Andrew Ellicott's map of the Mississippi River from the Lewis & Clark College Collections.   On March 6th Thomas Jefferson heard from Andrew Ellicott in southern Pennsylvania that he would be pleased to instruct Lewis in the arts of surveying and astronomy. Ellicott (1754-1820) was the son of a Quaker clockmaker, and at the age of fifteen studied mathematics with Robert Patterson, whom he had joined by 1803 as a member of the American Philosophical Society. Experienced in the manufacture of precision scientific instruments, and famous as the surveyor (among many other national boundaries) of the new territory that was to become Washington D. C., Andrew Ellicott was the ideal tutor for Lewis in the art of surveying. Lancaster March 6th 1803 Dear Sir . . . 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 42nd page of my printed observations made in our southern country, a copy of which I left with you [The Journal of Andrew Ellicott, Late Commissioner . . . for Determining the boundary Between the United States and the Possessions of His Catholic Majesty in America. . . . Philadelphia, 1803]. 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 with practice; in this he shall have all the assistance I can give him with the 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. [After a number of detailed instructions for calculating latitude and longitude, Ellicott closes with a practical suggestion:] It will be a necessary precaution, to have the Chronometer, with its case, tied up in a bladder when not in use,—it will privent its being injured if by accident it should be thrown in the water by the overturning of a canoe, or other accident. A. E. Source: Jackson, Letters, item 19