Benjamin Franklin Papers

To Benjamin Franklin from James Alexander, 29 January 1753

From James Alexander

Copies of letter and enclosure: New-York Historical Society

The rare transits of Venus across the sun’s surface were among the most important astronomical occurrences in the eighteenth century because they offered astronomers opportunities to calculate the solar parallax—the angle subtended at the sun by the earth’s radius—and thus not only provided a basis for computing the actual distance from the earth and other planets to the sun but completed the Newtonian system by establishing scalar distances. The last transits had occurred in 1631 and 1639, but the first was at night and the second was imperfectly observed. Late in the century Edmund Halley had devised a method to determine the parallax by using observations made at different points on the earth of the times of Venus’ transit. His method was modified and improved in the second quarter of the eighteenth century by Joseph-Nicolas Delisle, professor of mathematics in the Collège Royal de Paris. Astronomers looked forward to applying them to the transits which would occur in 1761 and 1769.2

Meanwhile in 1753 there was to be a transit of Mercury, and though observations of it would not yield the solar parallax, they would provide a check on astronomical calculations and a means of checking longitude. Moreover, as American philosophers realized, the observations would provide experience to use in the greater events of the next decade.

Franklin printed a calculation of the forthcoming transit in his almanac for 1753.3 Then in February 1753 he received from James Alexander translations of Delisle’s instructions to a Jesuit astronomer for making observations at Quebec. He printed fifty copies in a four-page leaflet,4 and distributed them among philosophical friends in other colonies,5 sending six, for example, to his nephew Benjamin Mecom, printer of the Antigua Gazette, just as Delisle was sending notices to European astronomers. Excerpts from the leaflet were printed in the Boston Weekly News-Letter, March 29; and the Pennsylvania Gazette, April 5, reinforced the notice by printing an appeal of the Royal Society to observe the transit and forward the results to London.6

Accordingly preparations were made. In Philadelphia Franklin proposed to use the telescope belonging to the Library Company. The New Yorkers thought themselves particularly well equipped, for they had three telescopes, Alexander’s watch with a second hand, “and the proprietors Quadrant will do to rectifie the Clock.”7 Professor John Winthrop was asked to be ready in Boston. As it fell out, however, the day was cloudy in New York and probably elsewhere on the coast, and no observations were made in North America. Only in Antigua, in the British American colonies, could the transit be observed: the results were sent to Franklin, who duly forwarded them to the Royal Society.8

New York. January 29. 1753


About a week ago the papers whereof that herewith Mark’t A is a Translation were sent to Peruse,9 and on my returning them With a Translation I beged Leave to Communicate Coppies of them to You and Dr. Colden with Liberty if you thought proper to print them or any Part of them. In Answer To which I obtained the Leave coppied at the end of Them and Struck out La Gallissonieres name, for the reason in the Leave.

The Delay of that Leave put me upon Dipping into the Abridgment of the Philosophical Transactions to see what [was] there said of the matters in those papers, and from thence I made the Extracts in the paper Herewith Mark’t B.1 by which I find that that which had baffled all the Art of man hitherto To discover with any Tolerable Certainty (Vizt. the Suns Distance from the Earth) may with great Certainty be Discovered by the Transit of Venus over the sun the 26 of May 1761 Old Stile, if well Observed in the East Indies and here and these Observations Compared Together.

It Would be a great honour To our young Colledges in America if they forthwith prepared themselves with a proper apparatus for that Observation and made it. Which I Doubt not they would Severally Do if they were Severally put in mind of it and of the great Importance that that Observation would be To Astronomy and that the missing that One Observation cannot be retrieved for 250 years To come.

You have on so many Occasions Demonstrated Your Love To Literature and the good of Mankind in General that I thought no person so proper as your self to think of the ways and means of perswadeing these Colledges to prepare themselves for taking that Observation and in order to it you may make what use you please of the papers herewith, only not my name.

Endorsed: Coppy Letter to Mr Franklin Janry 29 1753


[January 26, 1753]


What Put me upon giveing you the Trouble of this Was the Sight which a few Days agoe I had of some Letters from the Royall Academy of Sciences at Paris with directions for Observing in Canada the End of a Transit of Mercury over the sun Which is To happen on the 6. of may Next and by their Estimation about ¾ past five in the morning at Quebeck and that it Ends at or possibly before sun riseing at the Lac Superieur.

This made me Look into Lowthorpes Abridgment of the Transactions of the Royall Society at London page 431.3 and by that in the Longitude of 5°4 West of London, (Which I suppose is nearly the Longitude of Phileadelphia as New York by many Observations of the Eclipses of Jupiters first Satellite Was found in 4h. 56′ nearly) it would seem To me that the Transit would End three minutes after Five5 in the morning,6 but if it does, yet both Interior and Exterior Contacts at the end, may be seen at Phileadelphia and here the transition may be Seen some minutes before the Contacts, or yet Still Longer in Connecticut, Boston, Halifax, and Louisbourgh by reason of their greater Latitudes and Less Longitudes.

The Jesuits in China and the East Indies I doubt not will Observe it in Sundry places there as the Whole Continuance of Mercury on the sun will be there Visible and if well Observed there and the End well Observed in North America it will bid the fairest of anything hitherto had To give the Suns parrallax or distance from the Earth, for the Triangles made of the Observations in the East Indies With those here will have known Bases of above ¾ of The Diameter of the Earth; the Next Transit by page 432 is in 1756 but It will not be visible here.

But the Critical Time for Obtaining the suns Parrallax will Be the Transit of Venus over the sun (by page 436) the 26 of May 1761 Old Stile which in 5h. west Longitude I Esteem will end at 53′ after 4 in the Morning and be visible at Phileadelphia for ¼ of an hour after Sun rise and much Longer Northward and Eastward.

Jones’s Abridgment of the Transactions, Vol. 4 page 213 &ca. has a Discourse of Edmund Halley concerning that transit of Venus and a figure of it and by good Observations of that, he Says, the parrallax of the sun may be Ascertained to 1/500 whereas I doubt of it’s Being Certain now To ¼ for Halley there Supposes it May be 12½ Seconds Wheras Flamstead in Sundry places of the Transactions from his Observations of Mars when nearest the Earth Esteems it not To Exceed 10″.

By Transactions Vol: 1. 435 and 436 you’ll see that No Transit of Venus till the year 2004 Will be visible both here and in the East Indies at the Same time so that if this Opportunity be missed there Will be None so good for 250 Years To Come To Ascertain this Much Desired point of the Suns parrallax which being Known The parrallax of all the planets is of Course from thence found.

As there are Now Sundry Nurseries of Learning Springing Up in Pensilvania, Jersey, New York, Connecticut, and Boston; all ways Should be thought of To Induce each of those To provide a proper apperatus for makeing such Observations, Long before the year 1761 that they May be Expert at Takeing Observations of that Kind before that Transit Happens; and in the mean time To be Ascertaining the Longitudes of these places by Observations of the Eclipses of Jupiters first Satellite and the Latitudes of the places by good Quadrants for these things should Be Known and Ascertained to Render the Observations of the Transit of use for Ascertaining The parrallax.

Another Reason why preparation should be made in all these places for that Rare Event, is; that should it happen to be Cloudy in some of these places yet it may be Clear in Others and by means of them that have Clear weather the Great End may be Gained; that Event may Well be Called Rare as Horrox7 who says the Last Transit of Venus over the sun On November 24 1639 now 114 years ago is the Only one of the Posterity of Adam who has seen one of these Transits.

I have said before that the suns Parrallax is not Ascertained To ¼ part but it seems there is a much greater Difference in Opinion about it than that, for tho’ it be Generally Taken now to be 10″ yet the Book called Matho or the Cosmotheoria puerilis printed in 1740 Vol: 2 page 388 &ca.8 gives very plausible Reasons why it Cannot be Less than 20″ and he thinks 26″ a more Reasonable Number. For he shows that if the parrallax be Only 10″ then the point of Equall Attraction Between the Sun and Earth, is but 45 Semidiameters of the Earth Distant from the Earth but the moons Distance is about 60 and therefore 15 SemiDiameters in it’s Conjunction Nearer the Sun than the point of Equall Attraction and from thence he Concludes that as there the Attraction is greater To the sun than to the Earth it Could Never Adhere To the Earth but be Drawn from it Towards the sun and allowing the parrallax to be 20″ yet even in that Case the moon Would be sometimes Beyond the point of Equal attraction and 26″ Should be allowed To Keep it always within that Point, which Reasons prevailed with me to be of his Opinion Till I Saw Mr. Maclaurins Explanation of Sir Isaac Newtons philosophy9 which Demonstrates Clearly that tho’ the moon Goes Without the point of Equall Attraction as Matho says, Yet it Will Adhere To the Earth by Means of its projectile free in its Orbit Round the Sun.

To Point out what apparatus of Instruments should be procured I Know of Nothing Better than To referr to the Abridgments of the Philosophical Transactions where multitudes of Observations of the Heavenly Bodys appear and the Instruments they were Observed with and how, And as To the Observations of the Transit of Mercury in particular and Instruments the Observers made use of, You’ll see Largely in the Abridgment of Philosophick Transactions Vol: 8 page 194 To 208 and Vol: 4. 213 &ca. before mentioned.1

Each of these places which have not a Set of the Abridgments of the Philosophick Transactions should in the first place Get them Being in 9 Vol: quarto which as they will be Cheaper so will they be Much Easier To Turn To, than the Transactions themselves by Means of their Tables.

I find by the preface To Halleys Tables page 3 that No. 386 of the Transactions Contains Halleys Corrections of his Numbers of Mercury. But that Volume 6 Containing that Number I have Lent To a Gentleman Living at 70 miles Distance from here.2

I Find by Halleys Tables that Macao in China is in 7h. 35′ and Pekin in China in 7h. 45′ East Longitude yet by reason of their Northern Latitudes and the suns Northern Declination at the times of Both the preceeding Transits they may see the ends of them Before the Sun Setts in those places.

I have Computed what would be the Lengths of the Bases of triangles made of Observations at Phileadelphia and Observations in the following places in the East Indies, and found them as follows

Place Longitude Latitude Parts of the Earths Diameter Supposeing the whole to be 100,000
Longitudes and Latitudes Taken from Halleys Tables } Pekin in China 116° 20′ E. 39° 54′ N. 76,286
Macao in China 113° 46′ E. 22° 13′ N. 85,375
Longitudes and Latitudes Taken from a map of Asia by Bowain in 1714 } Pondicheri 79° 0′ E. 12° 30′ N. 87,551
Batavia 104° 20′ E. 6° 20′ S. 95,714

On the papers of Directions herewith Mark’t A I Would Observe that I think them very good and that I find all or most of them in the Transactions and that if the Observer is acquainted With makeing Observations of that Kind they Seem Rather Too particular. If he is not acquainted, then the Neglect of Mentioning the Smoaking the Eyeglass before he Looks at the Sun thro’ the Telescope and Quadrant (if they have not Smoak’t plain Glasses fitted without the Eyeglass) may run the Observer into the Danger of Looseing an Eye. If he is to Observe the figure of the Sun Cast on a paper thro’ the Telescope then the Darkning of the Room should have Been mentioned.

Endorsed: Coppy of a Letter to Mr Franklin concerning transits of Mercury and Venus over the Sun to put the Colledges on observing them Janry 26 1753  B

[Note numbering follows the Franklin Papers source.]

2I. Bernard Cohen, “Benjamin Franklin and the Transit of Mercury in 1753,” APS Proc., XCIV (1953), 222–32; Harry Woolf, The Transits of Venus (Princeton, 1959), pp. 24–35, 41–5.

3See above, p. 407. Nathaniel Ames’s almanac of 1753 also contained a notice of the impending transit. Samuel Briggs, The Essays, Humor, and Poems of Nathaniel Ames, Father and Son, of Dedham, Massachusetts, from their Almanacs, 1726–1775 (Cleveland, O., 1891), p. 241.

4Evans 7038. One copy is reproduced in facsimile in Cohen’s article. Delisle’s instructions were accompanied by two diagrams illustrating “the Situation of Mercury upon the Sun, at his going out of the Sun at Quebec.” BF had them copied in pen and ink on blank sheets of the printed leaflets.

5The copy James Bowdoin received is in Mass. Hist. Soc., and Rev. Samuel Johnson’s is in Harvard Coll. Lib.

6“The Royal Society in London, and the Foreign Professors in Astronomy, request the Curious in North-America, on the Continent, to observe the Transit of Mercury over the Sun, which will happen on the 6th of May next. First to settle the true Time and the Place of Observation, and be attentive in exactly determining the Time of Duration. And also to be careful in observing the Time of total Ingress, and also of total Egress; and that they will please to communicate their Observations to the Royal Society, or send them to some Friends that may do it for them.” Pa. Gaz., April 5, 1753. Alexander thought these directions, compared with Delisle’s, “another instance of their [the Royal Society’s] being asleep else they would never have directed us to Observe the beginning and Duration of that transit.” Colden Paps., IV, 385–6.

7Ibid., p. 368.

8See below, p. 508.

9Enclosure A had three parts: 1. a letter from the Marquis de la Galissonière, governor of Canada, to the governor of New York, dated Paris, Oct. 10, 1752, requesting the latter to forward a letter and its enclosure to Canada; 2. a letter from la Galissonière to Father Bonnécamp, professor of hydrography at Quebec, dated Paris, Oct. 10, 1752, requesting him to observe the transit of Mercury there on May 6, 1753, in accordance with the directions enclosed; and 3. instructions prepared by Delisle for making these observations. These papers are not reprinted here.

1Enclosure B, a memorandum from Alexander to BF, January 26, is printed below. Copies of this letter and of English translations of the letters in Enclosure A were sent to Colden (Colden Paps., IV, 346–51, 368) and to BF.

2The MS enclosure sent to BF has not been found; the text here printed is taken from the ms copy made for Cadwallader Colden (Colden Paps., IV, 368), in whose papers it survives. Differences between it and the printed version (Evans 7308) are noted, if significant. For clarity’s sake, punctuation, almost totally lacking in the MS, has been added from the printed version, but the meaningless capitalization is preserved.

3John Lowthorp and Henry Jones, eds., The Philosophical Transactions and Collections … Abridged, I, 431.

4An error in both MS and printed version for “five hours.”

5Alexander corrected this error in letters to Colden, February 12 (Colden Paps., IV, 369–70), and BF, who in the printed leaflet gives the correct time of 6:18 A.M.

6Instead of the remainder of this paragraph, the printed leaflet says simply: “So that it may very well be observ’d in all the English Colonies on the Main.”

7Jeremiah Horrocks, Opera Posthuma (London, 1678).

8Andrew Baxter, Matho: or, The Cosmotheoria Puerilis, a Dialogue (London, 1740).

9Colin Maclaurin, An Account of Sir Isaac Newton’s Philosophical Discoveries (London, 1748).

1John Martyn, ed., Philosophical Transactions … Abridged, VIII, 194–202.

2Ibid., VI, 273–8.

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