110 



Anniversary Meeting. 



[Nov. 30, 



predated. These considerations induced Maclear, shortly after his ap- 

 pointment to the Cape Observatory, to plan the verification which he has 

 now accomplished. Pursuing the still earlier inquiries of Sir George 

 Everest, he succeeded, though with considerable difficulty, in recovering 

 La Caille's terminal stations ; and, aided by the advice and encouragement 

 of Sir John Herschel (then at the Cape) and of the Astronomer Royal, he 

 commenced the work of a remeasurement in 1836. The proceedings were 

 necessarily tedious : the measurements of the base, of the triangles, and of 

 the zenith-distances were repeated to an extent and with precautions un- 

 practised at the earlier period. The zenith-distances were observed with 

 the sector with which Bradley discovered the aberration of light and the 

 nutation of the earth's axis, intrusted to Maclear by the Admiralty. The 

 terrestrial angles were taken with a 20-inch circle by Jones, and a smaller 

 theodolite by Reichenbach, both of remarkable precision. The base, from 

 which all the distances were derived, was measured with the compensation 

 bars used in the Irish triangulation. Thus, in respect to the means em- 

 ployed, this arc of the meridian may be regarded as inferior to none on 

 record. A full account of the whole was completed in 1S6G, and has been 

 published by the Admiralty in two quarto volumes. It does not confirm 

 the abnormal value obtained by La Caille, but shows a probable cause for 

 the discordance. La Caille's northern station was in a hollow surrounded 

 by mountains, one of which, half a mile distant to the north, was a mass of 

 rock 2000 feet high ; and others, at distances somewhat greater, were still 

 near enough to create disturbance. A station so situated was obviously ill 

 suited to be a terminal station ; and the triangulation was therefore ex- 

 tended across an immense plain of sand to a point without any visible source 

 of local attraction. By this extension, and by a similar one to the south, 

 Maclear's arc has an amplitude nearly four times as great as that of La 

 Caille, and is, on this account, as well as on account of the greater accuracy 

 in detail, far more deserving of confidence. The degree which is derived 

 from it is 1133 feet shorter than that of La Caille ; and as La Caille's is 

 1051 longer than that given by the spheroid which, according to Airy, 

 represents the average of northern arcs, Maclear's determination is evidently 

 a near approximation to the truth. This is even more distinctly shown by 

 the close agreement of the latitudes computed from the geodetic measure- 

 ments with those given by the sector — that of the north extremity being 

 0"4" in defect, that of the south extremity 0" 5 in excess. 



Captain Richards, 

 We should indeed have been happy to have had Sir Thomas Maclear's 

 presence among us ; but in his present unavoidable absence I will request 

 you to receive this Medal on his behalf, and to transmit it to him with the 

 assurance of the very great pleasure which it will give to the Society to 

 welcome him on his return to his native country. 



