Year: 1870

Jacob Abbott, in Harpers Magazine for December


Nothing is more wonderful than the
ingenuity which has been exercised by
scientific men, and the extent and varie.
ty of the resources which they have
called into action, for the purpose OF
eEciting from Nature secrets which she
would seem to have most effectually con
cealed. The method devised by a
French philosopher for measuring pre-
clsely the time required for the passage
of ray of light across a limited space
upon the earth's surface furnishes a
striking example of this. Some time
since an observation was made in Cali
fornia on the velocity with which the
electric force is transmitted along con-
ducting medium, by causing an electric
impulse to pass over the wires from San
Francisco to Cambridge, in Massachus-
etts, and back, without interruption and
noting precisely, at the place of obser-
vatlon, by means of a chronoscope. 1t
was found that the time required for this
SIX thousand miles run was eight t-tenths
of a second. Now the velocity of light
is such that if luminous impulse had
left the place of observation at the same
time with the electrical wave, and could
have pursued the same track, it would
have gone round the circuit five times
while it's competitor was making one
journey. When we contemplate this al
most inconceivable rate of motion, the
idea of devising any mode of actually
measuring with precision the time re-
quired for the passage of light across
any such narrow space as can be made
to intervene between any two stationed
observers on the earth's surface-as, for
example, the length of any line across
plain, or the distance from one emil
nence to another-would seem to be ut
terly hopeless Still the means have
been contrived for realizing it.


The principle on which the apparatus
was constructed is this:


If we suppose that an elastic ballot
ivory, for example, or steel-could be
projected perpendicularly against sol
id wall through small opening at a
known distance from it, and that the
action of gravitation upon the ball could
be suspended so that it might return to
the same orifce throught which it had
been projected; and if we could, more
over, find any way to close the opening
ot the instant that the ball reaches it
on it's return, SO as just to intercept it
in its passage, and then Immediately
open the way again for the passage of a
second ball-it is evident that if the ar
rangement of the apparatus for opening
and closing the orifice was such as to
measure precisely the time that inter
vened between the changes, we should
obtain from it the time required by the
ball for it's passage to and from the wall,
and SO could easily determined the We
locity of it's motion.


This process would, for obvious rea-
sons, be practically impossible in the
case of material missile rebounding
from wall. We can only imagine it,
as an aid to our conceptions in under
standing the analogous operation in the
reflection of light. For light can be SO
reflected as to return in precisely the
same path by which it came; and the
precise interval necessary between max
ing an opening, to allow it to pass, and
then closing the opening to intercept its
return, may be measured and marked
with as much accuracy as can be attain
ed by any measurement whatever.