Report of the Kew Committee. 49 o 



where it is now stored awaiting further disposal. A paper describing 

 the operations, and giving full details of the values of the vibration 

 numbers obtained at Kew and at Greenwich, has been contributed to 

 the Royal Society by General Walker (see ' Phil. Trans.,' vol. 181, A, 

 p. 537). The result shows the number of vibrations made by a 

 seconds' pendulum in one day to be 0'64 greater at Kew than at 

 Greenwich. 



Cloud Photographs. At the suggestion of General Strachey, Chair- 

 man of the Meteorological Council, a new departure has been made in 

 the photography of clouds during the past year, with the view of 

 simplifying the operations of determining the height and velocity of 

 their movement. Both cameras have been rigidly fixed on their 

 stands, with the axes of their lenses pointed directly to the zenith, 

 and photographs are now taken simultaneously of the area of the sky 

 surrounding the zenith within a circle of a radins of about 15. 

 These photographs are superposed one on the other, so tLat the 

 two pictures shall appear to coincide, and a simple measurement of 

 the distance between the images of the zenith points, which are 

 marked by intersecting lines, gives a means of readily determining 

 the height of the cloud above the surface of the ground. A second 

 measurement made in like manner of the displacement of the zeniths 

 in a second pair of photographs taken after a given interval of time 

 serves to show the rate of travel of the cloud and the direction in 

 which it is moving at the instant of observation. Twenty groups of 

 clouds, giving heights extending from If miles to 8 miles, and rates 

 of motion from 5 miles to 64 miles per hour, have been photographed 

 and measured in this manner during the past summer. 



A wooden frame, 12 feet in height, has been constructed, which is 

 occasionally erected above each of the cameras in order to verify the 

 position of their zenith points and the orientation of the cross lines on 

 the photographic plates. 



V. VERIFICATION OF INSTRTTMENTS. 



The following magnetic instruments have been purchased on com- 

 mission and their constants determined : 



1 Inclinometer, for Padre Denza, Rome. 



2 Collimator magnets for Professor Chistoni, Italy. 

 1 Pair of dip needles for Hong Kong Observatory. 



1 Pair of small dip needles for Senhor Capello. 



A single dip needle repaired for Professor Mielberg. 



Sextant Testing. This branch of the work of the Verification 

 Department has been very active during the past year, 346 instruments 

 having been examined and certified, and tables of corrections 

 supplied. Owing to the decay of the photographed scales of some of 



