394 Intelligence and Miscellaneous Articles, 



large blocks of the same rock also occurred. The author described 

 the beds displayed in a section close to the base of the great land- 

 slip, above which is a smooth surface of greenstone, covered 

 with prostrate trees and immense blocks of greenstone half buried 

 in yellow clay and sludge. The whole neighbourhood was described 

 by the author as presenting evidences of former landslips. The 

 author further described the appearances presented by the upper 

 winding part of the gully traversed by the torrent, and, in con- 

 clusion, noticed certain results of similar phenomena as displayed in 

 the same district. The paper was illustrated by three stereoscopic 



LI. Intelligence and Miscellaneous Articles. 



A NEW DETERMINATION OF THE VELOCITY OF LIGHT. 

 BY M. A. CORNU. 



1 HAVE the honour to present to the Academy the definitive re- 

 ■*- suit of my researches relative to the determination of the velocity 

 of light, undertaken three years since. In a preceding communica- 

 tion I have succinctly described the method of observation, which, in 

 principle, is that of the toothed wheel, due, as well as the improve- 

 ments which have been made in it, to M. Fizeau. Of the latter I 

 will mention the electrical registry of the velocity of the mechanism 

 — which it is necessary to ascertain at each instant in absolute 

 value, because it is with it that the velocity of light is directly com- 

 pared. By the regularity and concordance of the results of my first 

 essays I was led to hope that, by repeating the experiment between 

 two new stations at four times the distance from each other, I 

 should be able to obtain a determination sufficiently precise to de- 

 cide between the two values given, the one by the old astronomical 

 data (308000 to 310000 kilometres in a second), the other (298000) 

 by the experiments of Foucault with the rotating mirror. I am 

 happy to announce that the accuracy I expected (" probably to less 

 than a hundredth ") has been even exceeded ; and the question of 

 the absolute value of the velocity of light appears to me to be de- 

 cided in favour of the lower number, as will be seen from the 

 numerical results which I have obtained. 



I will first briefly describe the arrangements. ,The observing- 

 station is in an upper room of the pavilion of the Ecole Polytech- 

 nique ; the other, in the chamber of one of the barracks on Mont 

 Valerien. At the former station are fixed the observing-telescope 

 (aperture 180 millims., focal distance 2*4 metres), the toothed wheel 

 and its motive mechanism, the illuminating system, the apparatus 

 for registering the velocities, the electric wires, &c. The opposite 

 station contains only the reflecting collimator, composed of an ob- 

 jective (aperture 110 millims., focal distance 1*2 metre) mounted as 

 a telescope and with a small plane mirror of silvered glass in the 

 focal plane. 



