This is especially in the clearest weather, that these
tails exhibit pulsations, commencing from the head, as being the lowest
part, and vibrating in one or two seconds through the entire tail, which
thus appears rapidly to become some degrees longer, but again as rapidly
contracts. That these undulations, which were formerly noticed with
attention by Robert Hooke, and in more recent times by Schr??ter and
Chladni, 'do not actually occur in the tails of the comets', but are
produced by our atmosphere, is obvious when we recollect that the individual
parts of those tails (which are many millions of miles in length) lie 'at
very different distances' from us, and that the light from their extreme
points can only reach us at intervals of time which differ several minutes
from one another. Whether what you saw on the Orinoco, not at intervals of
seconds, but of minutes, were actual coruscations of the zodiacal light, or
whether they belonged exclusively to the upper strata of our atmosphere, I
will not attempt to decide; neither can I explain the remarkable 'lightness
of whole nights', nor the anomalous augmentation and prolongation of the
twilight in the year 1831, particularly if, as has been remarked, the
lightest part of these singular twilights did not coincide with the Sun's
place below the horizon.
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