lake, where the depth is not beyond the powers of natural vision, is
not that the rays of light reflected from the objects at the bottom
are so feeble as to be imperceptible to our sense, from their passage
through the denser medium of the water, but from the irregular
refractions given to the rays in passing out of the water into the
air, caused by the constant ripple or motion of the surface of the
water, where that refraction takes place. Reflections of light from
the surface also add to the difficulty; and before we can with any
just hope expect to see the objects distinctly at the bottom, these
obstructions must be removed.
This is done to a very great extent by the use of the instrument which
forms the subject of this notice; the tube serves to screen the eyes
from reflections, and the water being in contact with the glass plate,
all ripple is got rid of, so that the spectator, looking down the
tube, sees all objects at the bottom, whose reflective powers are able
to send off rays of sufficient intensity to be impressed on the
retina, after suffering the loss of light caused by the absorbing
power of the water, which obeys certain fixed laws, proportionate to
the depth of water passed through; for as light passing through pure
sea-water loses half its intensity for each 15 feet through which it
passes,[49] we must, from this cause alone, at a certain depth lose
sight of objects of the brightest lustre. The perfect purity of the
water, and its freedom from all muddy particles floating in it, form
an important element in the effective use of the water-telescope; for
example, in the Frith of Forth, and similar estuaries, where the
influx and reflux of the tide keep particles of mud in constant
motion, the instrument is of little or no use; for these act in
exactly the same way in limiting our vision through water, as a fog
does through the air: it is therefore only in the pure waters of our
northern and western shores that this contrivance is applied with any
advantage; and in such situations we can speak of its powers with
confidence. In a trial made with the instrument last autumn on the
west coast of Scotland, the bottom was distinctly seen (a white
bottom) at a depth of 12 fathoms; and on a black, rocky bottom, at 5
fathoms under water, objects were so distinctly seen that the parts of
a wreck were taken up--the exact place of which was not known previous
to its use. In these experiments a lenticular form of glass was made
use of at the bottom of the tube, having a plane surface to the water,
but no great or marked advantage was observable from this
construction. With respect to the history of this contrivance for
viewing the bottom of the sea, we are unable to assign any particular
date: so far as our information goes, it has been in use from a very
remote period. We are informed that it is in general use in
seal-shooting along our northern and western islands, where, sometimes
in the form of an ordinary washing-tub, with a piece of glass fixed in
its bottom, the shot-seal was looked for, and the grappling-hook let
down to bring him to the surface. It may not be generally known, that
in seal-shooting, the shot or wounded seal always seeks the bottom,
from which he never rises after death, till washed ashore by the
action of the sea: it is only when the fatal ball deprives him of the
power of diving that he is ever found at the surface. In such
employments, therefore, the use of this instrument, however modified,
must form an important auxiliary to the best rifle. Throwing oil over
the surface of the water is used in the same pursuits; but this only
so far stills the ripple, leaving the reflections. Our eminent
engineer, Mr Robert Stevenson, made use of the water-telescope more
than 30 years ago, in works connected with harbour improvement in the
north of Scotland; it has also been used to examine the sand-banks,
&c., at the bottom of the River Tay, but in this case the mud
prevented its use in any considerable depth of water. To obviate this
difficulty, the construction was modified thus: by making the tube of
considerable length, and placing the glass at the lower end, this tube
was thrust through the water till within a few feet of the bottom,
acting as a cofferdam to set aside the dirty water, and enable the
bottom to be seen; but in this method of application it was found very
difficult to hold the tube down in the water from its buoyant power,
and we are informed by Mr Thomas Stevenson, C. E., that, he understood
from this cause its use had been discontinued. He suggested a simple
remedy; viz., to fill up the empty tube with pure water. We are
indebted to Mr Mitchell, the gentleman already mentioned, for having
brought this instrument into notice in the public prints, under the
name of Norwegian water-telescope, on the shores of which country it
is stated to be much used in fishing--in particular, that of the
herring; but the herring-fishers on the east coast of Scotland inform
us, that they require no such auxiliary, as, from the surrounding
elevated grounds, they can tell the position of the shoal, and, from
their motions seen from such situations, they know where they are to
be found when they go out a-fishing.[50]
Footnote 49: Leslie's Elements of Nat. Phil., p. 19.
Footnote 50: _Norwegian Water-Telescope._
The water-telescope is thus noticed in a very promising
periodical, the American Annual of Scientific Discovery, just
published, of which a copy reached us a few days ago.--ED. _Phil.
Journal_.
The water-telescope is an instrument which the people of Norway
have found of so great utility, that there is scarcely a single
fishing-boat without one of three or four feet in length, which
they carry in their boats with them when they go a-fishing. When
they reach the fishing-grounds, they immerse one end of this
telescope in the water, and look through the glass, which shews
objects some ten or fifteen fathoms deep as distinctly as if they
were within a foot of the surface. When a shoal of fish comes
into their bays, the Norwegians instantly prepare their nets, man
their boats, and go out in pursuit. The first process is minutely
to survey the ground with their glasses, and where they find the
fish swarming about in great numbers, they give the signal, and
surround the fish with their large draught-nets, and often catch
them in hundreds at a time. Without these telescopes their
business would often prove precarious and unprofitable; as the
fish, by these glasses, are as distinctly seen in the deep, clear
sea of Norway, as gold-fish in a crystal jar. This instrument is
not only used by the fishermen, but is also found aboard the navy
and coasting-vessels of Norway. When their anchors get into foul
ground, or their cables warped on a roadstead, they immediately
apply the glass, and, guided by it, take steps to put all to
rights, which they could not do so well without the aid of the
rude and simple instrument, which the meanest fisherman can make
up with his own hands, without the aid of a craftsman. This
instrument has been lately adopted by the Scotch fishermen on the
Tay, and, by its assistance, they have been enabled to discover
stones, holes, and uneven ground, over which their nets travel,
and have found the telescope answer to admiration, the minutest
object in twelve feet of water being as clearly seen as on the
surface. We see no reason why it could not be used with advantage
in the rivers and bays of the United States.
_Experimental Investigations to Discover the Cause of the Change
which takes place in the Standard Points of Thermometers._
By JOHN ADIE, F.R.S.E., F.R.S.S.A.
Communicated by the Author.
It has long been known to experimentalists that, in thermometers
constructed with the greatest care, a change takes place after a lapse
of time in the standard points, as given by the melting of ice and
boiling of water under a fixed pressure; on this account it has been
recommended by most writers, where the employment of thermometers is
treated of, that they should from time to time be compared one with
another, and also at the freezing point. This change is a rising of
the mercury in the tube, so that, after a length of time, the mercury
will not sink to the point laid off in the construction of the
instrument. To investigate to what cause this change was due, formed
the object of my experiments: Was it a change in the glass of which
the bulbs are formed, or in the mercury with which they are filled? I
was aware that thermometers filled with alcohol were not subject to
this change, which would lead to the inference, that the change was in
the mercury and not the glass; but then, in the spirit-thermometer,
air is left above the column of spirit, whereas, in those constructed
with mercury, the air is expelled, and there is a vacuum above the
column; consequently, the bulb is pressed together with the force of
an atmosphere on all sides; might not this force, acting for a length
of time, cause some small alteration in the arrangement of the
particles forming the glass of the bulb?
This is the explanation accepted by most of the Italian and French
writers on this subject. Some suppose that the mercury may contain air
and moisture within its particles; but such a hypothesis I think
inadmissible, as in the case of a vacuum over the mercury, these
particles would seek the void, and cause rather a depression than a
rising of the freezing point. Mr Daniell, in his Essay on Climate,
adopts the same view; and Sir John Herschel, in his article "Heat," in
the Encyclopædia Metropolitana, says: "The freezing point upon the
mercurial thermometer has been supposed to undergo some slight
variation, so as to appear too low upon the scales of those
instruments which have been long made; and it is said that, in such
cases, the just indication was again recovered by breaking off the end
of the stem, so as to admit atmospheric air." But, as I had observed
that the change went on for a time only, after which it ceased, and
that it affected thermometers sealed with air over the mercury, as
well as those with a vacuum, I undertook the following experiments:--
In September 1848 I made four thermometers having long degrees,--such
that 1/10° might be easily noted, constructed of the same draft of
glass tube; two of these I placed in boiling water, and kept them at
that temperature for a week: my object in this was to learn if any
change in the form of the bulb would take place from this slow process
of annealing, as glass is known to undergo some change from such
exposure.
The four thermometers were now filled with pure mercury: two of these
were sealed with a vacuum over the mercury; one tube that had been
boiled, and the other not: the other two tubes were sealed with air
over their columns, and the freezing points of all were marked on the
tubes; after which they were placed in a window freely exposed to
light, where they were left till January 1849--a space of four
months--when they were again placed in melting ice, and the freezing
points marked; they had risen ·24°, ·24°, ·20°, ·06° parts of a
degree. The whole four thermometers were now placed in boiling water,
and kept there for a week, when the freezing points were again
observed to have risen respectively ·48°, ·41°, ·50°, ·45°.
The instruments were now left exposed to light as at first; and, in
January 1850, the freezing points were again observed, when they were
found to have farther risen ·12°, ·18°, ·20°, ·13°; and, lastly, they
were observed in May 1850, when no change from last observation was
notable.
The whole amount of rising of the freezing point in these four
thermometers, after a lapse of eighteen months, is respectively ·84°,
·83°, ·90°, ·65°; and these changes may be the full amount that would
take place were the instruments observed after a greater lapse of
time. From my experience, I know that there is a period after which no
change takes place; but, from the method in which these experiments
have been conducted, I am not at present in a condition to assign a
time; moreover, it is evident that this period will be much modified
by circumstances. The results above stated form the following Table:--
+-------------------------------------------------------------------------+
| No.| Description | Value of | Observed | Rise after | | |
| | of |one Degree|rise, Jan. | having | Rise at | Total |
| |Thermometer. | of Fahr. | 1849. |been boiled |Jan. 1850.| rise. |
| | | | |for a week. | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { | Sealed in | } | | | | |
|1.{ | vacuum, | } 0·166 | 0·24 | 0·48 | 0·12 | 0·84 |
| { | not boiled. | } | | | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { | Sealed in | } | | | | |
|2.{ | vacuum and | } 0·168 | 0·24 | 0·41 | 0·18 | 0·83 |
| { | boiled. | } | | | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { | Sealed with | } | | | | |
|3.{ | air, not | } 0·199 | 0·20 | 0·50 | 0·20 | 0·90 |
| { | boiled. | } | | | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { |Sealed with | } | | | | |
|4.{ |air, boiled. | } 0·154 | 0·06 | 0·45 | 0·13 | 0·65 |
+----+-------------+----------+-----------+------------+----------+-------+
From inspection of the Table, no very remarkable difference is
observable in the rising of these four instruments. No. 4 appears to
have risen less during the first period, but goes along with the
others afterwards. The effect of exposure to the temperature of
boiling water shews that, under high temperature, the change goes on
much faster than at the ordinary temperature of the air; from the
Table it will be observed, that about twice the amount of change was
caused by the boiling of the thermometers for a week, than had taken
place between the first and second observations, a period of four
months.
It does not appear that the boiling of the thermometer tubes for eight
days, previous to their being filled with mercury, had produced any
change on the form of the bulbs; we should at least infer this from
the change in their freezing points keeping pace so nearly with those
which had not been boiled.
I now come to the concluding experiment with these instruments, and,
it appears to me most interesting and anomalous. The four tubes being
placed in pounded ice, the columns stood at the points indicated in
the last column of the Table; in this situation the tops of the tubes
were broken off, so as to admit the free pressure of the air, and
instantly the thermometers fell, in the order of their numbers, ·54,
·43, ·40, ·35 of a degree, now indicating on their scales +·30, +·40,
+·50, +·35. The remarkable features shewn by this experiment are;
first, that the two thermometers sealed with vacuum, and the two
having air over their columns, should have risen nearly equally, when
two had their bulbs pressed with the whole force of an atmosphere,
while the other two had no pressure externally, farther than that
caused from changes in the pressure of the atmosphere. Next, that on
being opened, those with air over them should have started down nearly
as much as those with a vacuum; and on all these appears a permanent
change from three to five-tenths of a degree. I confess that I am very
much at a loss to account for these singular changes; atmospheric
pressure on the bulbs would account for the change in those sealed
with a vacuum; for we can easily suppose that a permanent form had
been taken from long exposure to that pressure by the glass forming
the bulbs: besides this permanent form, there appears to have been a
spring inwards, which instantly sprung out on removal of the pressure
by the admission of air over the mercury; but the same reasoning will
not apply to the thermometers having air over the mercury; and before
I attempt to make any suggestions as to the cause of these changes, I
propose to institute the following experiments. Having had three
thermometers blown and filled with mercury, I shall make one with a
perfect vacuum over the mercury, the next with air over it, and the
third with air condensed over it; and, noting the changes that may go
on in these, I hope to be able to assign a cause or causes for the
change. It is argued by some continental writers on this subject that
the reason why we do not perceive any change in the freezing point in
spirit-thermometers is from the great expansion of spirit above
mercury, volume for volume, thereby requiring a much smaller mass of
fluid to give the same length of a degree: this I propose to test by
making a thermometer with the same size of tube and bulb as those to
be experimented on with mercury. In mentioning these experiments to
Professor Forbes, he kindly put me in possession of some
spirit-thermometers, one of these, made in 1837, having a very large
bulb--this, with three others, shewed no change in the places of their
freezing points.
_Observations on the Discovery, by_ Professor LEPSIUS, _of Sculptured
Marks on Rocks in the Nile Valley in Nubia; indicating that,
within the historical period, the river had flowed at a higher
level than has been known in Modern Times_.
By LEONARD HORNER, Esq., F.R.S.S. L. & E., F.G.S., &c.
Communicated by the Author. With a Plate.
The recent archaeological researches of Professor Lepsius in Egypt,
and the Valley of the Nile, in Nubia, have given a deserved celebrity
and authority to his name, among all who take an interest in the early
history of that remarkable portion of the Old World. While examining
the ruins of a fortress, and of two temples of high antiquity at
Semne, in Nubia, he discovered marks cut in the solid rocks, and in
the foundation-stones of the fortress, indicating that, at a very
remote period in the annals of the country, the Nile must have flowed
at a level considerably above the highest point which it has ever
reached during the greatest inundations in modern times. This
remarkable fact would possess much geological interest with respect to
any great river, but it does so especially in the case of the Nile.
Its annual inundations, and the uniformity in the periods of its rise
and fall, have been recorded with considerable accuracy for many
centuries; the solid matter held in suspension in its waters, slowly
deposited on the land overflowed, has been productive of changes in
the configuration of the country, not only in times long antecedent to
history, but throughout all history, down to the present day. Of no
other river on the earth's surface do we possess such or similar
records; and moreover, the Nile, and the changes it has produced on
the physical character of Egypt, are intimately associated with the
earliest records and traditions of the human race. Everything,
therefore, relating to the physical history of the Nile Valley must
always be an object of interest; but the discovery of Professor
Lepsius is one peculiarly deserving the attention of the geologist;
for he does not merely record the facts of the markings of the former
high level of the river, but he infers from these marks, that since
the reign of Moeris, about 2200 years before our era, the entire bed
of the Nile, in Lower Nubia, must have been excavated to a depth of
about 27 feet; and he further speculates as to the process by which he
believes the excavation to have been effected.
It will be convenient, before entering upon the observations I have to
offer upon the cause assigned by Professor Lepsius for the former
higher levels of the Nile indicated by these marks, that I should give
the description of the discovery itself, by translating Dr Lepsius's
own account of it, in letters which he addressed to his friends,
Professors Ehrenberg and Böckh of Berlin, from the island of Philæ, in
September 1844.[51]
Footnote 51: Bericht über die zur Bekantmachung geeigneten
Verhandlungen der Königl. Preuss. Akademie der Wissenshaften zu
Berlin. Aus dem Jahre 1844.
"You may probably remember, when travelling to Dongola on the
Lybian side of the Nile, and in passing through the district of
Batn el hagér, that one of the most considerable of the cataracts
of the country occurs near Semne, a very old fortress, with a
handsome temple, built of sandstone, in a good state of
preservation; the track of the caravan passing close to it, partly
over the 4000-year-old artificial road. The track on the eastern
bank of the river is higher up, being carried through the hills;
and you must turn off from it at this point in order to see the
cataract. This Nile-pass, the narrowest with which I am
acquainted, according to the measurement of Hr. Erbkam, is 380
metres (1247 English feet) broad;[52] and both in itself, and on
account of the monuments existing there, is one of the most
interesting localities in the country, and we passed twelve days
in its examination.
Footnote 52: The breadth of the river itself. See Letter to Hr.
Böckh, p. 27.
"The river is here confined between steep rocky cliffs on both
sides, whose summits are occupied by two fortresses of the most
ancient and most massive construction, distinguishable at once
from the numerous other forts, which, in the time of the Nubian
power in this land of cliffs, were erected on most of the larger
islands, and on the hills commanding the river. The cataract (or
rapid) derives its name of Semne from that of the higher of the
two fortresses on the western bank; that on the opposite bank, as
well as a poor village lying somewhat south of it, is called
Kumme. In both fortresses the highest and best position is
occupied by a temple, built of huge blocks of sandstone, of two
kinds, which must have been brought from a great distance through
the rapids; for, southward, no sandstone is found nearer than
Gebel Abir, in the neighbourhood of Amara and the island of Sai
(between 80 and 90 English miles), and northward, there is none
nearer than the great division of the district at Wadi Haifa (30
miles distant.)
"Both temples were built in the time of Tutmosis III., a king of
the 18th dynasty, about 1600 years before Christ; but the
fortresses in which they stand are of a more ancient date. The
foundations of these are granite blocks of Cyclopian dimensions,
resting on the rock, and scarcely inferior to the rock itself in
durability. They were erected by the first conqueror of the
country, King Sesuatesen III., of the 12th dynasty, in order to
command the river, so easily done in so narrow a gorge. The
immediate successor of this king was Amenemha III., the Moeris of
the Greeks: he who accomplished the gigantic work of forming the
artificial lake of Moeris, in the Fayoum, and from whose time--the
most flourishing of the whole of the old Egyptian kingdom--the
risings of the Nile in successive years, doubtless by means of
regular markings, as indeed Diodorus tells, remained so well
known, that, according to Herodotus, they were recorded in
distinct numbers from the time of Moeris. It appears that this
provident king, occupied with great schemes for the welfare of his
country, considered it of great importance that the rising of the
Nile on the most southern border of his kingdom should be
observed, and the results forthwith communicated widely in other
parts of the land, to prepare the people for the inundations. The
gorge at Semne offered greater advantages for this object than any
other point; because the river was there securely confined by
precipitous rocky cliffs on each side. With the same view he had
doubtless caused Nilometers to be fixed at Assuan and other
suitable places; for without a comparison with these, the
observations at Semne could be of little use.