From Andrew Ellicott
Lancaster March 6th. 1803.
Your agreeable favour of the 26th. Ult has been duly received, and the contents noted.—I shall be very happy to see Captn. Lewis, and will with pleasure give him all the information, and instruction, in my power.—The necessary apparatus for his intended, and very interesting expedition, you will find mentioned in the last paragraph of the 42d. page of my printed observations made in our southern country, a copy of which I left with you.—But exclusive of the watch, I would recommend one of Arnolds chronometers, (if it could be had,) for reasons which I will fully explain to Mr. Lewis.—
Mr. Lewis’s first object must be, to acquire a facility, and dexterity, in making the observations; which can only be attained by practice; in this he shall have all the assistance I can give him with aid of my apparatus.—It is not to be expected that the calculations can be made till after his return, because the transportation of the books, and tables, necessary for that purpose, would be found inconvenient on such a journey.—The observations on which Arrowsmith has constructed his map of the northern part of this country, were all calculated in England.
The week before last I adapted a grid-iron pendulum to my regulator, it is the first ever made in this country, and was the work of six sundays, the duties of my office not allowing any other time:—the rods, and bob of this pendulum together, weigh 18 pounds.—
I had a midling good observation on the beginning of the eclipse of the sun on the evening of the 21st. of last month.
I am in hopes Mr. Madison forwarded my observations to the national institute by Mr. Munroe.—Those on the 4th. satellite of Jupiter, have been lately written for by both la Lande, and Delambre.
I have the honour to be with great respect and esteem your friend and hbl. Serv.
RC (DLC); at foot of text: “Thomas Jefferson President U.S.”; endorsed by TJ as received 18 Mch. and so recorded in SJL. PrC (DLC: Ellicott Papers).
favour of the 26th: a letter from TJ to Ellicott of 26 Feb. is recorded in SJL but has not been found.
Astronomical observations that Ellicott made during the survey of the southern boundary of the United States appeared in the Transactions of the American Philosophical Society in 1802 and as an appendix to his Journal of the survey published in 1803. With reference to a set of his lunar sightings on the Gulf Coast to calculate longitude, Ellicott made these recommendations about apparatus: “From this example it may be seen with what ease, both the latitudes, and longitudes of places may be determined on land for common geographical purposes with a good sextant, a well made watch with seconds, and the artificial horizon, the whole of which may be packed up in a box of 12 inches in length, 8 in width, and 4 in depth” (The Journal of Andrew Ellicott, Late Commissioner on Behalf of the United States … for Determining the Boundary between the United States and the Possessions of His Catholic Majesty in America [Philadelphia, 1803], appendix, 42; APS description begins American Philosophical Society description ends , Transactions, 5 , 202).
arnolds chronometers: for observations to find longitude, Ellicott recommended timepieces produced in England from John Arnold’s design (Vol. 34:118, 119, 120n).
calculations: Ellicott and Robert Patterson used a lunar distances method for finding longitude on land. They taught Meriwether Lewis how to measure the angular separation between the moon and the sun or another celestial body and to make multiple observations of the sun’s altitude each day. They did not expect him, however, to make the laborious mathematical calculations that their technique required for the computation of longitude from the observational data. In his orders for the western expedition drafted in April, TJ instructed Lewis to make careful observations, record the results accurately, and bring the information back to the War Department, which would have the responsibility of finding “proper persons” to make the lengthy computations (Richard S. Preston, “The Accuracy of the Astronomical Observations of Lewis and Clark,” APS description begins American Philosophical Society description ends , Proceedings, 144 , 168–91; Patterson to TJ, 15 Mch.; Document IV of the group of documents on drafting instructions for Lewis, at 13 Apr.).
Ellicott shared his information about the timing of the 21 Feb. eclipse of the sun with his acquaintance José Joaquín Ferrer y Cafranga, who had observed the eclipse in Havana, Cuba. Ferrer used the two sets of figures to calculate the longitude of Ellicott’s location in Lancaster, Pennsylvania (APS description begins American Philosophical Society description ends , Transactions, 6 , 158–64; Vol. 36:485).
During the survey of the boundary with Spain, Ellicott collected data on the disappearance and reappearance of moons of jupiter to determine longitude. A few of those observations were of the planet’s fourth moon (APS description begins American Philosophical Society description ends , Transactions, 5 , 188–9, 191).