Letter from Andrew Ellicott to Thomas Jefferson

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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.

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The week before last I adapted a gridiron 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 Londe, and Delambre.