i 
lil 
uly 3, 1873] 
NATURE 
185 
strong and small, for it is not bigger than the fists. He uses 
the works of a common clock, which do not cost more than 
asoyereign. He has only replaced the largest wheel of 
the scapement by another one, lighter and more finely 
toothed. Special experiments, not mentioned in his 
present memoir enabled him to choose the most proper 
diameter for that cog wheel. A strong spring drives the 
wheel 700 or 800 revolutions in a second. 
A drag has been added, in order to check the speed. 
By a special arrangement, the rotation of the wheel can 
be reversed, in order to eliminate certain errors that 
might result from the apparatus itself. 
In order to try the improvements of the apparatus, a 
first series of experiments was made between the Poly- 
technic School and a tower of the telegraph office, at a 
distance of about one mile and a half (2 kilometres and 
a half). The observer could perceive a window of this 
tower amid a forest of chimneys. The distance was too 
short : he prudently did not publish the result. 
‘A second series was attempted by him between the 
Polytechnic School and the Valérien Hill, at a distance 
of about six miles and a half (10 kilometres 310 metres). 
But a transparent atmosphere is seldom now to be ob- 
tainedin misty Paris If we goup tothe garret where the 
observer stands, we perceive asea of roofs below ; on the 
right Montmartre Hill, on the left the heights of Meudon, 
and in the front the Valérien fortress ; in one of the rooms 
in the barracks the mirror and the collimator were esta-- 
blished. 
The apparatus that sends forth the ray of light 
(an instrument with a large aperture) was laid on a 
solid timberwork ; in front of the eyepiece is the little 
machine; on the left side the source of light is esta- 
blished, a ray of which, reflected by a glass, is sent 
between two teeth of the wheel. 
But the Mont Valérien is concealed by mist ; the win- 
dow of the barrack is hardly distinguishable, although the 
sky is cloudless. Paris is covered with a damp and dusty 
veil. The sun sets behind the fortress, and suddenly the 
mist disappears and the air becomes transparent. The 
ray of light between the teeth of the wheel is to be seen 
in the telescope as a faint star in the midst of the inverted 
image of the window ; it is a star of the sixth magni- 
tude, the intensity of which increases and becomes of the 
first magnitude with the transparency of theair. Butit is 
necessary to make the experiments hastily, for that trans- 
parency will not last more than one hour. * 
‘An obstacle nearly checked the observer; the image 
often scintillated, and was agitated in such a manner that 
it was impossible to pursue the experiment. It was the 
warm air of a chimney unluckily standing in the way of 
the ray of light, the kitchen chimney of the Lycée Louis 
le Grand. M. Cornu waited for the holidays, and the 
operations were at last worked out. 
‘He thus made more than a thousand experiments, and 
calculated 690 of them. 
In order to determine the distance between the two 
stations, he compared the measures previously deter- 
mined, and made himself a triangulation ; the average of 
those numbers gave him the number above cited, about 
six miles and a half (10 kilometres, 310 metres). 
e did not at once take the average of the numbers of 
his experiments, but he gave a greater value to the num- 
bers obtained under the best circumstances. It appears 
evident that the results deduced from the fifth disappear- 
ance of the light are superior to those deduced from the 
first one, because of the more exact value of the velocity, 
of the wheel, and that the favourable atmospheric con- 
dition rendered the disappearance and reappearances 
of light more plain. 
The average thus obtained gives for the velocity of light 
* The source of light was Drummond’s lime-light, or only a petroleum 
lamp. Tt was necessary sometimes, in the finest weathers, to moderate 
it, in order to have a disappearance of light more* favourable to observations 
than a minimum of intensity, 
189,300 miles in a second ; by dividing the number by 
the refractive indices of the air (10003) we obtain the 
number 189,209 miles in a second in a vacuum; the 
possible error in this value is about 349 - 
M. Fizeau had found about 194,000 miles (312,000 kil.) ; 
Foucault 189,000 miles (298,000 kil.) The physi- 
cists will wonder at the concordance between M. Cornu’s 
number and that of Foucault, obtained by an entirely 
different method ; and so will the astronomers ; for this 
number of 189,090 miles gives by calculatinz the value of 
the parallax of the sun the number 8”.86; and it is 
exactly the one recently obtained by M. Leverrier as a 
consequence of three series of observations made on the 
movement of planets, particularly of Mars and Venus. 
If experiments on the velocity of light were made again 
under good topographic and atmospheric conditions, and 
between two stations, the distance of which would be 
known by a geodetic calculation, a value of this velocity 
would be obtained with an error less than ;ylyg. Astre- 
nomical methods do not easily perhaps give such ana 
approach. 
The author concludes his paper by saying : “It is to 
be desired for the honour of French science, that those 
great works relative to the velocity of light, begun by 
Roemer at the observatory of Paris, pursued and simpli- 
fied by some learned Frenchmen, should be finished in 
France with a precision worthy of their astronomical and 
physical importance,” M. C, 
Explanation of the Diagram (see next page) | 
A, Source of light; apetroleam lamp. B, a combination of 
lenses to direct and concentrate the light. C,D,E, F, areshown 
from above in order to show the direction of the ray of light :— 
C, glass plate, on the surface of which the light is reflected and 
sent into the telescope according to the direction of the arrows ; 
a isa little handle which permits of small motions being given to the 
little platein order to arrange it properly. D, works of a common 
clock drawn to the 4 of its linear dimensions. —It is used to put 
in motion the cog wheel Y, between the teeth of which the ray 
of light is sent forth. JV, wire touched by a cog d of the axis 
of the third wheel, at each revolution ; it is united to the electro- 
magnet z (of the plate G), and thus the number of revolutions 
during a second is registered by. 4,, 63, two barrels that give 
revolving motions in a contrary direction, in order to eliminate cer- 
tain errors that might result from the apparatusitself. 4, a wheel 
ontheside of whichadrag H_ bears (H has been drawn apart for 
greater clearness) ; 7, horizontal axis of rotation ; V, screw; when- 
ever bymeans of V the rod is brought to g, inthe same manner is 
brought to w, and the extremity ¢ does not rub on the side of the 
wheel. (Note.—A decreasing pressure is thus used, an in- 
creasing one is rendered impossible so as to prevent the delicate 
works from being broken.) D’, front view of the same work ; 
the same things designed by the same letters primed. D” 
shows the respective situation of the two barrels 6, and 4,. 
I, telescope ; the light is transmitted to a distance of six miles 
and a half, and comes back on the same path: the appa- 
ratus that reflects it back is a telescope like E, and performing 
the office of a collimator the eye-piece of which is replaced 
by a little mirror properly disposed. F, eye-piece of E, with 
which the ray of light is observed at its return; it is observed 
through the glass-plate C on which it has bees reflected. 
G, apparatus by which the various data of the experiments are 
registered. X, lamp-blacked cylinder. Y, moveable system 
bearing the electro-magnets /, m, 2. The cylinder revolves 
without changing its place with an uniform rotatory motion given 
by a special apparatus. The movable system slides by a uniform 
motion communicated by means of a stretching weight. The 
manner of giving this motion has not been represented ; the 
relative motion is the same as if the system were immove- 
able, and the cylinder going forwards and revolving in the 
same time. 4, m, 2, electro-magnets ; , 7, 7, armatures ; they 
terminate in needles and describe on the lamp-blacked paper the 
three lines drawn on the sketch. One extrenity of the wire of 
the electro-magnets communicates with the earth, the other 
with a pole of a special pile ; the other pole of the pile com- 
municates also with the earth. Oa the way of the current 
‘that ‘passes through from each particular pile to the three 
