1. _A landslip_ often occurs in mountainous regions, where strata,
dipping towards the valley, rest on soft layers; the hard rocks slip
into the valley after heavy rains, damming back the drainage, which
then forms a barrier-basin. Many small lakes high up in the Alps and
Pyrenees are formed by a river being dammed back in this way.
2. _By a Glacier._--In Alaska, in Scandinavia and in the Alps a
glacier often bars the mouth of a tributary valley, the stream flowing
therein is dammed back, and a lake is thus formed. The best-known lake
of this kind is the Märjelen Lake in the Alps, near the great Aletsch
Glacier. Lake Castain in Alaska is barred by the Malaspina Glacier; it
is 2 or 3 m. long and 1 m. in width when at its highest level; it
discharges through a tunnel 9 m. in length beneath the ice-sheet. The
famous parallel roads of Glen Roy in Scotland are successive terraces
formed along the shores of a glacial lake during the waning glacial
epoch. Lake Agassiz, which during the glacial period occupied the
valley of the Red River, and of which the present Lake Winnipeg is a
remnant, was formed by an ice-dam along the margin of two great
ice-sheets. It is estimated to have been 700 m. in length, and to have
covered an area of 110,000 sq. m., thus exceeding the total area of
the five great North American lakes: Superior (31,200), Michigan
(22,450), Huron with Georgian Bay (23,800), Erie (9960) and Ontario
(7240).
3. _By the Lateral Moraine of an Actual Glacier._--These lakes
sometimes occur in the Alps of Central Europe and in the Pyrenees
Mountains.
4. _By the Frontal Moraine of an Ancient Glacier._--The barrier in
this case consists of the last moraine left by the retreating glacier.
Such lakes are abundant in the northern hemisphere, especially in
Scotland and the Alps.
5. _By Irregular Deposition of Glacial Drift._--After the retreat of
continental glaciers great masses of glacial drift are left on the
land-surfaces, but, on account of the manner in which these masses
were deposited, they abound in depressions that become filled with
water. Often these lakes are without visible outlets, the water
frequently percolating through the glacial drift. These lakes are so
numerous in the north-eastern part of North America that one can trace
the southern boundary of the great ice-sheet by following the southern
limit of the lake-strewn region, where lakes may be counted by tens of
thousands, varying from the size of a tarn to that of the great
Laurentian lakes above mentioned.
6. _By Sand drifted into Dunes._--It is a well-known fact that sand
may travel across a country for several miles in the direction of the
prevailing winds. When these sand-dunes obstruct a valley a lake may
be formed. A good example of such a lake is found in Moses Lake in the
state of Washington; but the sand-dunes may also fill up or submerge
river-valleys and lakes, for instance, in the Sahara, where the Shotts
are like vast lakes in the early morning, and in the afternoon, when
much evaporation has taken place, like vast plains of white salt.
7. _By Alluvial Matter deposited by Lateral Streams._--If the current
of a main river be not powerful enough to sweep away detrital matter
brought down by a lateral stream, a dam is formed causing a lake.
These lakes are frequently met with in the narrow valleys of the
Highlands of Scotland.
8. _By Flows of Lava._--Lakes of this kind are met with in volcanic
regions.
C. ORGANIC BASINS.--In the vast tundras that skirt the Arctic Ocean in
both the old and the new world, a great number of frozen ponds and
lakes are met with, surrounded by banks of vegetation. Snow-banks are
generally accumulated every season at the same spots. During summer
the growth of the tundra vegetation is very rapid, and the snow-drifts
that last longest are surrounded by luxuriant vegetation. When such
accumulations of snow finally melt, the vegetation on the place they
occupied is much less than along their borders. Year after year such
places become more and more depressed, comparatively to the general
surface, where vegetable growth is more abundant, and thus give origin
to lakes.
It is well known that in coral-reef regions small bays are cut off
from the ocean by the growth of corals, and thus ultimately
fresh-water basins are formed.
_Life History of Lakes._--From the time of its formation a lake is
destined to disappear. The historical period has not been long enough to
enable man to have watched the birth, life and death of any single lake
of considerable size, still by studying the various stages of
development a fairly good idea of the course they run can be obtained.
In humid regions two processes tend to the extinction of a lake, viz.
the deposition of detrital matter in the lake, and the lowering of the
lake by the cutting action of the outlet stream on the barrier. These
outgoing streams, however, being very pure and clear, all detrital
matter having been deposited in the lake, have less eroding power than
inflowing streams. One of the best examples of the action of the
filling-up process is presented by Lochs Doine, Voil and Lubnaig in the
Callander district of Scotland. In post-glacial times these three lochs
formed, without doubt, one continuous sheet of water, which subsequently
became divided into three different basins by the deposition of
sediment. Loch Doine has been separated from Loch Voil by alluvial cones
laid down by two opposite streams. At the head of Loch Doine there is an
alluvial flat that stretches for 1½ m., formed by the Lochlarig river
and its tributaries. The long stretch of alluvium that separates Loch
Voil from Loch Lubnaig has been laid down by Calair Burn in Glen Buckie,
by the Kirkton Burn at Balquhidder, and by various streams on both sides
of Strathyre. Loch Lubnaig once extended to a point ¾ m. beyond its
present outlet, the level of the loch being lowered about 20 ft. by the
denuding action of the river Leny on its rocky barrier.
In arid regions, where the rainfall is often less than 10 ins. in the
year, the action of winds in the transport of sand and dust is more in
evidence than that of rivers, and the effects of evaporation greater
than of precipitation. Salt and bitter lakes prevail in these regions.
Many salt lakes, such as the Dead Sea and the Great Salt Lake, are
descended from fresh-water ancestors, while others, like the Caspian and
Aral Seas, are isolated portions of the ocean. Lakes of the first group
have usually become salt through a decrease in the rainfall of the
region in which they occur. The water begins to get salt when the
evaporation from the lake exceeds the inflow. The inflowing waters bring
in a small amount of saline and alkaline matter, which becomes more and
more concentrated as the evaporation increases. In lakes of the second
group the waters were salt at the outset. If inflow exceeds evaporation
they become fresher, and may ultimately become quite fresh. If the
evaporation exceeds the inflow they diminish in size, and their waters
become more and more salt and bitter. The first lake which occupied the
basin of the Great Salt Lake of Utah appears to have been fresh, then
with a change of climate to have become a salt lake. Another change of
climate taking place, the level of the lake rose until it overflowed,
the outlet being by the Snake river; the lake then became fresh. This
expanded lake has been called Lake Bonneville, which covered an area of
about 17,000 sq. m. Another change of climate in the direction of
aridity reduced the level of the lake below the level of the outlet, the
waters became gradually salt, and the former great fresh-water lake has
been reduced gradually to the relatively small Great Salt Lake of the
present day. The sites of extinct salt lakes yield salt in commercial
quantities.
_The Water of Lakes._--(a) _Composition._--It is interesting to
compare the quantity of solid matter in, and the chemical composition
of, the water of fresh and salt lakes:--
Total Solids by Evaporation
expressed in Grams per Litre.
Great Salt Lake (Russell) 238.12
Lake of Geneva (Delebecque) 0.1775
The following analysis of a sample of the water of the Great Salt Lake
(Utah, U.S.A.) is given by I. C. Russell:--
Grams per Litre. Probable Combination.
Na 75.825 NaCl 192.860
K 3.925 K2SO4 8.756
Li 0.021 Li2SO4 0.166
Mg 4.844 MgCl2 15.044
Ca 2.424 MgSO4 5.216
Cl 128.278 CaSO4 8.240
SO3 12.522 Fe2O3 + Al2O3 0.004
O in sulphate 2.494 SiO2 0.018
Fe2O3 + Al2O3 0.004 Surplus SO_3 0.051
SiO2 0.018
Bo2O3 trace
Br3 faint trace
The following analyses of the waters of other salt lakes are given by
Mr J. Y. Buchanan (Art. "Lake," _Ency. Brit._, 9th Ed.), an analysis
of sea-water from the Suez Canal being added for comparison:--
+-----------------------+---------+--------+-------------------+----------+---------+---------+-----------+
| | | | Caspian Sea. | | | |Suez Canal,|
| |Koko-nor.|Aral Sea+--------+----------+Urmia Sea.|Dead Sea.|Lake Van.| Ismailia. |
| | | | Open. |Karabugas.| | | | |
+-----------------------+---------+--------+--------+----------+----------+---------+---------+-----------+
| Specific Gravity | 1.00907 | .. | 1.01106| 1.26217 | 1.17500 | .. | 1.01800| 1.03898 |
| Percentage of Salt | 1.11 | 1.09 | 1.30 | 28.5 |22.28 | 22.13 | 1.73 | 5.1 |
+-----------------------+---------+--------+--------+----------+----------+---------+---------+-----------+
| Name of Salt. | Grams of Salt per 1000 Grams of Water. |
+-----------------------+---------+--------+--------+----------+----------+---------+---------+-----------+
| Bicarbonate of Lime | 0.6804 | 0.2185 | 0.1123 | .. | .. | .. | .. | 0.0072 |
| " Iron | 0.0053 | .. | 0.0014 | .. | .. | .. | .. | 0.0069 |
| " Magnesia | 0.6598 | .. | .. | .. | .. | .. | 0.4031 | .. |
| Carbonate of Soda | .. | .. | .. | .. | .. | .. | 5.3976 | .. |
| Phosphate of Lime | 0.0028 | .. | 0.0021 | .. | .. | .. | 5.3976 | 0.0029 |
| Sulphate of Lime | .. | 1.3499 | 0.9004 | .. | 0.7570 | 0.8600 | .. | 1.8593 |
| " Magnesia | 0.9324 | 2.9799 | 3.0855 | 61.9350 | 13.5460 | .. | 0.2592 | 3.2231 |
| " Soda | 1.7241 | .. | .. | .. | .. | .. | 2.5673 | .. |
| " Potash | .. | .. | .. | .. | .. | .. | 0.5363 | .. |
| Chloride of Sodium | 6.9008 | 6.2356 | 8.1163 | 83.2840 |192.4100 | 76.5000 | 8.0500 | 40.4336 |
| " Potassium | 0.2209 | 0.1145 | 0.1339 | 9.9560 | .. | 23.3000 | .. | 0.6231 |
| " Rubidium | 0.0055 | .. | 0.0034 | 0.2510 | .. | .. | .. | 0.0265 |
| " Magnesium | .. | 0.0003 | 0.6115 |129.3770 | 15.4610 | 95.6000 | .. | 4.7632 |
| " Calcium | .. | .. | .. | .. | 0.5990 | 22.4500 | .. | .. |
| Bromide of Magnesium | 0.0045 | .. | 0.0081 | 0.1930 | .. | 2.3100 | .. | 0.0779 |
| Silica | 0.0098 | .. | 0.0024 | .. | .. | 0.2400 | 0.0761 | 0.0027 |
+-----------------------+---------+--------+--------+----------+----------+---------+---------+-----------+
| Total Solid Matter |11.1463 |10.8987 |12.9773 |284.9960 |222.2600 |221.2600 | 17.2899 | 51.0264 |
+-----------------------+---------+--------+--------+----------+----------+---------+---------+-----------+
This table embraces examples of several types of salt lakes. In the
Koko-nor, Aral and open Caspian Seas we have examples of the
moderately salt, non-saturated waters. In the Karabugas, a branch gulf
of the Caspian, Urmia and the Dead Seas we have examples of saturated
waters containing principally chlorides. Lake Van is an example of the
alkaline seas which also occur in Egypt, Hungary and other countries.
Their peculiarity consists in the quantity of carbonate of soda
dissolved in their waters, which is collected by the inhabitants for
domestic and commercial purposes.
The following analyses by Dr Bourcart give an idea of the chemical
composition of the water of fresh-water lakes in grams per litre:--
+---------------+--------+--------+---------+-----------+
| | Tanay. | Bleu. |Märjelen.|St Gothard.|
+---------------+--------+--------+---------+-----------+
| SiO2 | 0.003 | 0.0042 | 0.0014 | 0.0008 |
| Fe2O3 + Al2O3 | 0.0012 | 0.0006 | 0.0008 | trace |
| NaCl | 0.0017 | .. | .. | .. |
| Na2SO4 | 0.0011 | 0.0038 | 0.0031 | 0.00085 |
| Na2CO3 | .. | .. | .. | 0.00128 |
| K2SO4 | 0.0021 | 0.0028 | 0.0044 | .. |
| K2CO3 | .. | .. | 0.0003 | 0.00130 |
| MgSO4 | 0.006 | 0.0305 | .. | .. |
| MgCO3 | 0.0046 | 0.0158 | 0.0008 | 0.00015 |
| CaSO4 | .. | .. | .. | .. |
| CaCO3 | 0.107 | 0.1189 | 0.0061 | 0.00178 |
| MnO | 0.001 | .. | .. | .. |
+---------------+--------+--------+---------+-----------+
(b) _Movements and Temperature of Lake-Waters._--(1) In addition to
the rise and fall of the surface-level of lakes due to rainfall and
evaporation, there is a transference of water due to the action of
wind which results in raising the level at the end to which the wind
is blowing. In addition to the well-known progressive waves there are
also stationary waves or "seiches" which are less apparent. A seiche
is a standing oscillation of a lake, usually in the direction of the
longest diameter, but occasionally transverse. In a motion of this
kind every particle of the water of the lake oscillates synchronously
with every other, the periods and phases being the same for all, and
the orbits similar but of different dimensions and not similarly
situated. Seiches were first discovered in 1730 by Fatio de Duillier,
a well-known Swiss engineer, and were first systematically studied by
Professor Forel in the Lake of Geneva. Large numbers of observations
have been made by various observers in lakes in many parts of the
world. Henry observed a fifteen-hour seiche in Lake Erie, which is 396
kilometres in length, and Endros recorded a seiche of fourteen seconds
in a small pond only 111 metres in length. Although these waves cause
periodical rising and falling of the water-level, they are generally
inconspicuous, and can only be recorded by a registering apparatus, a
limnograph. Standard work has been done in the study of seiches by the
Lake Survey of Scotland under the immediate direction of Professor
Chrystal, who has given much attention to the hydrodynamical theories
of the phenomenon. Seiches are probably due to several factors acting
together or separately, such as sudden variations of atmospheric
pressure, changes in the strength or direction of the wind.
Explanations such as lunar attraction and earthquakes have been shown
to be untenable as a general cause of seiches.
2. _The water temperature of lakes_ may change with the season from
place to place and from layer to layer; these changes are brought
about by insolation, by terrestrial radiation, by contract with the
atmosphere, by rain, by the inflow of rivers and other factors, but
the most important of all these are insolation and terrestrial
radiation. Fresh water has its greatest density at a temperature of
39.2° F., so that water both above and below this temperature floats
to the surface, and this physical fact largely determines the water
stratification in a lake. In salt lakes the maximum density point is
much lower, and does not come into play. In the tropical type of
fresh-water lake the temperature is always higher than 39° F., and the
temperature decreases as the depth increases. In the polar type the
temperature is always lower than 39° F., and the temperature increases
from the surface downwards. In the temperate type the distribution of
temperature in winter resembles the polar type, and in summer the
tropical type. In Loch Ness and other deep Scottish lochs the
temperature in March and April is 41° to 42° F., and is then nearly
uniform from top to bottom. As the sun comes north, and the mean air
temperature begins to be higher than the surface temperature, the
surface waters gain heat, and this heating goes on till the month of
August. About this time the mean air temperature falls below the
surface temperature, and the loch begins to part with its heat by
radiation and conduction. The temperature of the deeper layers beyond
300 ft. is only slightly affected throughout the whole year. In the
autumn the waters of the loch are divided into two compartments, the
upper having a temperature from 49° to 55° F., the deeper a
temperature from 41° to 45°. Between these lies the
discontinuity-layer (_Sprungschicht_ of the Germans), where there is a
rapid fall of temperature within a very short distance. In August this
discontinuity-layer is well marked, and lies at a depth of about 150
ft.; as the season advances this layer gradually sinks deeper, and the
layer of uniform temperature above it increases in depth, and slowly
loses heat, until finally the whole loch assumes a nearly uniform
temperature. Many years ago Sir John Murray showed by means of
temperature observations the manner in which large bodies of water
were transferred from the windward to the leeward end of a loch, and
subsequent observations seem to show that, before the
discontinuity-layer makes its appearance, the currents produced by
winds are distributed through the whole mass of the loch. When,
however, this layer appears, the loch is divided into two
current-systems, as shown in the following diagram:--
[Illustration: Current systems in a loch induced by wind at the
surface. (After Wedderburn.)
AB, Discontinuity layer.
C, Surface current.
D, Primary return current.
E, Secondary surface current.
F, Secondary return current.]
Another effect of the separation of the loch into two compartments by
the surface of discontinuity is to render possible the
temperature-seiche. The surface-current produced by the wind transfers
a large quantity of warm water to the lee end of the loch, with the
result that the surface of discontinuity is deeper at the lee than at
the windward end. When the wind ceases, a temperature-seiche is
started, just as an ordinary seiche is started in a basin of water
which has been tilted. This temperature-seiche has been studied
experimentally and rendered visible by superimposing a layer of
paraffin on a layer of water.
Wedderburn estimates the quantity of heat that enters Loch Ness and is
given out again during the year to be approximately sufficient to
raise about 30,000 million gallons of water from freezing-point to
boiling-point. Lakes thus modify the climate of the region in which
they occur, both by increasing its humidity and by decreasing its
range of temperature. They cool and moisten the atmosphere by
evaporation during summer, and when they freeze in winter a vast
amount of latent heat is liberated, and moderates the fall of
temperature.
Lakes act as reservoirs for water, and so tend to restrain floods, and
to promote regularity of flow. They become sources of mechanical
power, and as their waters are purified by allowing the sediment which
enters them to settle, they become valuable sources of water-supply
for towns and cities. In temperate regions small and shallow lakes are
likely to freeze all over in winter, but deep lakes in similar regions
do not generally freeze, owing to the fact that the low temperature of
the air does not continue long enough to cool down the entire body of
water to the maximum density point. Deep lakes are thus the best
sources of water-supply for cities, for in summer they supply
relatively cool water and in winter relatively warm water. Besides,
the number of organisms in deep lakes is less than in small shallow
lakes, in which there is a much higher temperature in summer, and
consequently much greater organic growth. The deposits, which are
formed along the shores and on the floors of lakes, depend on the
geological structure and nature of the adjacent shores.
_Biology._--Compared with the waters of the ocean those of lakes may
safely be said to contain relatively few animals and plants. Whole
groups of organisms--the Echinoderms, for instance--are unrepresented.
In the oceans there is a much greater uniformity in the physical and
chemical conditions than obtains in lakes. In lakes the temperature
varies widely. To underground lakes light does not penetrate, and in
these some of the organisms may be blind, for example, the blind
crayfish (_Cambarus pellucidus_) and the blind fish (_Amblyopsis
spelaeus_) of the Kentucky caves. The majority of lakes are fresh, while
some are so salt that no organisms have been found in them. The peaty
matter in other lakes is so abundant that light does not penetrate to
any great depth, and the humic acids in solution prevent the development
of some species. Indeed, every lake has an individuality of its own,
depending upon climate, size, nature of the bottom, chemical composition
and connexion with other lakes. While the ocean contains many families
and genera not represented in lakes, almost every genus in lakes is
represented in the ocean.
The vertebrates, insects and flowering plants inhabiting lakes vary
much according to latitude, and are comparatively well known to
zoologists and botanists. The micro-fauna and flora have only recently
been studied in detail, and we cannot yet be said to know much about
tropical lakes in this respect. Mr James Murray, who has studied the
Scottish lakes, records in over 400 Scottish lochs 724 species (the
fauna including 447 species, all invertebrates, and the flora
comprising 277 species) belonging to the following groups; the list
must not be regarded as in any way complete:--
_Fauna._ _Flora._
Mollusca 7 species Phanerogamia 65 species
Hydrachnida 17 " Equisetaceae 1 "
Tardigrada 30 " Selaginellaceae 1 "
Insecta 7 " Characeae 6 "
Crustacea 78 " Musci 18 "
Bryozoa 7 " Hepaticae 2 "
Worms 25 " Florideae 2 "
Rotifera 181 " Chlorophyceae 142 "
Gastrotricha 2 " Bacillariaceae 26 "
Coelenterata 1 " Myxophyceae 10 "
Porifera 1 " Peridiniaceae 4 "
Protozoa 91 "
----------- -----------
447 " 277 "
These organisms are found along the shores, in the deep waters, and in
the surface waters of the lakes.
The _littoral region_ is the most populous part of lakes; the
existence of a rooted vegetation is only possible there, and this in
turn supports a rich littoral fauna. The greater heat of the water
along the margins also favours growth. The great majority of the
species in Scottish lochs are met with in this region. Insect larvae
of many kinds are found under stones or among weeds. Most of the
Cladocera, and the Copepoda of the genus _Cyclops_, and the
Harpacticidae are only found in this region. Water-mites, nearly all
the Rotifers, Gastrotricha, Tardigrada and Molluscs are found here,
and Rhizopods are abundant. A large number of the littoral species in
Loch Ness extends down to a depth of about 300 ft.
_The abyssal region_, in Scottish lochs, lies, as a rule, deeper than
300 ft., and in this deep region a well-marked association of animals
appears in the muds on the bottom, but none of them are peculiar to
it: they all extend into the littoral zone, from which they were
originally derived. In Loch Ness the following sparse population was
recorded:--
1 Mollusc: _Pisidium pusillum_ (Gmel).
3 Crustacea: _Cyclops viridis_, Jurine.
_Candona candida_ (Müll).
_Cypria ophthalmica_, Jurine.
3 Worms: _Stylodrilus gabreteae_, Vejd.
Oligochaete, not determined.
_Automolos morgiensis_ (Du Plessis).
1 Insect: _Chironomus_ (larva).
Infusoria: Several, ectoparasites on _Pisidium_ and _Cyclops_,
not determined.
In addition, the following were found casually at great depths in Loch
Ness: _Hydra_, _Limnaea peregra_, _Proales daphnicola_ and _Lynceus
affinis_.
The _pelagic region_ of the Scottish lakes is occupied by numerous
microscopic organisms, belonging to the Zooplankton and Phytoplankton.
Of the former group 30 species belonging to the Crustacea, Rotifera
and Protozoa were recorded in Loch Ness. Belonging to the second group
150 species were recorded, of which 120 were Desmids. Some of these
species of plankton organisms are almost universal in the Scottish
lochs, while others are quite local. Some of the species occur all the
year through, while others have only been recorded in summer or in
winter. The great development of Algae in the surface waters, called
"flowering of the water" (_Wasserblüthe_), was observed in August in
Loch Lomond; a distinct "flowering," due to Chlorophyceae, has been
observed in shallow lochs as early as July. It is most common in
August and September, but has also been observed in winter.
The plankton animals which are dominant or common, both over Scotland
and the rest of Europe, are:--
_Diaptomus gracilis._
_Daphnia kyalina._
_Diaphanosoma brachyurum._
_Leptodora kindtii._
_Conochilus unicornis._
_Asplanchna priodonta._
_Polyarthra platyptera._
_Anuraea cochlearis._
_Notholca longispina._
_Ceratium hirundinella._
_Asterionella._
All of these, according to Dr Lund, belong to the general plankton
association of the European plain, or are even cosmopolitan.
The Scottish plankton on the whole differs from the plankton of the
central European plateau, and from the cosmopolitan fresh-water
plankton, in the extraordinary richness of the Phytoplankton in
species of Desmids, in the conspicuous arctic element among the
Crustacea, in the absence or comparative rarity of the species
commonest in the general European plankton. Another peculiarity is the
local distribution of some of the Crustacea and many of the Desmids.
The derivation of the whole lacustrine population of the Scottish
lochs does not seem to present any difficulty. The abyssal forms have
been traced to the littoral zone without any perceptible
modifications. The plankton organisms are a mingling of European and
arctic species. The cosmopolitan species may enter the lochs by
ordinary migration. It is probable that if the whole plankton could be
annihilated, it would be replaced by ordinary migration within a few
years. The eggs and spores of many species can be dried up without
injury, and may be carried through the air as dust from one lake to
another; others, which would not bear desiccation, might be carried in
mud adhering to the feet of aquatic birds and in various other ways.
The arctic species may be survivors from a period when arctic
conditions prevailed over a great part of Europe. What are known as
"relicts" of a marine fauna have not been found in the Scottish
fresh-water lochs.
It is somewhat remarkable that none of the organisms living in
fresh-water lochs has been observed to exhibit the phenomenon of
phosphorescence, although similar organisms in the salt-water lochs a
few miles distant exhibit brilliant phosphorescence. At similar depths
in the sea-lochs there is usually a great abundance of life when
compared with that found in fresh-water lochs.
_Length, Depth, Area and Volume of Lakes._--In the following table will
be found the length, depth, area and volume of some of the principal
lakes of the world.[1] Sir John Murray estimates The volume of water in
the 560 Scottish lochs recently surveyed at 7 cub. m., and the
approximate volume of water in all the lakes of the world at about 2000
cub. m., so that this last number is but a small fraction of the volume
of the ocean, which he previously estimated at 324 million cub. m. It
may be recalled that the total rainfall on the land of the globe is
estimated at 29,350 cub. m., and the total discharge from the rivers of
the globe at 6524 cub. m.