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night, it reached the end of the plain, and to the foot of the low hills situate
to the south. But as these hills are rugged with rocks, the greater part of the
torrent ran down the declivities between these rocks, and into two valleys ;
falling successively into the other plain, which forms the basis of the mountain ;
and after uniting there, it divided into 4 lesser torrents, one of which stopped

3 A 2



3(54 PHILOSOPHICAL TRANSACTIONS. [aNNO 1739.

in the middle of the road, a mile and half distant from the Torre del Greco.
The second flowed into a large valley. The third ended under the Torre del
Greco, near the sea ; and the fourth at a small distance from the new mouth.

The torrent, which flo\*ed into the valley, ran as far as between the church
of the Carmelites and that of the Souls of Purgatory, by the 8th hour on Tues-
day. The matter of the torrent ran like melted lead : in eight hours it ad-
vanced 4 miles. The trees, which the torrent found in its way, on the first
touch took fire, and fell under the weight of the matter.

The torrent which ran behind the convent of the Carmelites, after setting
the little door of the church on fire, entered not only by it, but also through
the windows of the vestry, and into two other chambers. In the refectory, it
burnt the windows ; and even the glass vessels, that stood on the tables, were
melted into a paste by the violent heat of the fire. Sixteen days afterwards,
the matter continued hot, and was very hard, but it was broken by repeated
blows.

A piece of glass fastened on the top of a pole, and thrust into this matter,
was in 4 minutes reduced to a paste. Under the mass of the torrent were heard
frequent reports, which made the church shake, as if by an earthquake. Along
the whole surface of the torrent, there appeared small fissures, out of which
issued smoke, that smelled of brimstone, mixed with sea-water ; yet these exha-
lations are not poisonous, but rather a remedy for some diseases. The stones
round about these fissures were observed to be covered with sublimed salts.

Iron, thrust into these fissures, was taken out moist ; though on thrusting in
paper, it was not moistened, but rather somewhat hardened.

At the same time when the new mouth opened, that on the summit of the
mountain vomited a vast quantity of burning matter, which, dividing into tor-
rents, and small streams, ran partly towards the Salvadore, and partly towards
Ottajano; and at the same time that this matter issued out, red-hot stones
were seen to be cast out of the mouth, in the midst of black smoke, frequent
flashes of lightning and thunder, all produced by the same matter.

These impetuous expulsions of fire continued till Tuesday, when the erup-
tion of the melted matter, the flashes, and thundering noise, ceased; but a
strong south-west wind arising, the ashes were carried in great quantities to
the utmost boundaries of the kingdom ; in some places very fine, in others as
coarse as Ischian sand: and in the neighbourhood they not only felt this plen-
tiful shower of ashes, but likewise pieces of pumice-stones, and other large
stones.

Tuesday night the fury of the mountain began to abate, so that on Sunday
there was scarcely any flame seen to break out of the upper mouth ; and on



VOL. XLI.] VHILOSOPHICAL TRANSACTIONS. 365

Monday but little smoke and ashes. This day it began to rain plenti-
fully, which continued to Tuesday, and afterwards for many days : a circum-
stance which has constantly happened after the eruptions of times past.

The damages done in the neighbourhood by this eruption of fire and ashes,
are incredible. At Ottajano, between 4^ and 5 miles from Vesuvius, the ashes
on the ground were 4 palms high. All the trees were burnt, or blasted, the
people terribly affrighted, and many houses crushed by the weight of the ashes
and stones that fell.

After the description of this fiery eruption, the Academy of Sciences at
Naples made an accurate analysis of the matter, and of the salts, that were
collected in great plenty near the fissures; and, towards the discovery of the
truth, they made the following experiments :

Exper. 1 . — Some of the stones of Vesuvius being pounded small, and the
loadstone applied to the powder, some few particles were attracted by it ; and
the same powder, put into aquafortis, caused a sensible effervescence; whence
it certainly contains no small quantity of iron : which was also found on trial
in another eruption by Tomaso Cornelio, But for the greater elucidation of
truth, one of these stones being applied to the magnetic needle, it turned to
the stone ; and then carrying it round to the opposite end of the needle, it
immediately turned from it, in the same manner as if iron was applied near
the compass.

Exper. 1. — ^The stones are not all of the same density or colour; but various,
and of different ponderosity. Some are composed of real talc, others full of
marcasites : some are almost all sulphureous, others nitrose; some of a grey
colour, others red.

Exper. 3. — The matter of the current is spongy at top, but very dense to-
wards the bottom ; which is a proof of its fusibility; the heavier bodies sub-
siding, and the lighter remaining at top.

Exper. 4. — After growing hard, it retained part of the heat above a month,
though unequally : for in the interior parts, where the air had no free access,
and the matter was more compact, the heat was much stronger, than towards
the surface.

Exper. 5. — ^Twenty days after the eruption, in divers parts of the mountain,
from the bottom to the top, there were seen to arise many pernicious damps,
[mofete] especially from the cavities, and the fissures of former torrents; as
also on the plain : but none were observed in the matter of this last eruption.
They issued out of the fissures under the appearance of a cold wind, and rose
about 3 palms high ; then they moved along the surface of the ground, and»



366 PHILOSOPHICAL TRAKSACTIONS. [ANNOl73t|.

after a progress of some paces, disappeared. Animals, which happened to
graze where these passed, were all killed by it ; and likewise a Teresian friar,
who inadvertently breathed the vapour of one of these damps.

Exper. 0. — Having placed the barometer in the vapour, it underwent no
change, but the thermometer fell somewhat more or less. A lighted torch,
thrust into them at two palms from the ground, was soon extinguished by the
action of the damp.

Exper. 7. — ^These damps grew gradually weaker in their pernicious effects,
for above 3 months, even to the subsequent autumn; as has been generally
found in other former eruptions, or when they happened to issue out of
their vents.

Exper 8. — Concerning the salts which are generated in abundance in Vesu-
vius, I have, by order of the Academy, examined them by accurate experi-
ments, My intention was to know, if besides sal ammoniac, there were also
sea-salt, vitriol, nitre, or any other salt. I thought there was no better way of
proceeding in this inquiry, than by crystallization ; because it is universally al-
lowed, that salts in crystallizing constantly retain one certain and determinate
figure ; sea-salt concreting into cubes, vitriolic salt into rhomboidal parallepi-
peds, alum into octaedrons, and nitre into rectangular prisms on hexagonal
bases. I imagined, that if the salt of Vesuvius happened to contain any par-
ticles of the salts abovementioned, it would discover them after crystallization.
This way of reasoning was confirmed by experiment : for the Vesuvian salt, in
crystallizing, left on the sides of the vessels small parcels of crystallized salts,
which, observed through a microscope, resembled a tree with its branches, on
the ends of which there appeared several pyramids of an irregular figure, but
very sharp-pointed ; and between the branches there were interspersed iti some
places a group of prisms, in others some small cubes: whence I inferred, that
the salt was ammoniacal, and indeed a genuine and efficacious sal ammoniac,
with insensible portions of nitre and sea-salt. Which coincides with the sen-
timents of the Royal Academy of Paris in 1 705 ; with those of Thomas Cor-
nelius in his Progymnasma de Sensibus; of Dominicus Gulielmini in his
Treatise de Salibus; of Dr. Boerhaave in his chemistry, and many other
writers.

Exper. 9. — In order to be convinced whether this salt was really ammonia-
cal, and of the nature of neutral salts, I mixed it with spirit of vitriol, and
spirit of salt, without producing the least fermentation. I afterwards put some
of it into oil of tartar per deliquium, but could not perceive any ebullition; so
that it is to be ranked among the neutral salts.



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. 367

Exper. 10. — Thrown upon red coals, it did not crepitate like sea-salt, but
it boiled and swelled, and after evaporating it dried up.

Exper. 1 ] . — It is of a very pungent taste, strongly pricking the tongue,
and of a bituminous smell of brimstone, which occasions a violent head-ach
by its volatile texture.

Exper. 12. — The salts taken from different stones are not all of the same
weight or colour : for some are yellow and unctuous, as if rubbed all round
with petroleum : others are very white, others blackish, and others of other
colours, according to the stones they adhere to.

Exper. 13. — I have likewise found by experience, that the sal ammoniac of
Vesuvius is much more efficacious, than any other salt known at this day, in
cooling liquors. On dissolving some of it in water, it makes the water so cold,
that the sides of the vessel which contains it, can hardly be touched without
uneasiness, through the excessive cold.

Exper. 14. — Mons. GeofFroy, a celebrated member of the Academy of
Sciences, thinks it a singular power of common sal ammoniac, that being
mixed with a certain quantity of water, it rendered the water so cold, that
it made the spirit of his thermometer, 18 inches high, fall 33 lines. But the
Vesuvian salt makes the liquor of a thermometer, like his, fall 44. inches ;
which is equal to 54 lines. So that the efficacy of this salt, in causing the fall
of the liquor, exceeds the efficacy of common sal ammoniac by 21 lines.

Exper. 15. — If round a vessel full of water cooled with snow, there be put
some of the salt of Vesuvius, the water freezes and grows hard in a very
little time.

Exper. 16. — If a good quantity of the salt of Vesuvius be put into snow,
set round a glass vessel full of water, and then stir the vessel, the contained
water becomes unfit to drink ; having acquired a very disagreeable acrid sul-
phureous taste; a manifest sign, that the salt is divided into small particles,
which passing through the insensible pores of the glass, enter into and mix
with the water.

Exper. 17. — Of all kinds of salts, this dissolves in the greatest quantity in
water; and perhaps the greater or less solubility of a salt in water, will be
found proportional to its greater or less effiict in cooling water.

Exper. 18. — Being put into brandy, or oil, besides that very little of it
is dissolved, it occasions no descent of the liquor in the thermometer.

Exper. 19. — Being mixed with blood lately drawn from the vein of a man,
but coagulated after settling, the blood was dissolved, and continued in that
state for the space of 24 hours.

Exper. 20. — A solution of this salt being injected into the vein of a dog, first



3(>8 PHILOSOPHICAL TRANSACTIONS. [aNNO IJSQ.

occasioned tremors, then universal convulsions, and lastly death. And 4 hours
afterwards, having opened the dog, the blood, which should have been coagu-
lated, was found fluid, both in the trunks of the veins, and at the ends of the
arteries.

Exper. 21. — It has all the properties of sal ammoniac to that degree, that on
substituting this Vesuvian salt, instead of common sal ammoniac, the strongest
sort of aqua regia may be had for dissolving gold ; which experiment was made
with success by Mons. Lemery, in the academy of France.

Exper. 11. — If a lump of the mineral matter be reduced to a fine powder,
and attentively viewed through a microscope, it appears very like the sand of
Ischia, and is very proper for writing-sand. Hence probably that sand is no-
thing else but the same matter for a long time comminuted by the action of
the sea.

Exper. 13. — In some of the stones there appear some few veins of gold, in
others of silver, but insensible ; and in others, which are very heavy, there is
some antimony.

Exper. 14. A great dispute arose in the academy on the rise of the (Mofete)
damps ; for what reason these should be seen only in the old strata of the mineral
substances, and not in the new, where by the action of the fire they ought to
issue ; which phenomenon, if I am not mistaken, may be accounted for in this
manner. As the cooling of the burning matter began at the surface, we may
think, that the more subtle heterogeneous particles, on the closing of the pores
at the surface, remained in quantities buried in the lower parts of the matter ;
which, in process of time, becoming acutangular and of deleterious figures, yet
cannot offend while imprisoned. But in new eruptions, when the shocks given
to the matter produce many fissures, the damps, meeting with less resistance
there, issue forth. As when the air is a long time pent up in some hollow, on
giving it vent, it generally comes out in a pernicious vapour.

Exper. 25. It was observed, that the greatest shocks happened to such
things as stood exposed to the volcano ; but that those things which were not
thus exposed to it, received but faint shocks : a manifest sign, that the vibration
of the air had a great share in the shocks of the earth : which circumstance is
taken notice of by Borelli with respect to Mount Etna.

An Abstract of a letter from an English Gentleman at Naples to his Friend in
London, containing an Account of the Eruption of Mount Fesuvius, May 18,
and the following Days, 1737, N. S. N° 455, p. 252.

I was lodged for some time at Chaja, and afterwards at Fontina Medina, in
the face of this surprising mountain, and at 2 or 3 miles distance.



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. 36O

By all accounts, there has not been any eruption remembered near so violent,
nor so furious ; and authors mention none to this degree later than above J 00
years since. On Friday, May 17, 1737, N. S. I observed, as far as I could
see round, that the mountain was covered with white ashes a great way down,
as it has been with snow in the winter. Pliny observes in these words :
" Praecesserat per multos dies terraemotus minus formidolosus, qui Campaniae
non solum castella, verum etiam oppida vexare solitus." (Plin. lib. vi. ep. 20.)
Other authors say the contrary ; though it may very likely be so, round and
near the foot of the mountain ; but this time I have not found any body sen-
sible of it here ; but it is certainly true, that our windows and doors shook
all the time of the violence of the eruption, which I take to be from the very
great concussion of the air on the violent explosions.

On Saturday night. May 18, this great phenomenon began, and increased
so much on Sunday, that it brought half the people out to gaze at it. There
were certainly, among some, great apprehensions, by their being employed in
processions, visiting their churches, and exposing their images of the Virgin
Mary.

I very boldly set out on Monday, about 2 hours before sun- set. It was a
melancholy sight, to see the road full of numbers of poor wretches, flying as
from Sodom. I stopped on the way, to observe the vast clouds of smoke
thrown up in a prodigious column, to an amazing height, which, by its gentle
waving and undulation, was a most beautiful sight ; and when it had mounted
so high, that it had lost the force of the protrusion, it was carried by the wind
a vast way ; but not too far for one to observe how its rolls began to break,
and, being dispersed and expanded, covered the country underneath with ashes
and darkness. Many great flashes of lightning were darted through this pillar
of smoke, and frequent discharges as of cannon or bombs, which were followed
by falling stars, such as we see from well-made rockets. We turned off" out of
Portici, to gain the north-side of the mountain, as far as we could, in chaises,
till we were forced to get upon asses or mules.

It was now growing dark, and the fire began to be visible, which it was not
in the day-time, the sun bearing no rival.

In a little time, by the light of the mountain, though that was much ob-
scured by the clouds and pillar of smoke, and the help of our torches, we
scrambled over very rough roads, till we got within about a quarter of a mile of
the great lava or current. But then we halted, as the scene on all sides became
so stupendous and terrible.

We returned to Portici, where we supped, and got home, much fatigued,
by 2 in the morning. The fury of this eruption was at its height this night, as

VOL. VIII. 3 B



370 PHILOSOPHICAL TRANSACTIONS. [aNNO l/Sp.

to burning ; but the next day, Tut'sday, the columns and bouYllons of sinoke
were as great, and thrown out with as much violence, which, as the wind sat,
carried its destruction, not of the large massy metallic bodies, but of infinite
quantities of ashes and cinders, all that day, and part of the night- Through
the columns of smoke was a continued lightning, the most beautiful sight
imaginable.

The following day, Wednesday, we set out again, to view the west-side of
the mountain at Torre del Greco, 8 miles from hence ; where the great lava
had stopped at the church of the Carmelites, but not without carrying part of it
away. This lava had, from the declivity, taken the water-course, which was
the preservation of the country from being drowned. This hollow, which was
for some miles between 30 and 40 feet deep, and as many wide, was not only
filled up, but the matter rose as many feet above the surface of the land about
it. We walked to view it on one side, but the heat was so intense, and the
sulphureous stench so suffocating, that we were obliged to keep at a good
distance ; and I was well informed by several, that it continued very hot 4 or
5 weeks after ; so long in cooling is that great quantity of bituminous and me-
tallic matter, with which this vomes is loaded.

As the fury of the expulsion and explosion was much abated on Tuesday
morning, the stop here was about 4 o'clock that day in the afternoon ; which
might be the more easily conceived, when no more of this vast metallic matter
was discharged, and the motion of all the rest was relented, for want of more
protrusion, and the bitumen growing a little cooler. As this stop was made
at the church, part of the lava took a turn into the great large road to Salerno,
to a great height ; which part is choaked up for ever, the expence being im-
mense to remove it.

Some persons say, that the matter discharged this time in the different cur-
rents or lavas round about, would make a mountain as large as their sire. The
Carmelites here soon fled, and were not come back 10 days afterwards, when
we returned that way, to visit the south-east side, to view the great devastation
which was made about Ottajano, ] 8 miles froui hence ; for though the great
discharge of the metallic body ceased on Tuesday, a vast destruction of the
country followed for a long time after ; for as the force of the explosion was
very great, it continued to throw out vast showers of cinders and ashes. The
lands indeed, where the lavas fall, are annihilated to the owners ; and the other
materials destroy all the fruit and produce of the earth where they fall, which
does not recover for a long time.

As we turned on the left from Torre del Greco towards Ottajano, we passed
all the way through their masserias (farms) ; and the mountain, being to the



_i_ .



VOL. XLI.l 1>HIL080PHICAL TRANSACTIONS. 371

windward of us for 3 or 4 miles, showered ashes plentifully upon us, and we lost
our sinell of every thing but brimstone. All the trees, vines, and hedges,
bent under the weight of these ashes; several arms, and even bodies of trees,
were broken with the weight ; so that in some narrow roads we had difficulty
to pass. Within a mile or two of the prince of Ottaja no's palace, one can
scarcely frame to one's self a sight of greater desolation ; 10 successive north-
ern winters could not have left it in a worse condition ; not a leaf on a tree,
vine, or hedge, to be seen all the way we went, and some miles further, as we
were informed. Here, and at the town, they had a new earth, about 2 feet
deep, some said more, by the account of the miserable inhabitants, who were
a dismal spectacle. The storm fell so thick and heavy for that time, that they
almost all fled, and many houses were beaten down. In one convent, two or
three nuns were buried in the ruins. At Somma, on the north-east side, it
has made great havock ; a monastery of nuns was destroyed. After a long day's
work, we returned at six o'clock.

Of the Lunar Atmosphere. By M. Jean Paul Grandjean de Fouchy, of the
Royal Academy of Sciences at Paris. N° 455, p. 26 1. Translated from the
Latin.

By an atmosphere is meant a certain assemblage of pellucid matter surround-
ing the planet, and capable of turning the rays of light, that pfTss through it,
from a right lined course. M. F. does not here inquire what may be an atmo-
sphere, in other respects, different from that of a refracting medium, but only
undertakes to prove, that the moon is not enveloped by any thing capable of
refracting the rays of light. He conceives an atmosphere, in this inquiry, to
be a homogeneous fluid, with a spherical surface, of a uniform density, which
is equal to the sum of the decreasing densities in the real atmosphere ; pur-
posely omitting the difference of density in the parts, as not disturbing the de-
monstrations.

Now if the moon be encompassed with an atmosphere, its diameter ought to
be found greater than in the naked planet. And that the quantity of the in-
crease may be known, let a i b, fig. 3, pi. 8, be the body of the moon, peg
its atmosphere ; then the angle a h l will be that of the real diameter of
the moon, and the angle e h l, made by the axis l h, and the ray a e h,
will be the observed diameter : so that the angle e h a will be the increase
of the moon's diameter by the atmosphere. But the angle e h a is opposite
to the side e a, of the triangle eh a; and the angle a e h, the supplement
to I80° of the horizontal refraction in the lunar atmosphere, is opposite to the

3b 2



372 PHILOSOPHICAL TRANSACTIONS, [aNNO 1739-

side A H, the moon's distance from the earth. Also the side e a is the half of a
chord of the lunar atmosphere touching the moon's body at a ; therefore the
sine of the increase e h a, will be to the sine of the horizontal refraction, as the
semichord a e is to a h, the moon's distance.

Hence it follows, that the increase of the moon's diameter is insensible ; for,
if it amounted to 2|", supposing the horizontal refraction 3', i.e. at least 30
times greater than it can be supposed, as will be proved hereafter, then the
semichord e a would be equal to 276 French leagues, and thus far exceed a like
chord of the terrestrial atmosphere. Therefore, whether the moon is covered
with an atmosphere or not, her diameter will always be observed the same : so
that the observation of the lunar diameter can never be sufficient for resolving
the problem.

But the solar eclipses afford better means for deciding the point ; for the ex-
treme rays bounding the cone of the lunar shadow, as they touch the moon's
body, and pass through her atmosphere, will be necessarily inflected toward
the axis of the cone ; hence the cone will become shorter and more obtuse.
But to know the quantity of that variation, it must be observed, that the ray
p A, fig. 4, or its parallel e g, which in case there be no atmosphere, would be
the limit of the lunar shade f a c, would be refracted towards the axis c A, at
the ingress of the atmosphere g, and at the egress h : hence the semi-angle of
the cone of the lunar shade will be increased by double the horizontal refraction
in the lunar atmosphere.

Hence it follows, that in the supposition of a lunar atmosphere, a total eclipse
of the sun will begin later, and end sooner, than without one ; also, that in
some certain cases, there would be no total eclipse ; which, yet, the diameters
of the sun and moon, observed in the same degree of anomaly, would require ;
for in these cases the cone of the lunar shade might be so constructed, as not
to reach the earth's surface.



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 43 of 85)