248 M.H. Fizeau on the Effect of the Motion of a Body 
The double journey performed by the rays had also the 
advantage of increasing the probable effect of motion; for this 
effect must be the same as if the tubes had double the length 
and were only traversed once. 
This arrangement also permitted the employment of a very 
simple method for rendering the bands broader than they would 
otherwise have been in consequence of the great distance (9 
millims.) between the slits. This method consisted in placing a 
very thick plate of glass before one of the slits, and inclining the 
same in such a manner that, by the effect of refraction, the two 
slits had the appearance of being very close to each other: in 
this manner the bands become as broad as they would be if the 
two slits were, in reality, as near each other as they appear to be ; 
and instead of the intensity of light being sensibly diminished 
by this expedient, it may, in fact, be greatly augmented by giving 
greater breadth to the source of light. By causing the inclina- 
tion of the glass to vary, the breadth of the bands may be varied 
at pleasure, and thus the magnitude most convenient for pre- 
cisely observing their displacement may be readily given to them. 
I proceed to describe the disposition of the tubes, and the 
apparatus destined to put the water in motion. 
The two tubes, placed side by side, were closed at each ex- 
tremity by a single glass plate, fixed with gum-lac in a position 
exactly perpendicular to their common direction. Near each 
extremity was a branch tube, forming a rounded elbow, which 
established a communication with a broader tube reaching to the 
bottom of a flask; there were thus four flasks communicating 
with the four extremities of the tubes. 
Into one flask, which we will suppose to be full of water, com- 
pressed air, borrowed from a reservoir furnished with an air- 
pump, was introduced through a communicating tube. Under 
the influence of this pressure the water rose from the flask into 
the tube, which it then traversed in order to enter the flask at 
the opposite end. The latter could also receive compressed air, 
and then the liquid returned into the first flask after traversing 
the tube in an opposite direction. In this manner a current of 
water was obtained whose velocity exceeded 7 metres per second. 
A similar current, but in an opposite direction, was produced at 
the same time in the other tube. 
Within the observer’s reach were two cocks fixed to the re- 
servoir of air; on opening either, currents, opposite in direction, 
were established in both tubes; on opening the other cock the 
currents in each tube were simultaneously reversed. 
The capacity of the reservoir, containing air at a pressure of 
about two atmospheres, amounted to 15 litres (half a cubic foot), 
that of each flask to about 2 litres ; the latter were divided into 
