520 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1915. 



International Astrographic Chart and Catalogue comes to be writ- 

 ten it will probabl}^ be found that much was due to GilFs initiative. 

 It may be difficult to trace whence the first suggestion arose, but at 

 least we know that he was in its councils from the very beginning 

 and gave his whole-hearted support to the great enterprise. His 

 measuring machine for photographic plates, designed by him and 

 constructed by Repsold, has been very generally copied in its main 

 features. Another work of great value which owes much to his coun- 

 sel and assistance is the chart of the sky made by the late J. Franklin- 

 Adams. Mr. Franklin- Adams, an enthusiastic amateur, who had 

 only recently applied himself to astronom}'', came to the Cape at an 

 early stage of the work to photograph the Southern Hemisphere. It 

 needs little imagination to realize how Gill, by his experienced ad- 

 vice and his insistence on a high standard of quality, helped to make 

 of this the valuable work that it became. 



In 1897 the necessary expenditure for a new reversible transit circle 

 at the Cape was at length sanctioned. Since his first appointment 

 Gill had lost no opportunity of urging the need for an instrument 

 which should be free from the defects which were obvious in the old 

 design. For the determination of fundamental right ascensions and 

 declinations the chief requirements are an extreme stability of the 

 instrument, means of eliminating or determining the flexures of the 

 various parts, and of guarding against the effects of temperature 

 changes both in the instrument and in the surrounding air. The 

 problem of equalizing the distribution of temperature was most care- 

 fully thought out. The piers were made hollow, covered externally 

 with nonconducting material, and filled with water. The telescope 

 tube was surrounded by a double envelope of copper to minimize the 

 effects of local heating, and the graduated circles were similarly pro- 

 tected by copper disks. Of special interest was Gill's method of ob- 

 taining fixed meridian marks for maintaining the azimuth of the 

 transit circle. Four deep pits, reaching down to the unweathered 

 rock, were constructed underneath the long- focus collimating lenses 

 and the marks respectively, and a simple method was devised by 

 which the apparatus above ground could be readily set in a definite 

 position with respect to the vertical collimating lines of object glasses 

 fixed in the rock below. So perfect is the stability of these marks 

 that it has been found possible to measure the movement of the North 

 Pole over the earth's surface by the apparent change of azimuth. It 

 is certain that the device will be widely imitated in future. 



On his appointment as H. M. astronomer, in 1879, Gill began to 

 consider the question of a geodetic survey of South Africa. His 

 previous experience of such work had been obtained on the occasion 

 of his visit to Mauritius. In connection with the Transit of Venus 

 expeditions of 1874, numerous longitude determinations were made 



