Your favor of Mar. 30. was recieved just as I was setting out for this place, 90. miles S.W from Monticello. I inclose an answer to the acceptable letter of your daughter. the sensibility expressed in hers for services so moderate shews a heart of great susceptibility, and which under your careful instruction promises to make happy parents and friends.

I am glad you have explained what I had considered as a breach of confidence in mr Canby, but which seems to have been a want of caution only in trusting my letter to a person capable of copying and publishing it without leave. this has so repeatedly happened to me that I have at times been near coming to a resolution never to answer a letter from any one whom I did not personally know and confide in. a similar abuse has been lately committed against mr Adams and myself by a person’s publishing our answers to a letter written by himself.

Our legislature has past a law for having an accurate map made of the whole state, by actual surveys of the roads, rivers, mountains Etc. to be corrected by astronomical observations. this last will be a distinct work. in the execution of it, I suppose, the person employed will travel with his assistant & instruments a gigg and strong horse, short journies of half a day at a time, from county to county, taking the Longitudes & Latitudes of the mouths of water courses, remarkable mountains, towns, Courthouses Etc. employing the intervening days of rain, or of cloudy noons or evenings in making his calculations. as in a former letter you stated to me that you had quitted the business at Baltimore, presuming you might be as yet disengaged, I proposed to our Governor the inviting you to undertake this work, provided they would give such compensation as would make it worth your while. on this point I have fears. I mentioned that a prime Hadley’s circle of Borda’s construction would be necessary and perhaps an Equatorial. should you undertake it, I would wish you to consider again the idea I had when Capt Lewis was about to undertake his Western journey, of relieving him from a dependance on any portable instrument of time, liable in such a journey to so many accidents, by substituting the use of a meridian, which, with an Equatorial, after ascertaining your latitude, can always be found, at land. I then explained this idea to you, and will here recall it to your recollection. the method I proposed was ‘to observe the moon’s distance from the meridian of the place at any moment and take by observation her right ascenscion at that moment. find from the tables her distance from the meridian of Greenwich at the moment she had that right ascenscion. if she was between the two meridians, add the two distances, if she was East or West of both, subtract one from the other, and you have the distance between the two meridians. in this way the appulse of the moon with a star, or the instant when she is in the same vertical with a star, may be advantageously taken for the moment of observation.’ that the meridian of the place of observation has never been resorted to in Europe as an element in taking the lunar distances, does not deter me from supposing it may be used to advantage because there they have no occasion for them, having every where observatories, with good timepieces and telescopes, with which they can practise the method by Jupiter’s satellites; and at sea they can have no meridian. but in our perambulations by land for the purpose of fixing our geography, where we can have no apparatus but what we can carry with us, I do not see why we should not avail ourselves of this important element of the meridian of the place of observation, it being so obvious that it’s distance from the moon, and her’s, at the same moment from the meridian at Greenwich give the difference of meridians. I am but a theorist, very little practised in these operations. but I wish you to turn your mind to it, and if it appears practicable, and to offer any advantages, tell me so, and whether you would undertake this business, and I will propose the providing an Equatorial. I suppose this operation will employ you a year at least, as we have about 100 counties in every one of which several points should be fixed: indeed I think the distances of our observations should not exceed 20. miles. I salute you with friendship and respect.

Th: Jefferson