256 M. H. Fizeau on the Effect of the Motion of a Body 
tube was 1 centimetre, and its effective length 1:495 metre; 
the direction of the motion in one tube was opposite to that in 
the other, and the pressure under which this motion took place 
was measured by a manometer placed at the entrance of the 
tubes ; it could be raised to 3 centimetres of mercury. 
The velocity of the air was deduced from the pressure and 
from the dimensions of the tubes, according to the known laws 
of the efflux of gases. The value thus found was checked by 
means of the known volume of the bellows, and the number of 
strokes necessary to produce a practically constant pressure at the 
entrance of the tubes. A velocity of 25 metres per second could 
easily be imparted to the air; occasionally greater velocities were 
reached, but their values remained uncertain. 
In no experiment could a perceptible displacement of the 
bands be produced: they always occupied the same positions, 
no matter whether the air remained at rest, or moved with a 
velocity equal or even superior to 25 metres per second. 
When this experiment was made, the possibility of doubling, 
by means of a reflecting telescope, the value of the displacement, 
and at the same time of completely compensating any effects due 
to accidental differences of temperature or pressure in the two 
tubes, had not suggested itself; but I employed a sure method 
of distinguishing between the effects due to motion, and those 
resulting from accidental circumstances. 
This method consisted in making two successive observations, 
by causing the rays to traverse the apparatus in opposite direc- 
tions. For this purpose the source of light was placed at the 
point where the central band had previously been, when the new 
bands formed themselves where the source of light had previously 
been placed. 
The direction of the motion of the air in the tubes remaining 
the same in both cases, it is easy to see that the accidental effects 
would in both observations give rise to a displacement towards 
the same tube, whilst the displacement due solely to motion 
would first be on the side of one tube and then on the side of 
the other. In this manner a displacement due to motion would 
have been detected with certainty, even if it had been accom- 
panied by an accidental displacement due, for instance, to some 
defect of symmetry in the diameters or orifices of the tubes, 
whence would result an unequal resistance to the passage of air, 
and consequently a difference of density. 
But the symmetry given to the apparatus was so perfect that 
no sensible difference of density existed in the two tubes during 
the motion of the air. The double observation was consequently 
unnecessary ; nevertheless it was made for the sake of greater 
security, and in order to be sure that the sought displacement 
