Royal Society (Great Britain).

The Philosophical transactions of the Royal society of London, from their commencement in 1665, in the year 1800 (Volume 8) online

. (page 44 of 85)
Online LibraryRoyal Society (Great Britain)The Philosophical transactions of the Royal society of London, from their commencement in 1665, in the year 1800 (Volume 8) → online text (page 44 of 85)
Font size
QR-code for this ebook


In the same manner, the duration and quantity of partial eclipses would be
also diminished ; and they would thus begin later, and end sooner, on the
supposition of a lunar atmosphere, than without one ; and in certain cases,
there would be no eclipse at all, when there would otherwise be one.

Such then would be the phaenomena in case there were an atmosphere about
the moon. Let us now consider what is really observed. In the first place, as
the axis of the lunar shade extends to 55 semidiameters of the earth, when
greatest, and to 52-l when least ; and as the least distance of the moon from
the earth is 54 of the earth's semidiameters ; if the lunar atmosphere were
capable of a horizontal refraction of 8", the semi-angle of the conical shadow
would be increased by double the quantity, that is 16"; therefore it would be



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. 373

equal to 1 6' 4 1" when most open, and to l6'5"when narrowest. Also, the
least semi-angle of the cone being supposed l6' 5", its axis will be less than
the least distance of the moon from the earth, or 54 terrestrial diameters ; and
therefore the point of the lunar shade will not extend to the earth. So that, if
there were an atmosphere about the moon, in which the horizontal refraction is
8", there would be no total eclipse on the earth. There is, therefore, either
no atmosphere about the moon, or else it produces a horizontal refraction less
than 8*.

But certain total eclipses of the sun are observed with some duration of the total
darkness. For instance, in the eclipse of 1724, the duration of total darkness
amounted to "i*" l6^ The moon, at that time, ran over l'l5''in her horary motion,
and her shadow, with a parallel motion, a space 54 times greater on the earth's disk,
that is, equal to 1° ^' 30" ; from which, if there be deducted the diurnal motion of a
habitation, by which the duration of the eclipse can be prolonged, it gives the dia-
meter of the shadow equal to 47'30", or 45 1 73 toises, or 11 Paris leagues. Hence,
by calculation, it is found, that the axis of the cone of the lunar shade is greater
by at least one diameter of the earth, than the moon's distance from it, which
was then the least, the moon being then about the perigeum. Further, from
the given diameters of the luminaries, observed in the same degree of anomaly,
the axis of the cone of the lunar shade is found at least equal to 55 semidi-
ameters : hence it follows, that the spot of the lunar shade on the earth's disk,
and the axis of the cone, are found to be exactly the same, as the distances of
the moon and the observed diameters of the luminaries seem to require. There
is therefore no atmosphere about the moon, or, if there is any, it produces no
sensible refraction.

But to leave no room for doubt, we may consider the reason for those phae-
nomena, observed in solar eclipses, which have given cause to suspect a lunar
atmosphere.

And first, that very faint light, observed in total eclipses, is no proof of any
refraction in a fluid about the moon ; for, by Maraldi's experiments, success-
fully repeated by M. Fouchy, it appears that the shadows of bodies that have
no atmosphere, when they are exposed to the sun, are bright about the axis of
the cone; and the more so as it is farther from the body itself. And the situa-
tion of an observer, in a total eclipse, is about the axis of the cone of the lunar
shade, and near its vertex. It is no wonder therefore, that the middle of the
shadow exhibits a kind of gloomy light, which may also be augmented by the
rays being reflected by an illuminated air surrounding the shadow about the
middle.

2dly. The lucid annulus about the moon, in total eclipses, by no means



374 PHILOSOPHICAL TRANSACTIONS. [aNNO 173Q.

proves the existence of a lunar atmosphere, as will be plain to any one who
hides the sun froni him by balls of wood, or other opaque matter. Hence this
is not to be ascribed to a lunar, but to a solar atmosphere; as has been proved
by M. Mairan, in his treatise on the Aurora Borealis, p. 14, i.

3dly. The diminution of the lunar diameter, which in solar eclipses is ob-
served to be about 30' less than when the moon shines with a full orb, in the
same degree of anomaly, by no means proves a lunar atmosphere; though some
inequalities of mountains are observed in the circumference of the moon's disk,
which quite disappear in the full moon ; for lucid objects strike the fibres of the
eye so strongly, that their motion is communicated to the neighbouring fibres,
and so the image of the lucid body is increased beyond the due quantity, as is
known by common experience : for if a stick be placed between the eye and
the moon, the diameter of the interposed stick will seem to be diminished; but
if at that time any cloud pass before the moon, the diameter of the stick will
appear less diminished; and if the moon be quite obscured, there will be no
diminution at all; and lastly, the diminution will vary according to the various
intensity of the moon's light.

As for the inequalities of the mountains, they are, for the same reason, ob-
served least in the full moon; for the lunar mountains, obscure of themselves,
and seen in the bright orb of the sun, escape the eye much less, than when,
shining in the full moon, they are extinguished in the splendour of that lumi-
nary ; especially as the lunar light is so intense, that a star of the 3d magnitude
can hardly be seen, when near it. But, to remove all doubt, if the lin)b of
the moon, when in opposition to the sun, were the bound of an atmosphere,
and not of her very body itself, the mountains in her circumference would
never be observed by the longer telescopes with narrow objective apertures.
Whereas M. Fouchy had often observed several inequalities of mountains in
the disk of the full moon, with a telescope of 3d Paris feet, and an objective
aperture of 1 inch : hence it follows that the disk of the full moon is bounded
by the periphery of her body, and not of her atmosphere.

4thly. We must not omit to speak of that remarkable observation, in 1715,
of the lunar corusaitions, made by M. Delouville, in the presence of many
astronomers of the Royal Society, and seen by Dr. Halley, N° 343, vol. vi, p.
158 of these Abridgments. We may suppose that the moon's visible limb is
composed of the tops of mountains; which, in a total eclipse, hide the sun
from an observer, in the same manner as the trees in great woods obstruct the
sight. Hence if some rows of mountains, on the moon's surface, afford a
free and direct passage for the solar rays, they must imitate a kind of corusca-
tions, like as when, i« a camera obscura, a ray of the sun is suddenly admitted



VOL. XLI.] PHILOSOPHICAL TRANSACTION-S. 375

bv means of a speculum, and the picture of the external objects, drawn on the
focus of the lens, is taken away, it will be enlightened with luminous traces,
much resembling lightning.

From all tliese it is manifest, that there is nothing like a lunar atmosphere in
solar eclipses. We may now speak of the occupations of the stars and planets
by the moon.

If the moon be surrounded by an atmosphere, the planets and fixed stars
must be seen, by an observer on the earth, to immerge behind the moon later,
and to emerge from it sooner, than if she have no atmosphere; and even in
some plnces where such occultations ought to appear, there would be none. To
make this plain, let abc, fig. 5, be the moon's body, and s a star at immense,
or as at an infinite distance; then the parallel rays lv, mx, touching the moon's
body on all sides, inclose a cylindric space, of which the base vzx includes all
the parts of the earth's surface to which the star or planet is occulted by the
moon. The observer therefore will see the beginning of the eclipse at v, and
the end at x, and will measure the duration of time in which the moon may
run through her diameter, or an equal space. But if we suppose an atmos-
phere of the moon, the ray iw will not remain parallel to the axis of the
cylinder, but this will now become a cone, of which the section ytu will mark
the earth's surface where the eclipse will be seen. Now the base ytu being
contracted, the point y wilj come on any part later than the point v, and the
point u will quit it sooner than x; therefore the occultation will begin later,
and end sooner, by supposing an atmosphere about the moon, than the con-
trary; and there will be no occultation in some places, where it ought to be
observed without an atmosphere; for the place c, being included in the base
vzx of the former cylinder, is without the conical section ytu. Besides, sup-
posing the horizontal refraction in the lunar atmosphere equal to 8'', then vy
will be 1384 toises, or a quarter of a Paris league; and hence it follows, that
no eclipse must be observed in places according to calculation, whenever they
are without a circle of a quarter of a league radius.

Another phenomenon also takes place on the supposition of a lunar atmos-
phere: in the part of the cylinder yr the star indeed will always be seen, but it
will be through that medium : hence it will acquire a motion and colour different
from the genuine, and that in all eclipses whatever, whether the star be one of
the largest or smallest.

Besides, the duration of such occultations does not seem at all diminished,
but is always found to be exactly agreeable to the moon's diameter and motion.
As to those observations in which the star, after the contact, is seen to proceed
a little in the moon's disk before the occultation, the whole cause of them may



376 PHILOSOPHICAL TRANSACTIONS. [aNNO 1739-

be referred to the increased diameters of the moon and star; for if the lunar
atmosphere were the cause of this appearance, it would always be observed the
same in all stars, and in any apertures of objectives. Besides, there has not as
yet been observed the progression of any star in the moon's disk, unless it be
of the first, or perhaps of the second magnitude, and that by only the half of
it at most, and it is well known, that their whole diameter is insensible, and is
only increased by spurious rays; whence the adventitious rays, both of the
moon and star, are mixed in the bottom of the eye, before the true conjunc-
tion of their bodies; and if the visible limb of the moon were the limit of the
atmosphere and not of the body, no mountains would be observed on its peri-
phery with the larger tubes and narrower objective apertures ; which, however,
as beforesaid, are seen plainly enough.

From all these then it is manifest, that the moon is not surrounded with a
refracting atmosphere, or one capable of being observed; though there might
be one producing a horizontal refraction of l" or 2"; and this seems to be coun-
tenanced by the larger spots in the moon, which cannot by any means be taken
for woods, as Hartsocker and some others have imagined; for the shadows of
the edges are always observed nearer to the bright limb of the moon ; whence it
is rightly concluded that they are cavities, and not woods, as these would pro-
ject a shadow from the other side. Further, some fluid may well be supposed
to be in them; in which case it would be very agreeable to philosophy, that
they should emit some vapours, the congeries of which would represent a kind
of atmosphere ; which would not be found very dense, since, by Sir I. Newton's
demonstrations, it could hardly equal a third of that of the terrestrial vapours,
nor be observed alike at different times, those vapours being destitute of any
other addition.

^n extraordinary sinking down and sliding away of some Ground at Pardines

near Auvergne. By M. T. Communicated by Phil, Henry Zollman,

- Esq. F. R. S. N° 455, p. -ITI.

The village called Pardines, was composed of 46 buildings ; the ground on
which it was built, as well as that of the whole hill, is a good and light earth,
mixed with a little white clay; there are also in it some stones and rocks of a
middling size. This land was very well cultivated, and very fruitful, consisting
of fields sowed with corn, of orchards, but for the most part of vineyards; the
whole ground was overspread with fruit-trees, particularly walnut-trees. This
earth used to dry soon, and chap froii) the heat; some clefts of a considerable
depth, which growing wider and wider, often formed several gullies.



VOL. XLI.J PHILOSOPHICAL TKANSACTIONS. 377

On the 23d of June 1733, about Q in the evening, the inhabitants of the
village saw the walls of their houses shake sensibly ; on which they all withdrew
out of them, and saw that the hill visibly melted away, as it were, the greater
part of the land sliding along towards the vale; other parts subsided sensibly; in
some places the earth, opening, formed new gulls, and those that were there
before, grew much wider ; sometimes the ground which slid along in large
pieces, stopped, and tumbled one piece over another ; and the rocks, which
broke loose from that rolling earth, precipitated themselves into the valley,
which became quite filled up with them, as well as with the earth which rolled
down, by which the neighbouring road becaine impassable.

All this was done very gently, and even sometimes almost imperceptibly ; a
sensible motion was observed during the space of 3 or 4 days at different times;
one house even did not fall till the 10th of July. During all that time no noise
was heard, any otherwise than what proceeded from the rocks falling into the
valley, and from some large masses of earth, which loosening themselves from
the steeper parts, fell down with precipitation. By this rolling were carried
away 26 buildings, some of which subsided with the ground, and, being shaken
at their foundations, tumbled on a heap ; the remains of some others appear
yet, on those pieces of ground that rolled down into the valley. It is com-
puted, that the ground which slid away, or was lost by being buried under the
rubbish of the others, amount to the number of 150 acres of Paris measure.
It is observable, that in this number were comprised several orchards, besides
that the whole ground was covered with trees, either walnut-trees on the hill,
or willows and poplars in the valley, about 4000 in all.

If one may conjecture what was the cause of so dismal an accident, it seems
it proceeded from the situation of the ground, and the nature of the soil. The
first surface of the hill, about 4 or 6 feet deep, was a pretty light earth, easily
dried by the heat of the sun ; under this first layer there was a stratum of fat
clay, which at present lies open in several places, and which is very moist, so
that the water is seen bubbling out of it in some places.

The great rains that fell in the beginning of the spring, soaked through and
diluted this stratum of clay, which retained and gathered all the waters of tlie
hill running between the two layers ; the heat of the summer ensued, which
dried up the upper surface, and formed it into a sort of solid crust, which rest-
ing on a fat and moist clay, and by its steep situation being inclined to slide to-
wards the valley, its whole surface loosened itself by great pieces, and break-
ing in several places, slid along towards the place whither its declivity would
naturally carry it. There are some parts which moved almost insensibly, and
only sunk or subsided, either because the rolling of the neighbouring soils made

VOL. VIII. 3 C



378 PHILOSOPHICAL TRANSACTIONS. [aNNO \73Q.

room, that what was under this surface might slide off, or perhaps because the
parts under this surface had been hollowed a long while before, by the waters
which passed between this surface and the stratum of fat clay. Other parts,
which were much more in number, rolled all together towards the valley, and
whole pieces of vineyards are still seen, with the props remaining upright.
There are again other parts, which in tumbling were overturned in ditferent
manners.

This accident is not without example in the province of Auvergne : there has
not indeed been so considerable a one before, yet it has often happened, that
pieces of earth, of a quarter or half an acre, have separated all in one piece,
from the top of a hill, and slid down visibly on the lands below.

On the Worms* which destroy the Piles on the Coasts of Holland and Zealand.
By Job Baster,f M. D. F. R. S. N° 435, p. -276.

In the year 1730, the persons appointed to take care of the dykes on our
coasts, observed that the piles made of the hardest oak, defending the coasts of
the Netherlands against the sea, were eaten through in a few months, so as to
be broken by the least external force. Surprised at this uncommon and danger-
ous phenomenon, they inquired into its cause, and saw that a kind of worms,
before that time very scarce, but now increased to an incredible number, had
in so short a time eaten into those piles, between the highest and lowest water-
marks, and threatened very great damage to the inhabitants of these countries.

If a pile of the hardest oak has stood 6 months on the shore, and be taken
out in summer or autumn, there appears mud and filth sticking to its outer
surface ; which being scraped off with a knife, discovers a vast number of holes,
hardly so large as pins heads.

Viewing this mud through a microscope, there are seen, 1. A number of
whitish points, not larger than grains of sand. — 2. Some very small worms.

The whitish points seem to be the eggs of this insect, and the worms to be
such as are already hatched from them ; and these worms gradually perforating

* The animal here described is the teredo navalis of Linnaeus.

t Job Basterj an ingenious Dutch physician and naturalist, distinguished himself by a controversy
with the celebrated Mr. Ellis, relative to the animal nature of the corallines, which Dr. Baster con-
sidered as rather the habitations of the inclosed polypes than as forming a constituent part of the ani-
mal. His chief work however is his Opuscula Subseciva, containing observations on various marine
animals. These interesting observations were continued, in distinct parts, from the year 1759 to
1765, and are illustrated by very instructive plates. They relate to the propagation and ovaria of
shell-fish in general; the ostrea, mytuli, pholades, and tellinae; and the structure of several specie*
of the testacea is explained in a satisfactory manner.



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. ' 379

the outer surface of the wood, rendered soft by lying in the water, made the
aforesaid holes, and through them worked their way into the substance of the
wood.

A small style of whalebone or lead, thrust into these small holes, runs
straight into them for 3 or 4 lines, so that its outer end always makes a right
angle with the pile : but afterwards, if the style be gently pushed forward, it
does not continue in the straight line, but runs either way, generally upward.

But if one of these piles be split lengthwise, with a hatchet or wedge, it is
found full of passages, or hollow cylindrical ducts, each of which contains a
worm, surrounded with a thin testaceous substance, exactly filling the ductj,
and forming its involucrum or sheath, in which it can move with freedom.

The ducts, beginning at the outer surface by a narrow hole, grow gradually-
wider, and run either straight, oblique, upward or downward. But what is
most surprising, these ducts never run into each other, nor communicate; but
each continues separate for every single worm. Over the worm's head there
are 2 or 3 drops of a salt liquor, thicker than water, but not the least appear-
ance of the dust of the corroded wood.

Whence it appears, that all the vvood, which had before filled up the place
of the duct, in which the worm with its covering is now found, was eaten
and consumed by the worm : and as it seems quite incredible, that an ani-
mal, which appears soft, and almost as fluid as the white of an egg. should
be able to eat through such hard wood; the description of this xylophagous
worm is offered to the Royal Society, in order to give them some knowledge
of this water-insect, which has done so many millions damage to these
countries.

They are found of various sizes and thickness. There are some of the
younger ones not above an inch or two in length ; some of a middle size, such
as represented in figures 6 and 7, pi. 8 ; and some 13 or J 4 inches long.

But in order to a more accurate description, we will divide the animal into
head, body, and tail.

The head is of a most wonderful structure, being covered with two hai-d
shells or hemicrania, of a substance neither testaceous nor osseous, securing
their softer contents : and being viewed through a microscope, they appear as
in fig. 8, as well as they could be drawn. These hemicrania are two white bodies,
much harder than the substance which forms the testaceous covering ; the
inner surface hollow and smooth ; the outer, convex and rough, with 3 fibres
running different ways ; and both together perfectly represent a double bit, of
that kind of borer, called an auger,

3 Ci



360 PHILOSOPHICAL TRANSACTIONS. [aNNO 173Q.

The upper part of the external convex surface, a, fig. 8, has a very sharp
edge, in which the first series of fibres begins from a point ; which fibres
gradually dilating, and running lengthwise, end about the middle part of it,
which makes a right angle with the upper part. In this part the fibres being
elevated, run crosswise, e, fig. 8. The lower part is thicker than the upper,
but softer and less compact. In this part the fibres are raised up and rough,
first curved, then straight, and, like the others, run lengthwise to the lower
edge of this part, which is strongly fastened to the head by various ligaments
c, D.

The concave or inner part of these hemicrania, which contains the softer
parts of the head, is very smooth ; but almost in the middle has a very small
and tender eminence or process, in shape much like Dr. Rau's process in the
organ of hearing, fixed at one end, and loose at the other, running almost the
whole width, and doubtless destined for supporting some of the inner parts of
the head. See a, fig. g.

These two hemicrania, connected together by strong ligaments, and as it
were by a small hinge, by means of which they can dilate without separating,
besides their defending the soft head from external injuries, are the instruments
with which the animal gets its food. For whatever way it turns its head, the
raised and rough fibres, running either lengthwise or crosswise, always rub off
some of the wood.

These hemicrania being carefully removed, the contained parts, fig. 1 1 , are
laid open to view ; but they are so soft, and of so wonderful a structure, that
the eye, though armed with a microscope, can neither discern their true make
nor use. First indeed there appears a membrane, enveloping the whole head ;
in the middle and anterior part, which is not covered by the said hemicrania, it
appears as if raised by a tubercle c, and in that place it is of a red colour ; but
the lower ligamentous edge firmly adheres, both to the small process, and to
the lower edge of the hemicranium.

This membrane carefully separated and removed, a, a, fig. 12, in the middle
of the subjacent pulp is a small pear-like body, perfectly pellucid, somewhat
protuberant above the other parts, which made the tubercle in the membrane.
It is much harder than the other contained parts of the head and body ; so that
it will bear cutting with the scalpel. It is of a red colour, as perfectly pellucid
as a drop of water; of the shape of a pear, from a larger basis terminating in a
point. It cannot be better resembled to any thing, than to the crystalline lens
of the eye : yet in spirit of wine it preserved its transparency, but its bulk was
diminished, as b, fig. 12.

At the sides, where the lower edges of the hemicrania do not touch each



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. 381

other, there is a sort of cavity ; and in these sides the harder fibres may be
distinguished, disposed in such a manner, as perfectly to resemble the gills of
fish : and through them the worm seems to breathe.

The extreme softness of the other parts of the head prevents our coming at
the knowledge of the use of the membranes, furnished with fibres of different
tendencies, or inquiring by what organs the worm takes the wood shaved off by
the hemicrania, or rough shells ; whether it does this by suction, or not ; by
what muscles, or how acting, this wonderful head is moved. It is probable in-
deed, that its motion consists in the opening and closing these shells, that shave
off the wood ; and that the inner parts have a power to move on all sides, as



Online LibraryRoyal Society (Great Britain)The Philosophical transactions of the Royal society of London, from their commencement in 1665, in the year 1800 (Volume 8) → online text (page 44 of 85)