Speculation on the Speed of Ships
MS: Library of Congress
During his homeward voyage Franklin took time off from writing his journal of the peace negotiations in London to return to a question that had intrigued him for years: why do westbound ships have a longer crossing of the Atlantic than eastbound? Almost three decades earlier he had advanced a tentative answer in terms of the earth’s rotation;5 he now sketched a new one that was essentially a recapitulation of the old, and apparently prevailed on his grandson to write it out.6 It remained with him for another decade until August, 1785, when he was again homeward bound after his years in France; he then embodied this paragraph of speculation in a long set of maritime observations. By January, 1786, however, he convinced himself that the earth’s rotation was irrelevant,7 and appended a note to that effect when his observations were published.8
April 5. 1775.
Suppose a Ship to make a Voyage Eastward from a Place in Lat: 40. North, to a Place in Lat 50. North, Distance in Longitude 75 Degrees.
In sailing from 40 to 50 she goes from a Place where a Degree of Longitude is about 8 miles greater than in the Place she is going to. A Degree is equal to 4 Minutes of Time. Consequently the Ship in the Harbour she leaves partaking of the Diurnal Motion of the Earth, moves two miles in a minute faster, than when in the Port she is going to; which is 120 miles in an Hour.
This motion in a Ship and Cargo is of great force; and if she could be lifted up suddenly from the Harbour in which she lay quiet, and set down instantly in the Latitude of the Port she was bound to, tho’ in a Calm, that Force contained in her would make her run a great Way at a Prodigious Rate. This Force must be lost gradually in her Voyage, by gradual Impulse against the water, and probably thence shorten the Voyage. In returning just the contrary must happen and her Voyage be retarded and lengthned.
Endorsed: Voyages from & to America why not of equal Length
5. Above, III, 67–8.
6. The heading and date, and the endorsement at the end, are in BF’s hand; the body of the text appears to be in WTF’s.
7. BF to Jonathan Williams, Jr., Jan. 19, 1786, APS.
8. In the form of a letter to Julien-David (misnamed Alphonsus) LeRoy, Jean-Baptiste’s brother, which the editor or editors entitled “Maritime Observations”: APS Trans., II (1786), 294–324; BF’s note is on p. 317.