We may remark, again, that if we produce a single wave upon water, it
will be propagated in a uniform motion, and will form in front of it
successive waves whose velocity of propagation is accelerated.
This may explain why sounds perceived at great distances are briefer
than at small ones. A detonation that gives a quick dead sound at a few
yards is of much longer duration, and softer at a great distance.
The laws that govern the system of wave propagation are, then, very
complex.
[Illustration: II]
II.--If an obstacle be in the way of the waves, there will occur in each
of them an _alteration_, a break, which it will carry along with it to a
greater or less distance. This succession of alterations forms a trace
behind the obstacle, and in opposition to the line of the centers.
Finally, if the obstacle itself emits waves in space that are of less
intensity then those which meet it, these little waves will extend in
the wake of the large ones, and will form a trace of parabolic form
situated upon the line of the centers.
[Illustration: III]
III.--Let us admit, then, that the sun, through the peculiar energy that
develops upon its surface or in its atmosphere, engenders in ethereal
space successive waves of varying nature and intensity, as has been said
above, and let us admit that its _mechanical_ waves are traversed
obliquely (Fig.
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