102 



KEPORT 1882. 



mucli larger amplifying power, so as to try to register the variations of 

 the plumb-line.' 



In a letter to me, M d'Abbadie mentions an attempt by Brunner to 

 improve M. Bouquet de la Grje's apparatus, but considers that the attempt 

 was a failure. 



He also tells me that Delaunay directed M. Wolf to devise an 

 apparatus for detecting small deviations of the vertical, and that the 

 latter, without M. d'Abbadie's knowledge, adopted his rejected idea of 

 a pendulum, about 30 meters long, bearing a prism at the end by 

 reflection from which a scale was to be read by means of a distant small 

 refractor. The pendulum was actually set up, but the wire went on 

 twisting and untwisting until Delaunay's death, and no observations were 

 made with it. Our own experience is enough to show that nothing could 

 have been made of such an instrument. 



M. d'Abbadie gives further explanations of a passage in his own paper 

 about the arrangements for the staircases for access to and observation 

 with his Nadirane. In writing the Report of 1881 I had found the 

 description of the arrangements difficult to understand. 



The woodcut below is a copy of the rough diagram that he sent me. 



There were three staircases : — 



T cut in the rock; C B to ascend from the cellar-flags C D; and, lastly, 

 A S to mount from the boarded ground floor, A B, to the small floor S N, 

 which was hung from the roof. The two upper staircases did not touch 

 the truncated cone of concrete anvwhere. 



Judging from this figure, I imagine that the concrete cone has an 

 external slope of ten in one, instead of one in ten as stated in last year's 

 Report ; the French expression was ' une inclinaison d'un dixieme.' 



M. d'Abbadie informs me that the apparently curious phenomenon of 

 the ' ombres fuyantes,' which were observed in the reflection from the 

 pool of mercury, to which we drew attention last year, was of no signifi- 

 cance. It arose from the currents of air caused by a candle left standing 

 on the staircase T cut in the rock. The light was required for pouring 



