

 nvisible when observed directly overhead. This was notably the 

 ;ase with cirrus, some forms of which, especially those possessing the 

 lature of cirro-stratus, appear as practically structureless masses 

 vhen seen in this position. But notwithstanding these drawbacks, 

 tome of which, it is hoped, may be obviated, the advantages of this 

 nethod of observing seem to be sufficient to lead to its adoption in 

 reference to any other yet suggested. 



To adapt the cameras for work in this manner, both altitude and 

 .zimuth circles were permanently clamped, rendering them immovable 

 n both vertical and horizontal pla.nes, and the locking plates were 

 hifted on the pedestals, so that, while the fiducial lines on the 

 tictures intersect at the zenith, the direction of one of them is that 

 f the line joining the two stations, or the base, the other being at 

 ight angles to it. 



With the object of ensuring the proper adjustment of the optical 

 xes of the cameras, a tripod stand 12 feet in height was made, which 

 ras temporarily erected immediately over them. A plummet was 

 uspended directly above the lens-centre, from the point of intersec- 

 ion of two horizontal wires fixed at right angles to one another, one 



