Geoscience Reference
In-Depth Information
The works of Perrault and Mariotte promptly crossed the Channel and were deemed
remarkable enough to be reported in the Philosophical Transactions of the Royal Society
(Anonymous, 1675; 1686) immediately after their publication. But it is clear that not every-
body accepted Mariotte's theory there. Edmond Halley's (1656-1741) reaction is a case
in point. Without a doubt Halley was thoroughly familiar with developments in France. In
1681 he had already spent 6 months in Paris where he had become acquainted with several
members of the Academie and other learned persons, and had purchased many topics of
interest to ship back to England (see Cook, 1998); in 1686, at the time of the publication
of Mariotte's topic, Halley was Clerk of the Royal Society and maintained an extensive
international correspondence; he was also editor and publisher of the sixteenth volume of
the
Philosophical Transactions
, which contained the review of Mariotte's topic. All this
makes him almost certainly the author of Anonymous (1686); it must also be his familiarity
with this topic, no doubt combined with his experiences at sea, which prompted Halley
(1687) to engage in the study of evaporation, an aspect of the water cycle, which both
Perrault and Mariotte had only dealt with obliquely in qualitative terms. From weight
changes during evaporation of water from a small pan he deduces that, on warm days, evap-
oration amounted to approximately 2.5 mm (0.1 in) in 12 h; this was a reasonable result, as
can be seen in Figure 4.16. Halley next uses Mariotte's method to determine the discharge
rate of the Thames at Kingston Bridge; the determination of the flow rate this way was far
from obvious at the time, as witnessed by the fact that some 15 years earlier Perrault had
not quite known how to deal with this same problem. Estimating that the Mediterranean is
fed by nine rivers, each of which is ten times larger than the Thames, he concludes that the
total inflow into that sea amounts to hardly more than one third of the daily evaporation
of 2.5 mm. At a first glance this conclusion is but a confirmation of Aristotle's (correct)
explanation of why the sea does not overflow, some 20 centuries earlier. What was new,
however, was that now an earnest attempt was made to base Aristotle's speculation on exper-
imental evidence, and not just on everyday observation on a kitchen table. Although his pan
evaporation measurements could provide only rough estimates of the actual values for the
Mediterranean, Halley's study was probably the first in which evaporation was considered
quantitatively in relation to streamflow.
What happens to this evaporated seawater in the global water cycle was the subject
of a second paper (Halley, 1691). In brief, all of these vapors are eventually returned to
the sea in various ways and this explains why the sea does not decrease even though the
evaporation is so much larger than the river inflows. The greater part of these vapors is
returned immediately to the sea as rains or dews without ever touching land. Part of the
vapors, which are blown off the sea, falls on the lower lands where either it nourishes plants
and is exhaled again, or it finds its way into the rivers, after the earth is saturated with
moisture, to return to the sea. But most of these vapors are carried by the winds over the low
lands to mountain ridges, where part of them precipitates “. . . gleeting down by the crannies
of the stone . . .”, and part enters the caverns of the hills, inside of which the vapors are
collected “...asinanalembic, into basins of stone they find there . . .”; this condensed water
then breaks out through the hillsides to form springs, which unite further down into rivulets,
and eventually into rivers. (Halley's ideas on the origin of springs are also detailed in the
Journal Books of the Royal Society
(MacPike, 1932, pp. 217, 227).) Thus rain is not the only
source of all springs. One may wonder why Halley rejected the explanation of Mariotte,
whose topic most likely had prompted his study in the first place, and why he was misled into
invoking, beside rain, direct condensation on the ground and also Aristotle's underground
