116 REPORT— 1881. 



great breakers on the shore at a distance of two miles caused a deviation 

 of the vertical of l"'l. On one occasion the vertical seems to have varied 

 through 3" '2 in 3| hours. 



M. d'Abbadie also quotes Elkin, Yvon Villarceau, and Airy as having 

 found, from astronomical observations, notable variations in latitude, 

 amounting to from 7" to 8". 



As M. d'Abbadie did not consider levels to afford a satisfactory 

 method of observation of the presumed changes of horizon, he deter- 

 mined to proceed in a different manner. 



The site of his experiments was Abbadia, in Subernoa, near Hendaye. 

 The Atlantic was 400 meters distant, and the sea level 62 meters below 

 the place of observation. The subsoil was loamy rock (joclie marneuse), 

 belonging to ci-etaceous deposits of the South of France. Notwithstanding 

 the steep slope of the soil, water was found at about 5 meters below the 

 surface. 



In this situation he had built, in 1863, a concrete cone, of which tho 

 external slope was one in ten (una inclinaison d'une dixieme). The con- 

 crete cone is truncated, and the flat surface at the top is 2 meters in 

 diameter. It is pierced down the centre by a vertical hole or well 

 1 meter in diameter. This well extends to within half a meter of the top, 

 at which point the concrete closes in, leaving only a hole of 12 centi- 

 meters up to the flat upper surface. 



From the top of the concrete down to the rock is 8 meters, and the 

 well is continued into the rock to a further depth of 2 meters : thus 

 from top to bottom is 10 meters. 



A tunnel is made to the bottom of the well in order to drain away the 

 water, and access of the observer to the bottom is permitted by means of 

 an underground staircase. Access can also be obtained to a point half. 

 way between the top and bottom by means of a hole through the con- 

 crete. At this point there is a diaphragm across the well, pierced by a 

 hole 21 centimeters in diameter. The diaphragm seems to have been 

 originally made in order to support a lens, but the mode of observation 

 was afterwards changed. The diaphragm is still useful, however, for 

 allowing the observer to stand there and sweep away cobwebs. 



The cone is enclosed in an external building, from the roof of which, 

 as I understand, there hangs a platform on which the observer may 

 stand without touching the cone ; and the two staircases leading up to 

 the top are also isolated.' 



On the hole through the top of the cone is riveted a disk of brass 

 pierced through its centre by a circular hole 21 mm. in diameter. The 

 hole in the disk is traversed across two perpendicular diameters by fine 

 platinum wires ; at first there were only two wires, but afterwards there 

 were four, which were arranged so as to present the outline of a right- 

 angled cross. The parallel wires were very close together, so that the 

 four wires enclosed in the centre a very small square space. 



At the bottom of the well is put a pool of mercury. The mercury 

 was at first in an iron basin, but the agitation of the mercury was found 

 sometimes to be so great that no reflection was visible for an hour to- 

 been made with pendulums in Italy, but that it does not distinctly appear that the 

 variations of level are simultaneous over wide areas. No reference is given as to 

 the observers. 



' This passage appears to me a little obscure, and I cannot quite understand the 

 arrangement. ■ - ' 



