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then, suddenly spreading out, these vast clouds give rise to the well-
known appearance of the Italian pine-tree. At the same time, from
every available fissure are seen to issue little columns of vapor, adding
their small share to the grandest visible display of force that Nature
lias provided for our amusement or peril.

This brings us to the next point of interest, the formation of fu-
maroles, which may be considered as the effect of these two agents



52 THE POPULAR SCIENCE MONTHLY.

last spoken of acting together. In Fig. 2 arg seen two varieties, one
assuming a spire-like form. These may make their appearance any-
where on the volcano, but are usually situated in close proximity to
the vent. Their size varies from twelve inches in height upward ;
generally from three to thirty feet. They are commenced in the major
number of cases in the fissured crust recently formed over still-flowing
lava. Here the vapor escapes in spasmodic puffs, and by its force a
small quantity of lava is forced up and spread out around the aper-
ture, which rapidly cools. It is followed by another puff and another
oozing of lava above and around the aperture of the first. In this
manner layer by layer is built up, thus giving an irregular, imbricated,
roll-like appearance to the exterior. The surface is rapidly covered by
brilliantly-colored sublimates, and the fumarole then presents a very
pretty spectacle. The author lately was able to thoroughly watch the
formation of such a fumarole some twenty feet high, its decadence
and disintegration extending over a period of eight months. On pass-
ing the arm down the central tube (i. e., the fumarole was extinct), it
could be felt very regular and smooth, and having a pretty uniform
bore of about nine inches.

After one slight eruption, the fumarole in question presented a
very curious phenomenon. Immediately (about two or three seconds)
after the explosion from the main vent, there came three terrific bangs,
with a spout of vapor from its apex, the last one shooting out small
fragments of still liquid lava.

This continued without variation for six hours that the author re-
mained in the crater. The spire-like form may be varied according to




Fig. 3.— Cratek of Deposition.

surrounding circumstances. If the escape take place along a fissure, it
will assume on occasions a miter-like form. There are many other
varieties in form, depending on the variability of surrounding circum-
stances.

It is now necessary to draw attention to the great difference of
opinion which has been expressed upon a point for which we have
very little data to support either of two views of the question.

Vulcanologists were for a long time divided into two schools,
which often waged war against each other with considerable fierceness.
The so-called npheavalists were led by such eminent men as Von
Buch, Elie de Beaumont, and Humboldt ; whereas those who held the



ORIGIN AND STRUCTURE OF VOLCANIC CONES.



53



opposite view, which will be immediately explained, claimed as their
adherents Sir Charles Lyell, Poulett Scrope, and others.

In Figs. 3 and 4 are diagrammatic representations of the two the-
ories.

The upheavalists believed that the earth-crust actually surrounding
the vent was bodily lifted up by the subterranean igneous forces into
a dome-shaped or bubble-like mass, thus forming the main mass of the




Fig. 4.— Crater of Upheaval.



cone, of which the center was the point of fracture, and therefore the
vent. The ejecta were therefore considered to form only a thin super-
ficial crust covering this. The subjacent rock which had been elevated
would thus have a quaquaversal or periclinal dip away on all sides
from the chimney (Fig. 4).

The opponents to this view attribute the entire bulk of the moun-
tain to the ejecta, as seen in Fig. 3, the only change in the basement
beds being those produced by pressure and excavation, both of which
tend to make them dip toward the vent, thus producing quite a con-
verse effect to the former.

This latter view certainly seems the most feasible, and, after a care-
ful examination of many of the old craters brought forward by the
upheavalists as evidence, one becomes satisfied that they have wrongly
interpreted facts, which the more advanced state of knowledge at the
present day and the collected experience of subsequent observers make
easy to our perception. On the other hand, it would be undoubtedly
rash to conclude that all craters were formed entirely on one or the
other model. Jorullo, in Mexico, for instance, has many points about
it to support the upheaval theory. David Forbes, that clear observer,
mentions many facts about South American volcanoes that should
deter us from admitting the formation of cones and craters by the de-
position of ejecta only.

The rapidity with which a volcanic cone may be raised is a point
of great interest. We hear every now and then of some small island
appearing and again disappearing below the sea almost as rapidly as
it rose. Probably, however, the best illustration is that of Monte
Nuovo, four hundred and fifty-six feet high, situated in the Campi
Phlegraci, about eight miles west of Naples. This was raised from a



54 THE POPULAR SCIENCE MONTHLY.

marshy plain almost level with the sea in about four days, commenc-
ing on September 30, 1538.

The whole hill is, therefore, the product of one eruption. It is an
interesting fact that no stream of lava was developed. It would seem
that the explosions were so intense that the fluid rock was entirely
broken up and ejected in a fragmentary condition, of which there are
great quantities forming the slopes.

The cone of Vesuvius proper, fifteen hundred feet high above the
lowest edge of the crater of Somma, has entirely been built up of the
ejecta thrown out at the time of and since the memorable eruption of
A. D. 79, in which Ilerculaneum, Pompeii, Stabia, etc., were destroyed.
Besides the bulk of the mountain now seen, we must not forget the
vast quantity that has been required to fill up the crater of Somma,
much enlarged by the eruption spoken of. It is said that no lava ran
from Vesuvius till the tenth century ; this probably would be explained
by the fact that all the earlier streams were occupied in filling up the
great crater. — Science- Gossip.



EYES A^D SCHOOL-BOOKS.

By Professor HERMANN COHN.

IT was formerly considered, and some recent text-books have re-
peated the error, that the qualities of near-sighted and long-sighted
eyes were opposed. The investigations of Professor Donders, of
Utrecht, have, however, shown that not only is long-sightedness not
the opposite of near-sightedness, but that the two defects may be
associated in the same individual. The real opposite of short-sighted-
ness, according to Professor Donders, is over-sightedness. He dis-
tinguishes three kinds of eyes : 1. Those whose axis is of the proper
length from front to rear, normal-sighted or emmetropic {ev fitrpcj wi/),
seeing at the right distance) ; 2. Those whose axis is too long, short-
sighted, or myopic (from ^veiv, to blink, from the habit common to
near-sighted persons of partly closing the eyelids in looking at distant
objects) ; and, 3. Those whose axis is too short, over-sighted, or hyper-
metropic, seeiyig beyond the measure. To see at a distance, the em-
metrope needs no glass, the myope a concave glass, the hyperope a
convex glass.

All three kinds of eyes may become far-sighted or old-sighted as
their near vision becomes weaker in old age. This kind of far-sight-
edness is no more a disease than the turning gray of the hair ; it
depends upon the diminished force of the muscle that curves the
crystalline lens for near vision.



EYES AND SCHOOL-BOOKS. 55

Myopy is seldom, congenital. All experts remark that it is rarely
found in children of less than five years of age. All agree likewise
that it arises from a too steady application of the eyes to close objects,
especially during the school age. The attention of the authorities in
Baden was directed to this fact forty years ago, by the number of
students in the gymnasia who wore spectacles. Their inquiries were
followed up by Dr. Szokalsky in Paris. Professor C. von Jager, of
Vienna, in 18G1, was the first person who made a systematic examina-
tion of the eyes of children in reference to this point. Out of two
liundred children, he found fifty-five per cent, of those in an orphan
liouse, and eighty per cent, of the pupils in a private school, to be
short-sighted. He did not, however, consider his investigation ex-
tended enough to justify his di-awing a general conclusion.

I began in 1865 to examine the school-children of my native city,
and believed, after I had gone through thirty-three schools of all
grades, up to the gymnasium, containing 10,060 children, that -I was
justified in announcing the three following laws : 1. Short-sightedness
iiardly exists in the village schools — the number of cases increases
steadily with the increasing demands which the schools make upon the
eyes, and reaches the highest point in the gymnasia ; 2. The number
of short-sighted scholars rises regularly from the lowest to the highest
classes in all institutions ; 3. The average degree of myopy increases
from class to class — that is, the short-sighted become more so.

My investigations have been repeated in many cities of Europe
and America, and my conclusions have been everywhere confirmed.
I may cite the examinations of Dr. Thilenius at Rostock in 1868 ; of
Dr. Schultz at Upsala in 1870 ; of Dr. Crismann at St. Petersburg, and
Dr. Maklakoff at Moscow, in 1871 ; of Dr. Kriiger at Frankfort, and
ilerr von Hoffmann at Wiesbaden, in 1873 ; of Dr. A. von Reuss in
Vienna, Dr. Ott and Dr. Ritzmann in Shaffhausen, Dr. Burgl in Mu-
nich, and Professor Dor in Bern, in 1874 ; of Dr. Conrad in Konigs-
l)ei"g in 1875, of Dr. Scheiding in Erlangen, Dr. Koppe in Dorpat,
Professor Pfliiger in Lucerne, and Drs. Loring and Derby in New
York, in 1876 ; of Dr. Emmert in Bern, Drs. Kotelmann and Classen
in Hamburg, Professor Becker in Heidelberg, Drs. William, Agnew,
and Derby, in Cincinnati, New York, and Boston, in 1877 ; Dr. Nie-
mann in Magdeburg, Dr. Seggle in Munich, Processor Dor in Lyons,
Dr. Haenel in Dresden, and Dr. Reich in Tiflis, in 1878 ; Dr. Just in
Zittau, and Dr. Florschutz in Coburg, in 1879. We have in all more
than thirty accurate reports of competent oculists, giving the results
of the most careful investigations among more than forty thousand
scholars.

The final results of all these observations, when combined, show
that in the village schools hardly one per cent., in the elementary
schools five to eleven per cent., in the girls' schools ten to twenty-four
per cent,, in the real schools twenty to forty per cent., and in the



56 THE POPULAR SCIENCE MONTHLY.

gymnasia between thirty and fifty-five per cent, of the pupils are
myopic.

University students have so far been examined only in Breslaxi
and Tubingen. I found in 18G7 fifty-three per cent, among the Cath-
olic theologues, fifty-five per cent, of the law students, fifty-six per
cent, of the medical students, sixty-seven per cent, of the evangelical
theologues, and sixty-eight per cent, of the students of philosophy, to
be short-sighted. In July, 1880, I again examined our medical stu-
dents, and found that fifty-two per cent, of thosb who had not passed
the exarnen physicum, and sixty-four per cent, of the candidates who
had already stood the examination, w^ere myopic ; and I am convinced
that the work of preparing for the examination in this as well as in
the other departments contributes to the increase of near-sightedness.
Dr. Gartner, between 1861 and 1879, examined six hundred and thirty-
four students of the Evangelical Theological Seminary in Tubingen,
and found that seventy-nine per cent, of them were myopic.

If we inquire into the bearing of nationality on the development
of the affection, we find that in the gymnasia at TJpsala thirty-seven
per cent., at St. Petersburg thirty-one per cent., at Dorpat fifty-five
percent., at Lyons twenty-two per cent., at Tiflis thirty-seven per cent.,
at New York twenty-seven per cent., at Boston twenty-eight per cent.,
of the students are myopic. In the gymnasia of St. Petersburg thirty-
four per cent, of the Russian, and only twenty-four per cent, of the
German scholars ; at Tiflis, thirty per cent, of the Russians, thirty-
eight per cent, of the Armenians, and forty-five per cent, of the Geor-
gians, were short-sighted. Of five hundred and twenty-nine teachers
in Lucerne, fourteen per cent, of the Latin Swnss, twenty-four per
cent, of the German Swiss, were affected. Loring and Derby ob-
served in New York, in 1876, that fourteen per cent, of the children
of Irish, twenty per cent, of American, and twenty-four per cent, of
German parents, were near-sighted. At the International Congress
of Physicians, held in Paris in 1867, 1 confidently addressed every one
who wore spectacles in German, and was sure to receive a German
answer. It is possible that the Germans have become more than
ordinarily predisposed to short-sightedness, by the operation of com-
pulsory education through several generations ; but this can not yet
be taken for granted, for relatively only a small proportion of non-
German school children have been examined. The statements of all
the authorities establish, however, that everywhere, and in all institu-
tions, the number of myopes increases from class to class, and becomes
really formidable in the secunda and prima of the gymnasia and real
schools, and the corresponding classes of other schools. It ranges at
between thirty-five and sixty per cent, of the whole number of schol-
ars ; but the proportion has been found to exceed sixty per cent, in
the prima of several German gymnasia, and to rise to eighty per cent,
at Erlangen, and one hundred per cent, at Heidelberg. Taking the



EVES AND SCHOOL-BOOKS.



57



average of the results of the examinations in twenty-five German and
Swiss gymnasia with 9,096 scholars, the percentage of short-sighted
pupils rose from the sexta to the prima as follows : 22, 27, 33, 46,
52, 53.

These numbers speak plainly enough. Still there are persons who
doubt that children become short-sighted at school. In order to make
this more clear, I examined the pupils of the Friedrichs Gymnasium
at Breslau in 1871, and repeated the examinations upon the same per-
sons three semesters afterward. Seventeen pupils who had been
found normal-sighted at the first examination had become short-
sighted, and more than half of those who had appeared near-sighted
at first had become more so. Similar results have been obtained
by Dr. A. von Reuss in the Leopold Stadt Gymnasium at Vienna,
by Dr. Seggle in the Cadet Corps at Munich, and by Dr. Derby at
Boston.

It is evident that we are threatened with a great national affliction,
which is likely not only to be detrimental to all peaceful occupations,
but to impair the military efficiency of our people. It is important to
seek out the causes of this ever-growing evil and contest them with
energy. We can not discuss here all the causes that tend to produce
myopy. All protracted looking at close objects may contribute to it.
Among the more active causes may be mentioned badly-constructed
school-benches, imperfect lighting, too much reading, bad writing, and
bad type. The matter of the style of typography which is most com-
patible with the preservation of the eyesight deserves especial con-
sideration. The most important point is the size of the letters. We
can not determine this by the measurement of the em, as the printers
do, for that regards the shank of the type, of which readers know noth-
ing ; but it must be judged by a special measurement of the visible
letter. I have adopted as the standard of measurement the letter n,
that being the most regular and symmetrical in shape in both the
Roman and German alphabets. I have found that the n in pearl
type is about 0-75 millimetre (or about y^o o^ ^^ inch) high, in
nonpareil 1 millimetre (or about ^ of an inch), in brevier (petitschrift)
\\ millimetre (or about -^ of an inch), in long primer (corpusschrift)
H millimetre {^ inch), and in pica (Ciceroschrift) If millimetre
(^ inch).

We have hitherto had no definite rules concerning the smallest size
of letters which should be permitted for the sake of the eyes. The
distance at which a letter of any particular size can be seen does not
afford a guide to it, for it does not correspond at all with the distance
at which matter printed in the same type can be read steadily, at the
usual distance in reading. I believe that letters which are less than a
millimetre and a half {^ inch) high, will finally prove injurious to the
eye. IIow little attention has hitherto been paid to this important
subject is exemplified in the fact that even oculistic journals and books



58 THE POPULAR SCIENCE MONTHLY.

frequently contain nonpareil, or letters only a millimetre (3^5 inch)
high.

Many of the text-books required by the school authorities are bad-
ly printed. The officers should go through every school-book with a
millimetre-rule in their hands, and throw out all in which the letters
are less than a millimetre and a half high, and should give the prefer-
ence to those establishments which do not use letters of less than two
millimetres (^j inch).

The distance between the lines is an important factor in respect to
ease in reading. As is well known, the compositors often insert thin
leads between the lines so that the letters which project above the
average height and those that fall below the line shall not touch.
Every one knows that legibility is improved by contrast ; the darker
the print and the clearer the paper, so much easier is the reading.
AY hen the lines are close together, or the matter is printed " solid," the
eyes become tired sooner, because the contrast is lessened. The lines
tend to run together, and the effort to separate them strains the eyes.
In fine editions the lines are widely separated. I consider a book well
leaded in which the interlinear space, measured by the shorter letters,
amounts to three millimetres (^ inch). The lines will really seem to
be closer, for the projections of the longer letters will encroach upon
the interlinear space ; and cases may occur, when those letters pre-
dominate, in which the space may seem to be only one millimetre.
The narrowest interval that should be permitted is, in my opinion, two
and a half millimetres {^ inch).

The thickness of the strokes should also be regarded, far it is ob-
vious that the form of the letter is more readily and more clearly im-
pressed on the retina when the stroke is broad and distinct than when
it is fine. Letters having a stroke of less than one fourth of a milli-
metre (7^ of an inch), in thickness should not be admitted into school-
books. Ample space should be allowed between the letters. Labou-
laye recommended that every two letters should be separated by a
clear space at least as broad as the distance between the two strokes
of the n.

Javal believes that the extension of the lines beyond a certain limit
of length contributes to myopy, by forcing the eye to endeavor to
adjust itself to the varying distances from the eye of the ends and the
middle of the line. This has not been demonstrated, but it is not
improbable. Every near-sighted person is aware of the pain it occa-
sions him to read a number of long lines without spectacles. The
shorter the lines, the more easily they are read, because the eye does
not have to make wide excursions. The most suitable length of lines
for school-books appears to be about ninety millimetres, or three and
a half inches.

Javal has observed that the rectangular Roman letters are liable to
be reduced in apparent size, and have their corners seem rounded by



DEEP-SEA INVESTIGATION. 59.

irradiation from the white paper, and recommends a thickening of the
cross-strokes at the ends to obviate this defect. This observation is
less applicable to the German letters, for they already have broken
lines and knobbed expansions at the ends of the strokes. Many
physicians, particularly those who are not Germans, believe that tho
shape of the German letters is more tiresome to the eyes than that of
the Roman letters. I have never been able to perceive this, nor any
reason why it should be so, provided the German print is large and
thick enough, and the lines are far enough apart. Use has doubtless
much to do with the matter. For myself, it is always pleasant, after
a long reading of the monotonous Roman print, to return to " our
beloved German."

Even the thickest and largest letters, the shortest and best sepa-
rated lines, and the most excellent printing, may speed the progress of
myopy if the light is bad. At home, every one can find a light place
to read — by the window on dark days, by a bright lamp at night. It
is different in schools and ofiices. Fifteen years ago, after measuring
the ratio of the wnndow-space to the floor-space in the schoolhouses of
Breslau, I declared that there could never be too much light in a
schoolroom, and estimated that unless the house could be furnished
with a glass roof, at least thirty square inches of window-space should
be provided for each square foot of floor-space. In many schoolrooms
as at present arranged, the pupils nearest the windows may be sitting
in a glare of light, while those farthest away are not able to study for
the obscurity. Notwithstanding all that has been written and all that
has been done in the last fifteen years for the improvement of school-
rooms, enough is still left to be done in nearly every town. — Deutsche
Hundschaii.



DEEP-SEA INYESTIGATIOK*

By J. G. BUCHANAN, F. E. S. E.,

OF THE CHALLENGER EXPEDITION.

THE first problem of deep-sea investigation is to determine the
extent of the ocean, its size, its volume. The superficial extent
and limits are determined by the surveyor. In order to map out the
bottom of the sea, there is only one method, namely, the direct de-
termination of the depth at as many places as possible. When a ship
is " in soundings," the depth is ascertained by the ordinary hand lead-
line, which is from twenty to twenty-five fathoms long, and is con-
ventionally marked at stated intervals with bits of leather, white,

* Abridged and condensed from an address delivered before the Society of Arts, Feb-
ruary 24, 1881.



6o THE POPULAR SCIENCE MONTHLY.

red, and blue bunting, and knots. The lead is a long, finely-tempered
block, generally weighing fourteen pounds, which has a recess at the
thick end, and is perforated at the other end for the reception of the
line. This instrument is chiefly used while the vessel is in motion.
The leadsman swings the lead vigorously, so as to give it momentum
enough to carry it well in advance of the ship before it touches the
water. It sinks rapidly while the leadsman's position is advancing to
the spot where it touched the water. The depth is ascertained by
looking at the marks on the line. This method is effective and cor-
rect enough for ordinary purposes, in depths of not more than twelve
or fifteen fathoms. Accurate soundings may be obtained by reducing
the speed of the vessel as much as possible, in depths which do not
much exceed thirty or forty fathoms. In ocean-water, where depths
of two or three thousand fathoms are met, the vessel must be kept
stationary, and heavier weights than are found sufticient for shallow
soundings must be employed.

Deep-sea soundings have received much attention during the last
thirty years. The first attempt at them appears to have been made
by Captain Constantino John Phipps, during his Arctic Expedition in
1773, He sounded a depth of six hundred and eighty-three fathoms
with a lead Aveighing one hundred and fifty pounds, which appears
to have sunk about ten feet into the mud. Determinations of the
temperature of the sea water and of its density were made at the same
time. Captain John Ross employed, during his Arctic voyage of 1818,
one of the earliest satisfactory instruments for bringing up a consider-
able quantity of the bottom mud in deep water, with which he was
able to ascertain the temperature at any depth.

A contemporary of Ross, the younger Scoresby, observed that,
Avhen in sounding at great depths the ordinary deep-sea line and lead
are used, the increasing weight of line, in proportion as more of it is
required, renders less certain the determination of the moment when
bottom is reached. He has also left the record of the first observa-



Online LibraryD. S. (David Samuel) MargoliouthThe Popular science monthly (Volume 19) → online text (page 7 of 110)