Geoscience Reference
In-Depth Information
the rain flows out on the surface of the earth into deep hollows, it forms the lakes & the swamps there,
from which then rivers are born, which also owe their origin to the waters gushing from the fountains.
Consequently, river water is either rain water, or fountain water, or both together.
In a later treatment of the same topic Van Musschenbroek (1769, p. 281) seems to have
become aware that this rainfall penetration had presented some difficulties with others in
the past. He now addresses the controversy, listing Seneca, Varin, de La Hire, and Buffon,
as those who claim that rain cannot penetrate the earth beyond 4-10 ft; he then counters
them with his own experience in Holland, as well as with that of Erndetl in Poland, and le
Monnier in Auvergne, and repeats essentially his earlier description.
But in spite of the frequent appearance of the hydrologic cycle in physics and in physical
theology alike, in none of the treatments reviewed here was there even a hint of calcula-
tions, of the kind made earlier by Perrault, Mariotte and Halley; in fact, during the century
following their writings the basic notions on the origins of springs and streams did not
undergo drastic changes, and many of the disagreements and uncertainties lingered on, it
seems. This is brought out in Dalton's (1802a) paper, which he presented in 1799 before the
Manchester Literary and Philosophical Society, and which he starts off with the following
observation.
Naturalists, however, are not unanimous in their opinions whether the rain that falls is sufficient to
supply the demands of springs and rivers, and to afford the earth besides such a large portion for
evaporation as it is well known is raised daily.
This is followed by Dalton's rough estimates for all of England and Wales of average
annual precipitation as
P
=
787 mm (31 in), on the basis of measurements at some 23
different sites; of annual dew, as 127 mm (5 in), on the basis of measurements by one
Dr. Hales (probably Stephen Hales (1677-1761)); of river runoff, as
R
=
330 mm (13 in),
by extending and correcting Halley's estimate for the Thames; and finally of evaporation,
as
E
=
635 mm (25 in) per year, on the basis mainly of his own measurements with a
simple lysimeter overa3yperiod at Manchester, which combined with the dew amounts
to 762 mm annually “raised into the air.” Combining these terms in a water budget (cf.
Equation (1.1)), in which the dew fluxes cancel out, Dalton ends up with an annual deficit
of (330
+
635
−
787)
=
178 mm (7 in); he attributes this failure to close the budget to a
possible underestimate of the average precipitation and, which he feels is more likely, to
certain features of his lysimeter, which somehow lost water in heavy storms and which
usually kept the soil surface more moist, and therefore must have evaporated more, than the
earth around it. He summarizes this part of the paper.
Upon the whole then I think we may fairly conclude - that the rain and dew of this country are equivalent
to the quantity of water carried off by evaporation and by the rivers. And as nature acts upon general
laws, we ought to infer, that it must be the case in every other country, till the contrary is proved.
All this is fair enough, but evidently in Dalton's opinion, the closure of the water budget
is a separate issue from the origin of springs and not a persuasive argument to prove that
precipitation is their sole source. Thus he points out next (p. 367) that at the time
. . . There are three opinions respecting the origin of springs which it may be proper to notice.
1st.
That they are supplied entirely by rain and dew.
2d.
That they are principally supplied by large subterranean reservoirs of water.
3d.
That they derive their water originally from the sea, on the principle of filtration.
It is obvious that before we pay any attention to the latter two opinions, the causes assigned in
the first ought to be proved insufficient by direct experiment. M. de la Hire is the only one who has
attempted to do this...
