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Page 46
It is improbable, however, that even these nearer stars are
practicable subjects for the direct determination of annual parallax.
By indirect means, however, we can obtain some idea of their distance.
All that we want to know for the purpose is the _rate_ of the sun's
motion; its _direction_ we may consider as given with approximate
accuracy by Airy's investigation. Now, spectroscopic measurements of
stellar movements of approach and recession will eventually afford
ample materials from which to deduce the solar, velocity; though they
are as yet not accurate or numerous enough to found any definitive
conclusion upon. Nevertheless, M. Homann's preliminary result of
fifteen miles a second as the speed with which our system travels in
its vast orbit inspires confidence both from the trustworthiness of
the determinations (Mr. Seabroke's) serving as its basis and from its
intrinsic probability. Accepting it provisionally, we find the
parallax of Alcyone = about 0.02', implying a distance of
954,000,000,000,000 miles and a light journey of 163 years. It is
assumed that the whole of its proper motion of 2.61' in forty-five
years is the visual projection of oar own movement toward a point in
R.A. 261�, Decl. +25�.
Thus the parallax of the two stars which we suspect to lie between us
and the stars forming the genuine group of the Pleiades, at perhaps
two-thirds of their distance, can hardly exceed 0.03'. This is just
half that found by Dr. Gill for [xi] Toucani, which may be regarded
as, up to this, the smallest annual displacement at all satisfactorily
determined. And the error of the present estimate is more likely to be
on the side of excess than of defect. That is, the stars in question
can hardly be much nearer to us than is implied by an annual parallax
of 0.03", and they may be considerably more remote.
Dr. Elkin concludes, from the minuteness of the detected changes of
position among the Pleiades, that "the hopes of obtaining any clew to
the internal mechanism of this cluster seem not likely to be realized
in an immediate future;" remarking further: "The bright stars in
especial seem to form an almost rigid system, as for only one is there
really much evidence of motion, and in this case the total amount is
barely 1 per century." This one mobile member of the naked eye group
is Electra; and it is noticeable that the apparent direction of its
displacement favors the hypothesis of leisurely orbital circulation
round the leading star. The larger movements, however, ascribed to
some of the fainter associated stars are far from harmonizing with
this preconceived notion of what they ought to be.
On the contrary, so far as they are known at present, they force upon
our minds the idea that the cluster may be undergoing some slow
process of disintegration. M. Wolf's impression of incipient
centrifugal tendencies among its components certainly derives some
confirmation from Dr. Elkin's chart. Divergent movements are the most
strongly marked; and the region round Alcyone suggests, at the first
glance, rather a very confused area of radiation for a flight of
meteors than the central seat of attraction of a revolving throng of
suns.
There are many signs, however, that adjacent stars in the cluster do
not pursue independent courses. "Community of drift" is visible in
many distinct sets; while there is as yet no perceptible evidence,
from orbital motion, of association into subordinate systems. The
three eighth-magnitude stars, for instance, arranged in a small
isosceles triangle near Alcyone, do not, as might have been expected
_a priori_, constitute a real ternary group. They are all apparently
traveling directly away from the large star close by them, in straight
lines which may, of course, be the projections of closed curves; but
their rates of travel are so different as to involve certain
progressive separation. Obviously, the order and method of such
movements as are just beginning to develop to our apprehension among
the Pleiades will not prove easy to divine.--_A.M. Clerke, in Nature._
* * * * *
DEEP SEA DREDGINGS: EXAMINATION OF SEA BOTTOMS.
By THOMAS T.P. BRUCE WARREN.
I believe Prof. Ehrenberg was one of the first to examine,
microscopically, deep sea dredgings, some of which were undertaken for
the Atlantic cable expedition, 1857.
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