Tidal Movements, the British March, and the Midnight Ride, April 18-19, 1775
Shakespeare observes that “there is a tide in the affairs of men; which, taken at the flood, leads on to fortune” (Julius Caesar, IV, iii, 217). So it was for Paul Revere, but General Gage would have been luckier at the ebb.
Tidal movements in the Charles River and Boston’s Back Bay had an important impact on these events. Paul Revere remembered that when he crossed the Charles River at approximately 10:30 to 11:00 p.m., “It was then young flood.” The strong Boston tide was flowing into the harbor and running westward up the estuary of the Charles River.
At that same hour, the British troops were moving across the Charles River to Lechmere Point in Cambridge. Lt. William Sutherland wrote that when they marched along the river at 2 a.m., “the tide being in we were up to our middles.” Mackenzie remembered that the men were “obliged to wade, halfway up to their thighs, through two inlets, the tide being by that time up.”
Modern computations confirm the accuracy of these accounts. Professors Olson and Doescher, using a method of harmonic analysis, estimate that on the afternoon of April 18, 1775, high tide in Boston harbor occurred at 1:14 p.m. local apparent solar time. Low water followed that evening at 7:19 p.m. and high tide again at 1:26 a.m. in the morning. These results confirm that the tide was rising when Revere and the British soldiers were crossing the river.
Early American almanacs were even closer to the descriptions of the participants. Three colonial almanacs variously estimated the time of high tide on the morning of April 19 at approximately 2:34 (Low) 2:36 (Isaiah Thomas), and 2:39 (Bickerstaff).
All this was an advantage for Paul Revere, and a problem for the British expedition. At the point where Revere crossed, the river flowed west around a bend. His course to Charlestown took him upstream. The Regulars crossed further west where the river was flowing southwest. Their course to Cambridge was downstream. Revere’s passage was comparatively short, and as his route took him diagonally upstream he was moving with the tide. The British troops were moving diagonally downstream against the tide over a longer distance. Revere was traveling in a small rowboat with two experienced Boston watermen. The British troops were in heavy, overloaded longboats. The tide gave Revere an important advantage, and was a factor in speeding him on his way, while retarding the progress of the British troops.
See Donald W. Olson and Russell L. Doescher, “Astronomical Computing: Paul Revere’s Midnight Ride,” Sky and Telescope, April 1992, pp. 437-40.