upon the Velocity with which it is traversed by Light. 249 
equal volumes, and the velocity of the water was deduced from 
the section of the tubes, and from the time of efflux of half a litre. 
The apparatus above described was only employed for the ex- 
periments with water in motion: with some modifications it 
might also be used for air; but my experiments on moving air 
had been previously made with a slightly different apparatus, of 
which more hereafter, and the results had been found quite con- 
clusive. I had already proved that the motion of air produces no 
appreciable displacement of the bands. But I shall return to this 
result and give further details. 
For water there is an evident displacement. The bands are 
displaced towards the right when the water recedes from the ob- 
server in the tube at his right, and approaches him in the tube on 
his left. 
The displacement of the bands is towards the left when the direc- 
tion of the current in each tube is opposite to that just defined. 
During the motion of the water the bands remain well defined, 
and move parallel to themselves, without the least disorder, 
through a space apparently proportional to the velocity of the 
water. With a velocity of 2 metres per second even, the dis- 
placement is perceptible ; for velocities between 4 and 7 metres 
it is perfectly measureable. 
Tn one experiment, where a band occupied five divisions of the 
micrometer, the displacement amounted to 1:2 divisions towards 
the right and 1-2 divisions towards the left, the velocity of the 
water being 7:059 metres per second. The sum of the two dis- 
placements, therefore, was equal to 2°4 divisions, or nearly half 
the breadth of a band. 
In anticipation of a probable objection, I ought to state that 
the system of the two tubes and four flasks, in which the motion 
of the water took place, was quite isolated from the other parts 
of the apparatus: this precaution was taken in order to prevent 
the pressure and shock of the water from producing any acci- 
dental flexion in parts of the apparatus whose motion might in- 
fluence the position of the bands. I assured myself, however, 
that no such influence was exerted, by intentionally imparting 
motions to the system of the two tubes. 
After establishing the existence of the phenomenon of dis- 
placement, I endeavoured to estimate its magnitude with all 
possible exactitude. To avoid all possible sources of error, I 
varied the magnification of the bands, the velocity of the water, 
and even the nature of the divisions of the micrometer, so as to 
be unable to predict the magnitude of the displacements before 
measuring them. For in measuring small quantities, where our 
own power of estimating has to play a great part, the influence 
of any preconception is always to be feared; I think, however, 
