Monticello Dec. 17. 19.
I have duly read the pamphlet you were so kind as to put into my hands on the new method you propose of observing longitudes both at sea and land. the subject, you know, is one of the difficult ones of Astronomy, and, to be investigated understandingly would require time, consideration and research. these neither the state of my health, of my mind, or occupations permit me to give to it, increased as they would be by the rust of 50. years, employed in matters of a very different character. I can only therefore hazard such suggestions as occurred, primâ facie, on reading your paper. a general observation is, that, your own mind being familiar with the subject from long contemplation, you have supposed it as obvious to others, and therefore have not given it all the developement and explanation it may require with those to whom it is new. I presume for instance that you propose the simple process of a watch for the time, and a meridian drawn on a board, by way of example only: whereas in taking observations of longitude at land, a standing clock of superior workmanship and accuracy is deemed indispensable; because, as you justly observe, an error of one of time, is an error of 15 miles of place we read of clocks used for this purpose varying not more than 4. or 5″ in a year. again, I suppose the meridian drawn on a board, is merely exempli gratiâ: because, to regulate these clocks, and to place them on true time, with the necessary accuracy, requires a meridian of elaborate construction. where Gnomons are used for this purpose, those of Europe are from 30. to 100.f high generally. that of Ste Sulpice at Paris is 80. feet, and that of the cathedral of Florence is largely upwards of 200f. when a Transit instrument is used for the meridian. a horizontal line is always sought, of the greatest length the situation will admit. this depends on the site of the observatory or place of observation. mr Rittenhouse’s meridian was of ¼ of a mile only. that of Paris, I think is upwards of 3. miles. at the site of our University we shall be able to have one of 8. or 10. miles at Monticello a mountain in the meridian gives me a line of 40. miles. and certainly the greater the length the less the danger of error. altho’ you mention taking an arbitrary prime meridian, as Staunton for example, whose position in longitude is unknown, yet I do not understand you as preferring such an one to those of Greenwich, or Paris, for example, for which almanacs are prepared 2. or 3. years beforehand, in which elements for calculating the position of the sun, moon, & remarkable stars, are given for every day of the year and even portions of days, which saves the necessity of a 2d place of observation. and altho’ you do not notice, in the observation of the star, the corrections for parallex, refraction Etc necessary to deduce it’s true from it’s apparent place, yet no doubt this is intended. but to give a fair opportunity of estimating the merit of your proposition, a full development of all the details of the process, as you propose them in practice, should, I think be given. that the longitude may be well found from the passage of a star over the meridian, at land, has never, I believe, been doubted; and the reason, I suppose, why it has been little, if ever, practised, is the superior convenience and accuracy of observations & calculation of the eclipses of Jupiter’s satellites, altho’ these also require clocks and meridians of the highest order. when Lewis was sent to trace the Missouri & Columbia to the Pacific ocean, as course was almost uniformly West, so that he would change his longitude by nearly the length of his journey daily, it was proposed he should ascertain his longitudes by the passage of stars over his meridian, or their distance from it, by the aid of an Equatorial, which, by it’s combined motions in the three plains of the horizon, equator, & the circle of latitude, enables you to ascertain your latitude and meridian accurately in any place, in 2. or 3. hours. the lunar observations however with a good chronometer were preferred, as more convenient and, since the introduction of the repeating circle of Borda have come much into use at land as well as at sea.
Explanation however becomes much more necessary when we proceed to consider the application of your method to find the longitude at sea, where there can neither be a standing clock, nor a meridian, nor a telescope of any power, because of the motion of the ship and the impracticability that occasions of keeping the star in the field of the glass, with the steadiness requisite, were I to presume advice on a subject I have so hastily considered, it would be that you should give a full developement of the whole process in all the details of practice, as well that you may see yourself whether any of these minutiae may have escaped your attention, as to satisfy the doubts of others. and, to recommend it to practice. it’s advantages over the method of Jupiter’s satellites, if any it has, should be shown.
Be pleased, Sir, to accept of these suggestions as meant only for your own consideration, and not to engage further discussion which might employ your time, as well as mine unprofitability. if they are unsound, you will percieve it; if sound they may serve for the improvement of your proposition. as to myself I have long laid aside these speculations, as entirely uninteresting to the views of age, and I will trespass no further on your time than to assure you of my great respect and esteem.
DLC: Papers of Thomas Jefferson.