480 



Cloud 



[Apr. 30, 



vertical lines from M and N at points P and Q ; and the length of the 

 parallax p, on the horizontal line from M towards N, at a point R. 

 Join P, B, intersecting the vertical through N at S. Then QS 

 represents the drift on the scale assumed to represent 1 mile. Let 

 this be marked off upwards on the vertical line drawn on the scale of 

 proportional velocities, fig. 3, from the seconds division correspond- 

 ing to the time interval between the pictures, and the velocity of 

 drift will be indicated by the radiating line nearest to the mark thus 

 made. 



The scales above described for the graphical determination of the 

 cloud heights and velocities are engraved and printed on sheets of 

 paper, which, after the computations are completed by their aid, will 

 serve as convenient records of the observations. 



After a little practice, the whole of the processes requisite for these 

 determinations from the glass plate -negatives of a complete set of 

 four pictures will not exceed 20 minutes. Quite sufficient accuracy 

 is ensured, and the labour and risk of error arising from the use of 

 tables is entirely avoided. 



Although the cameras now in use only embrace a circle of angular 

 diameter of about 30, trials have been made with a lens which gives 

 satisfactory pictures of double that extent, which is probably as much 

 as could be desired. 



The following is a list of the determinations made during the past 

 year by the methods now described : 



