234 HEADQUARTERS OF SURVEY DEPARTMENT. 



the three pendulums to officers of the United States Survey, who took 

 them round the world and swung them at Auckland, Sydney, Singa- 

 pore, Tokio, San Francisco, and finally at Washington. When Colonel 

 Herschel's observations came to be finally reduced, it was found that 

 the relation of Kew to Greenwich by one of the pendulums differed 

 by more than six vibrations from the values by the two other 

 pendulums. Revisionary swings were therefore made at the two 

 observatories, with all three pendulums, by the observatory staff 

 at each place. The final result gives the daily vibration number at 

 Kew an excess of - 64 of a vibration over that at Greenwich. 

 Geueral Walker has given an account of Colonel Herschel's work and 

 the revisionary operations at Kew and Greenwich, with an abstract 

 of his results and those of the officers of the United States Coast 

 and Geodetic Survey at other stations, in the Philosophical 

 Transactions, Vol. 181 (1890) A., pp. 537-558. 



At the Dehra office the solar photographs referred to at page 340 

 of the Memoir on the Indian Surveys, 2nd edition, were taken under 

 the care of an observer, Mr. C. Meins, who had been trained by 

 Mr. Norman Lockyer. Mr. Meins died on the 31st March 1879, 

 and nine months later the observations were resumed by Mr. L. H. 

 Clarke. The photo -heliograph employed gave 4-inch images of the 

 sun, but by means of an enlarging combination, 8-inch pictures 

 were obtained, while the definition in no way suffered. In July 1882, 

 however, the great photo-heliograph sent out by the Secretary of 

 State, and capable of taking 12-inch pictures of the sun, was received, 

 and after some difficulty connected with the period of exposure, and 

 some delay entailed by the construction of tho necessary rotating 

 dome, good and regular pictures were eventually secured. The 

 difference between the amount of invisibility in India and in the 

 sunless climate of England is very striking ; in 1880-81 at Dehra Dun 

 the percentage of dark days was 15 ; in 1881-82 it was 10, and in 

 1882-83 it was 13, as against 50, which was the average percentage 

 of " invisible" days at Greenwich during the same period! The 

 8-inch pictures have been continued jmri passu with the 12 -inch 

 photographs, the former being used for the measurement of the 

 areas of spots and facula?, while the latter are specially suited for 

 the study of the mottling or granular appearances of the normal 

 photosphere and structure of the penumbra of the spots. The results 

 of tho observations are included by the Astronomer Royal in the 

 yearly volumes of the Greenwich observatory. 



