Meteorology. 299 



is melted and added to that which had been caught in the tiask. [Z. 

 G.M., XVIII, p. 68.) 



149. Dr. W. Zenker states that after using for many years a method of 

 determining the heights of clouds, which afterwards developed what is 

 known as Braun's nephoskope, he then devised a method for photograph- 

 ing the clouds, which, however, he was not able to put in practice until 

 1882, and with his present experience he is able to describe an arrange- 

 ment which he thinks will be thoroughly satisfactory, and which is 

 about as follows : Two or three photographic cameras of the same focal 

 length are needed, two of which are placed on the ends of a base line, their 

 axes parallel to each other, directed toward the cloud, and simultaneously 

 exposed and closed. His own apparatus has the focal length 0.5 meter, 

 the objective being an achromatic telescope lens, the sensitive plate 

 being set at the chemical focus for parallel rays. The plates are gelatine 

 dry plates ; the simultaneous opening and closing of the a])paratus must 

 be obtained by electric contact, so that only a momentary exposure will 

 be made ; a third camera, stationed alongside of these two and parallel 

 to them, can be exposed ten seconds later. By this third photograph 

 the movements and changes in the clouds can be determined. We can 

 also obtain this third photograph by a second exposure of either one of 

 the first two cameras, which method has the advantage of being inde- 

 pendent of the adjustment of the two axes. The distance of the two 

 cameras from each other may ordinarily be 100 meters. There are three 

 methods of pointing the cameras, each of which has its advantages : {!) 

 if they are pointed towards the zenith we have the best position for 

 determining the height of the clouds, while the direction and velocity 

 of movement are determined directly without computation; the direc 

 tion of the four principal points of the compass can be photographed 

 upon the plate by the shadows of wires placed immediately above it; 

 (2) pointing the camera towards the sun : this has the great advantage 

 that we can utilize the picture of the sun's disk to obtain the direction 

 or the i^arallelism of two directions with greatest safety ; measurement 

 from the sun to the cloud gives us the parallax of the cloud to a minute 

 of arc; (3) pointing the camera toward the horizon: this has the ad- 

 vantage that the clouds are photographed in vertical section, and, 

 moreover, there is no danger that any cloud feature appearing in one 

 photograph will be obscured in the other by intervening portions of the 

 same or other clouds ; but this advantage is balanced by the fact that 

 the distance of the clouds in the horizon is alwaj^s great, and the base 

 line of 100 meters must be proportionately increased ; an error in the 

 direction of the axes of the cameras is therefore here of greater impor- 

 tance, and to counteract this it is important to photograph on the 

 same plate some distant object (e. g., a church spire), from which as a 

 base the horizontal and vertical angular measurements can be made. 

 Zenker states that we can thus follow in the cirrus clouds the sinking, 

 step by step, of the upper warm and moist current of air ; the rising of 



