the ichthyosaurians, in the pterodactyls or in the megalosaurians,
seem to me to be clearly of a different character, and to differ from
simple progressive types, inasmuch as those which appear earlier,
combine peculiarities which, at a later period, appear separately in
distinct forms. For instance, the reptilian characters which we
recognise in the sauroid fishes, are developed at a later period in
animals no longer belonging to the class of fishes, but constituting
by themselves new types, provided with additional peculiarities which
separate them fully from the fishes in general, as well as from those
fishes in which we recognise some relation to reptiles during a period
when no reptile existed.
Again, the ichthyosaurians, though true reptiles appearing long after
fishes had been called into existence, and during an early period of
the history of the reptiles, still shew their relation to fishes by
the character of their vertebral column, and foreshadow, as it were,
in their form, the cetacea of later ages, as well as many forms of the
gigantic saurians of the secondary period. The same may be said of the
pterodactyls, which are also true reptiles, but, in which the anterior
extremity foreshadow peculiarities characteristic of birds and bats.
Such types I shall call _Prophetic Types_.
To an analytic mind the examination of the peculiarities of such
animals may foretell a higher progress of development, carried out in
real existence, only during a later period, even if he had never seen
the later ones; for in such types the germs of a future development
may be recognised, and upon close examination, truly referred to the
peculiarities of other higher groups, even if the intermediate links
remained unknown, which, however, as the matter now stands, can leave
no doubt in our mind that these prophetic types really foreshadowed
that diversity of forms which has been created since they have gone
by. We may also say that these prophetic types lay before us the
course of thoughts which has been carried out in the plan of creation
by the Supreme intelligence, who called them into existence in rich
order of succession, and in so diversified relations. The recognition
of this prophetic character of certain types of extinct animals is not
only important in a philosophical point of view; I have no doubt it
will ultimately and rapidly lead to a better, fuller, higher, and
deeper understanding of the various relations which exist between
animals. Let me at once point to some of these relations which might
never have been understood but for this appreciation.
Among Crinoids, we have not only progressive types, as I have already
quoted, but we have also prophetic ones. The Cystidæ are truly
prophetic of the Echini proper. I may only mention the genus
Echinocrinus to shew the link.
The Pentremites, again, are the prophetic type foreshadowing the
star-fishes. And often in subordinate groups we may find such close
relations between genera of the same minor divisions; such, for
instance, as the genus Encrinus, in which the genera Apiocrinus and
Pentacrinus, are simultaneously foreshadowed. Perhaps, in this case, a
distinction might be introduced between truly prophetic types and
synthetic types, in which the characters of later groups are rather
more combined than really foreshadowed.
As for the relation between older types and the embryos of the living
representatives of the same families which are so extensively observed
in almost all groups of the animal kingdom, which have existed during
earlier periods, it may best be expressed if we call those fossils
which exemplify, in full grown animals, forms which exist at present
only in the earliest stages of growth of our living animals,
_Embryonic Types_, in counterdistinction from the progressive types,
and from the prophetic types. These embryonic types may be purely
such, or they may be at the same time either progressive types, or
even prophetic types. I shall call purely embryonic types those in
which we recognise peculiarities characteristic of the embryo of the
same family. For instance, the older Sauroids, which have the upper
lobe of the tail prolonged, or the common Crinoids provided with a
stem, which resemble the young Comatulæ, &c., &c. I shall distinguish,
as progressive embryonic types, those in which we recognise
simultaneously a relation to the embryo of the same family, when they
form besides a link in the natural chain of progressive development.
Such, for instance, as the oldest Salamanders, or the earliest
Sirenoid Pachyderm. Finally, I shall call prophetic embryonic types
those in which we have embryonic characters, combined with the
peculiarities which stamp the type as a prophetic one, such, for
instance, as the Echinoid and Asteroid Crinoids of the former ages.
The fact that these different types may thus present complications of
their character, or appear more or less pure and typical, goes further
to shew how deeply diversified the plan of creation is, and how many
relations should be simultaneously understood before we are prepared
to have a full insight into the plan of creation. There we see one
type forming simply, and alone, the first link of a progressive
series. There we see another which foreshadows types, which appear
isolate afterwards. There we see a third, which, in its full
development, exemplifies a state which is transient only in higher
representatives of the same family. And then, again, we see these
different relations running into each other, and reminding us that,
however difficult it may be for us to see at one glance all this
diversity of relations, there is, notwithstanding, an intelligence
which not only conceived these various combinations, but called them
into real existence in a long succession of ages.--_L. Agassiz in the
American Association for the Advancement of Science_, August 1849.
_On a new Analogy in the Periods of Rotation of the Primary
Planets discovered by Daniel Kirkwood of Pottsville, Pennsylvania._
At the recent meeting of the Association for the Advancement of
Science, an announcement was made, which, if it is found to be
correct, will be regarded as relating to one of the most important
discoveries which have been made in astronomy for years. It is no less
than a new law of the solar system, closely resembling those of
Kepler, which form the groundwork of many of the problems of
astronomy. Mr S. C. Walker read to the Association a letter from Mr
Daniel Kirkwood, of Pottsville, Pa., the discoverer of this new law,
from which we make some extracts, omitting all that refers to the
higher branches of mathematics.
"While we have in the law of Kepler a bond of mutual relationship
between the planets, as regards their revolutions around the sun, it
is remarkable that no law regulating their rotations on their axes has
ever been discovered. For several years I have had little doubt of the
existence of such a law in nature, and have been engaged, as
circumstances would permit, in attempting its development. I have at
length arrived at results, which, if they do not justify me in
announcing the solution of this important and interesting problem,
must at least be regarded as astonishing coincidences."
After stating some equations, he gives the following tables as the
data on which he has proceeded:--
+---------+---------------+-------------+-----------+----------------+
| | Mean dist. | | Square |No. of rotations|
|Planet's | from the sun | Mars. | root of | in one sid. |
| name. | in miles. | | Mars. | period. |
+---------+---------------+-------------+-----------+----------------+
|Mercury, | 36,814,000 | 277,000 | 526·3 | 87·63 |
|Venus, | 68,787,000 | 2,463,836 | 1·569·6 | 230·90 |
|Earth, | 95,103,000 | 2,817,409 | 1·678·5 | 366·25 |
|Mars, | 144,908,000 | 392,735 | 626·7 | 669·60 |
|Jupiter, | 494,797,000 | 953,570,222 | 30·879·8 | 10·471·00 |
|Saturn, | 907,162,000 | 284,738,000 | 16·874·1 | 24·620·00 |
|Uranus, | 1,824,290,000 | 35,186,000 | 5·931·5 | |
+---------+---------------+-------------+-----------+----------------+
From these data he deduces the following law:--"The square of the
number of a primary planet's days in its year, is as the cube of the
diameter of its sphere of attraction in the nebular hypothesis."
"The points of equal attraction between the planets severally (when in
conjunction), are situated as follows:--
Miles from the Miles from the
former. latter.
Between Mercury and Venus, 8,029,600 23,943,400
" Venus and the Earth, 12,716,600 13,599,400
" Earth and Mars, 36,264,600 13,540,400
" Jupiter and Saturn, 266,655,000 145,710,000
" Saturn and Uranus, 678,590,000 238,538,000
"It will be seen from the above, that the diameter of the earth's
sphere of attraction is 49,864,000 miles. Hence the diameters of the
respective spheres of attraction of the other planets, according to my
empirical law, will be found to be as follows:--
Diameter of sphere
of Attraction.
Mercury, 19,238,000
Venus, 36,660,000
Mars, 74,560,000
Jupiter, 466,200,000
Saturn, 824,300,000
"The volumes of the sphere of attraction of Venus, Mars, and Saturn in
this table, correspond with those obtained from the preceding one;
that of Mars extending 61,000,000 miles beyond his orbit, or to the
distance of 206,000,000 miles from the sun. This is about 2,000,000 or
3,000,000 miles less than the mean distance of Flora, the nearest
discovered asteroid. That of Mercury extends about 11,000,000 miles
within the orbit; consequently, if there be an undiscovered planet
interior to Mercury, its distance from the Sun, according to my
hypothesis, must be less than 26,000,000 miles. Jupiter's sphere of
attraction extends only about 200,000,000 of miles within his orbit,
and leaving 89,000,000 miles for the asteroids. It is only in the most
distant portion of this space, where small bodies would be likely to
be detected, that none have yet been discovered."
Mr Kirkwood then modestly concludes:--
"The foregoing is submitted to your inspection with much diffidence.
An author, you know, can hardly be expected to form a proper estimate
of his own performance. When it is considered, however, that my
formula involves the distances, masses, annual revolutions, and axial
rotations of all the primary planets in the system, I must confess I
find it difficult to resist the conclusion, that the law is founded in
nature."
After this letter had been read, Mr Walker said, that, induced by the
importance of the subject, he had at once proceeded to verify the data
and conclusions of Mr Kirkwood, and had found that there was nothing
in them requiring modification, except, perhaps, the substitution of
some more recent values for the masses of Mercury and Uranus. This
theory and that of Laplace, with reference to nebulæ, mutually
strengthen each other; although the latter has been a mere
supposition, while the former rests upon a mathematical basis. In a
later letter, which was also read, Mr Kirkwood says that he has
pursued this subject for the last ten years, it having been first
suggested to him by the nebular hypothesis, which he thought could be
established by some law of rotation.
Mr Walker then entered into a lengthened examination of the data on
which the law rests, and seemed to come to the conclusion, that, as
far as we know at present, everything is in favour of the truth of the
law, except that it requires the assumption of another planet between
Jupiter and Mars.
Mr Walker closed his examination by saying, "We may, therefore,
conclude, that, _whether Kirkwood's analogy is or is not the
expression of a physical law, it is, at least, that of a physical fact
in the mechanism of the universe_. The quantity on which the analogy
is based has such immediate dependence upon the nebular hypothesis,
that it lends strength to the latter, and gives new plausibility to
the presumption that this, also, is a fact in the past history of the
solar system.
"Such, then, is the present state of the question. Thirty-six elements
of nine planets (four being hypothetical) appear to harmonize with
Kirkwood's analogy in all the four fundamental equations of condition
for each planet.
"To suppose that so many independent variable quantities should
harmonize together by accident, is a more strained construction of the
premises than the frank admission that they follow a law of nature.
"If, in the course of time, the hypotheses of Laplace and Kirkwood
should be found to be the laws of nature, they will throw new light on
the internal organization of the planets in their present, and in any
more primitive, state through which they may have passed.
"For instance, we may compute the distance from the centre at which
any planet must have received its projectile force, in order to
produce, at the same time, its double movement of translation and
rotation.
"If the planet, in a more primitive state, existed in the form of a
ring revolving round the Sun, having its present orbit for that of the
centre of gravity of the ring, the momentum of rotation must, by
virtue of the principle of conservation of movement, have existed in
some form in the ring. It is easy to perceive that this momentum is
precisely the amount which must be distributed among the particles of
the ring, in order to preserve to all the condition of dynamical
equilibrium, while those of each generating surface of the ring were
wheeling round with the same angular velocity.
"If the planets have really passed from the shape of a revolving ring
to their present state, the prevalence of Kirkwood's analogy shews a
nice adaptation of parts in every stage of the transition.
"If the primitive quantity of coloric (free and latent) had undergone
a very great change beyond that now indicated in the cooling of their
crusts; if the primitive quantity of movement of rotation had been
different from its actual value for any planet; if the law of
elasticity of particles for a given temperature and distance from each
other varied from one planet to another in the primitive or present
state; in either of these cases, the analogy of Kirkwood might have
failed. As it is, no such failure is noticed; we are authorised,
therefore, to conclude, that the primitive quantity of coloric, the
law of elasticity, the quantity of movement of rotation, the past and
present radii of percussion, the primitive diameter of the generating
surface of the rings, and the present dimensions and density of the
planets, have been regulated by a general law, which has fulfilled for
all of them the four fundamental conditions of Kirkwood's hypothesis.
"We may extend the nebular hypothesis and Kirkwood's analogy to the
secondary system. If they are laws of nature, they must apply to both.
In the secondary systems, the day and month are the same. This fact
has remained hitherto unexplained. Lagrange shewed that if these
values were once nearly equal, a libration sets in round a state of
perfect equality; but he offered no conjecture as to the cause of the
primitive equality. On the nebular and Kirkwood's hypothesis, it would
only be necessary that, upon the breaking up of the ring, the
primitive diameter of the generating figure and law of relative
density of layers should be preserved."
Professor Peirce, whose opinions will probably be regarded as of more
value on such a subject than that of any other man in this
country,--especially since his successful discussion with
Leverrier,--remarked, that Kirkwood's analogy was the only discovery
of the kind since Kepler's time that approached near to the character
of his three physical laws. Bode's law, so called, was at best only an
imperfect analogy. Kirkwood's analogy was more comprehensive, and more
in harmony with the known elements of the system. The diameter of the
sphere of attraction, a fundamental element in this analogy, now for
the first time gave an appearance of reality to Laplace's nebular
hypothesis which it never had before. The positive testimony in its
favour would now outweigh the former negative evidence in the case,
however strong it may have been. It follows at least from Kirkwood's
analogy, that the planets were dependent upon each other, and
therefore connected in their origin, whatever may have been the form
of the connection, whether that of the nebular hypothesis, or some
other not yet imagined.
At a later period of the meeting, M. B. A. Gould junior, stated that
he had gone through the necessary calculations, using different
quantities, and had come to the same conclusions as Mr Walker. He
expressed his opinion, that at some future day the world will "speak
of Kepler and Kirkwood as the discoverers of great planetary laws."
The members generally expressed the opinion, that Laplace's nebular
hypothesis, from its furnishing one of the elements of Kirkwood's law,
may now be regarded as an established fact in the past history of the
solar system.--_American Annual of Scientific Discovery_, p. 335.
NOTE.--Such, at least, is rather a representation of American opinions
than of our own. We are inclined to compare it more with Bode's law
than with Kepler's. The former is a mere arithmetical accident,
applying indifferently well to a portion only of the planets, and
having nothing of reason to advance for its establishment. The latter
are essential parts of mechanics and gravitation, and precisely and
perfectly, and necessarily true, not only in every part of the solar
system, but through the whole universe.
The fact of axial rotations being the groundwork of Kirkwood's analogy
seems fatal to it, for gravitation takes no more account of the time
of rotation of a planet than it does of specific gravity; all
calculations of the movement of the body in space are equally
independent of the one and the other.
Under these circumstances, the degree of accuracy with which it may be
found to apply is the only saving clause. Messrs Walker and Gould
investigating the subject independently, and with better constants of
mass and distance than Kirkwood had been able to procure, declare that
it appears _perfectly_! We are sorry that the late hour at which we
have received this paper has prevented us either from giving it in
full, or from testing the theory rigidly.
It will be observed that, according to Kirkwood's theory, in order to
compute the time of axial rotations of any planet, it is necessary to
have its mass and mean distance, together with the same quantities for
the planets on either side of it. Now, these quantities are only
obtainable for Venus, the Earth, Saturn, and Uranus (a planet being
lost between Mars and Jupiter); and the rotation of Uranus not having
been obtained as yet, there remains only the three first by which the
theory can be tested.
In a preliminary calculation which we have instituted, we do not find
the results so accordant as we had been led to expect, but still
sufficiently so to give a certain probability of the approach to
truth, in a case where the quantity had not been observed.
Viewed in this light, some very interesting results are obtained.
_1st_, The idea entertained by Bianchini and other observers, that the
rotation of Venus is nearly 24 times as long as hitherto supposed, is
utterly untenable.
_2d_, The time of rotation of Uranus, a quantity never yet observed
(but doubtless capable of being observed by a telescope of Lord
Rosse's calibre, _removed to a table-land in a tropical country_) is
given; and appears so very different from any other yet observed,
especially so from those of its neighbours Saturn and Jupiter, being =
1·396779, earth's = 0·997270 (sidereal rotation in mean solar days.)
_3d_, Knowing the rotations of Jupiter and Mars, we may supply, by
using the analogy conversely, the _diameters of their spheres of
attraction_, and thus get at the elements of the lost planet between
Mars and Jupiter, and these appear to be:--mean distance = 2·9085111
(earth unity), mass in terms of Sun 1/1353240, sidereal rotation in
earth's mean solar days 2·406104, and diameter of sphere of attraction
0·830951, in terms of earth's distance. The size is thus a little
larger than Mars. The slowness of rotation is remarkable, especially
in the case of a planet which is supposed since to have burst into
pieces: the Americans have called it Kirkwood. P. S.
SCIENTIFIC INTELLIGENCE.
METEOROLOGY.
1. _Use of Coloured Glasses to assist the View in Fogs._--M. Lavini of
Turin, in a letter to the editor of _L'Institut_ at Paris, makes the
following curious observation, which, if confirmed, may prove to be of
great importance:--"When there is a fog between two corresponding
stations, so that the one station can with difficulty be seen from the
other, if the observer passes a coloured glass between his eye and the
eye-piece of his telescope, the effect of the fog is very sensibly
diminished, so that frequently the signals from the other station can
be very plainly perceived; when, without the coloured glass, even the
station itself is invisible. The different colours do not all produce
this effect in the same degree, the red seeming to be the best. Those
who have good sight prefer the dark-red, while those who are
short-sighted like the light-red better. The explanation of this
effect seems to depend upon the fact, that the white colour of the fog
strikes too powerfully upon the organ of sight, especially if the
glass have a somewhat large field. But by the insertion of the
coloured glass, the intensity of the light is much diminished by the
interception of a part of the rays, and the observer's eye is less
wearied, and, consequently, distinguishes better the outlines of the
object observed."
2. _Ozone._--Chemists are not yet fully agreed concerning the nature
or production of this singular substance, ozone. To Schonbein and
Williamson we are indebted for most of our knowledge concerning it.
The latter has supposed it to be a compound of oxygen and hydrogen,
from the fact, that, when the ozone completely freed from moisture was
passed over ignited copper, water was produced. De la Rive produced it
by passing a current of electricity through pure dry oxygen gas
contained in a receiver. It is also obtained in large quantities by
passing oxygen gas over moistened phosphorous, and afterwards drying
it. Thus prepared, it is a powerful chemical agent, possesses
bleaching properties, oxidises the metals with rapidity, and destroys
India-rubber. The hydrogen acids of sulphur are decomposed by it,
water being formed by uniting with the hydrogen of the acid, and
sulphur being set free. Professor Horsford has observed that ozone,
subjected to a heat of 130° Fah., entirely loses its properties.
Ozone, like chlorine, precipitates iodine, colouring a solution of
iodide of potassium, and starch a deep blue colour. The peculiar
smell, prevalent in the vicinity of objects struck by lightning, as
well as that occasioned by the excitation of an electrical machine,
and by the striking of two pieces of silica together, it is believed
to be occasioned by ozone.--_Editors._--_Annual of Scientific
Discovery_, p. 219.
_Method of Determining the Amount of Ozone in the Atmosphere._--At the
meeting of the American Association, an instrument for determining the
relative quantity of ozone in the air was presented by Professor
Horsford. It consisted of a tube, containing at one end a plug of
asbestus, moistened with a solution of iodide of potassium and starch.
This plug within the tube, attached to an aspirator, would, as air
passed over it, become blue. If much water flowed from the aspirator,
and of course much air flowed over the asbestus before it became blue,
the quantity of ozone indicated would be small. If but little water
flowed (and this could be measured), the quantity of ozone indicated
would be greater. The quantities of ozone would be inversely as the
volumes of air passing through the tube before blueness is
produced.--_Annual of Scientific Discovery_, p. 219.
HYDROGRAPHY.