| The construction
of dams and sea walls in the nineteenth century have created
a Charles River basin which does not fluctuate with the tides.
This was not the case during the American Revolution and before.
The height and flow of the Charles River varied greatly over
the course of the day.
David Hackett Fischer's book, Paul Revere's Ride
(Oxford: Oxford University Press, 1994) includes an appendix
on "Tidal Movements, the British March, and the Midnight
Ride, April 18-19, 1775". Fischer analyzes the tides
on the night of April 18, 1775 when first Paul Revere and
then British troops crossed the Charles. His analysis uses
written accounts from Revere and several British soldiers,
as well as modern astronomical calculations, to show that
the tide was rising as Revere and the British crossed the
river. Fischer argues this was an advantage for Revere, whose
upstream crossing to Charlestown was made more easy, but created
difficulties for the British who were moving downstream against
the tide.
For more information on crossing the Charles this evening
see Fischer, pp. 104-106, 115-117 and 312-313 as well as Donald
W. Olson and Russell L. Doescher, "Astronomical Computing:
Paul Revere's Midnight Ride," Sky and Telescope,
April 1992, pp. 437-40.
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