142 ESSAY ON THE VELOCITY OF LIGHT. 
nished by the discovery of aberration, made fifty years later by the celebrated 
English astronomer Bradley. 
This phenomenon of aberration consists in an apparent displacement which 
all ‘the stars and planets experience on account of the combination of the 
velocity of the earth with the velocity of light. In order to explain this we 
will take another example from among familiar things, and with which we are 
all acquainted. 
Suppose that we are in a railroad car during a rain, without wind, so that 
the drops falls vertically. When the train stops we see the rain-drops moving, 
as they really do, each in a vertical line. But when the train runs on the rails, 
things are completely changed. The drops now seem to fall obliquely, as if a 
strong wind blew in a direction contrary to that of the train’s motion, and 
obliged the vertical lines, naturally described by the rain-drops, to be inelined 
in the direction of its action. It is easy to explain this. From the interior of 
the car we observe what passes outside through the rectangular opening of the 
window. Let us consider specially a drop of rain which we perceive near the 
upper angle or corner of the window on the side in the direction of the motion. 
If the car was stationary we would see the drop descend along the vertical 
side of the window of which that corner is the upper extremity. But the car 
moves at the same time as the rain-drop, and at the moment it arrives at the 
bottom edge of the window sash it is appreciably behind the vertical edge; 
the quantity it is behind is exactly the distance run over by the car while the 
rain-drop was descending the whole height of the window-sash. The rain- 
drop seems to run over a straight line which joins the point where it enters 
the space of the window-pane and the point where it goes out; and as this 
straight line has necessarily a certain obliquity, the rain-drop appears to descend 
obliquely ; and all the other drops of rain moving absolutely the same as the 
one we have just considered, seemed also to move in this oblique line, which 
is but an appearance caused by our own motion while we look at the falling 
rain. If, for example, the car progressed exactly the breadth of the window 
while the rain-drop fell through a distance equal to its height, a drop appearing 
in the upper corner of the window would seem to run over the diagonal drawn 
to the opposite lower corner of the pane. 
We will remark, in passing, that if we measure the apparent obliquity of the 
rain, we can thence deduce the ratio of the velocity of the rain-drops to that 
of the train, since we know the lengths of the paths run over in the same time 
by the drops and by the car; and if we know one of these velocities, for 
example that of the train, we can calculate the other, that of the rain. This 
method, entirely practical, was proposed several years ago as furnishing the 
means of accurately measuring the velocity of falling rain. 
Thus from the simple fact that the observer is in motion, the bodies which 
move in his neighborhood do not appear to have the motion which they have 
in reality, the direction of their motion being altered by this circumstance, and 
that so much the more as the velocity of the observer is greater. | 
Keeping always our illustration of vertically falling rain, observed from 
the interior of a railroad car in motion, suppose that the train gradually 
changes its direction, and from running north to south it changes to run from 
east to west. On account of the motion of the car the rain appears driven 
against the train by a wind blowing from west to east. If the railroad is built 
in the form of a circumference of a circle, so that the train in running over its 
whole length comes again to the point of departure, the rain which really falls 
vertically appears from the interior of the car to fall obliquely under the action 
of a wind blowing successively from all points of the horizon. 
Similar circumstances are presented when the rain, instead of falling verti- 
cally, falls with a certain obliquity, caused by a wind blowing regularly. If 
the wind throws the rain in a direction contrary to that of the car, the motion 
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