Dumfries 2d July 1820
altho there is no persons opinion I would prefer to yours on any Invention of mine as none so favourable Could be more useful to me, I should not have Ventured to trouble you, If the particular situation in which I am placed had not renderd it necessary. A few days past I enclosed the Instrument (I now Enclose to you) to a Mr Grolick of Fredericksburg, he on returning it wrote me amongst other things in the following words to Wit “the Idea of the Mapper I am Inclined is not a new one W Gerrardin was at my house 6 or seven years ago he lived at that time in the neighbourhood of Monticello and was Intimate with Mr Jefferson as near as my recollection serves me at so remote a period I think I was Informd that Mr Jefferson used something similar to your mapper in platting fields &c” as I Could Conscienciously have honor that I believed I was the original Inventor of the Enclosed Instrument before the Receipt of this letter: I Cannot now do it unless I hear from you altho I believe Mr Grolrick has mistaken Mr Girardins description of your Instrument. I have deemd it the Correct Course to Enclose it. may I ask the favour of you after Examining of it and If not your Invention. to say your opinion of it and whether I may make that Opinion Publick practical surveying here have born testemony to its usefulness. I have for many years platted by Latitude & departure instead of Course and distance. by this means. the distance was a Check upon my work. it occurd to me that an Instrument on the plan sent would facilitate mapping as the this protractor, sweep, hole, or Even dividers would be necessary. after the mapping Sheet is prepared like the one Enclosed which by the by Should Cost a surveyor his life time as I have made dozens of maps on the same Sheet. by drawing the outlines of the map with a black lead pencil when the map is pressd off on a Clean Sheet with a rubber the lines are rubbed out and the prints stopd by pressing on them the sheet is ready for a new map. this sheet should be Carefully laid of and to prevent Blunders in Calculating the Contours of your map Each Inch should be a red line as the map lies before you you may in a few minutes add up the Contents of your map the mapper is on a scale of Forty poles to an Inch but may be made to any scale, in useing this Instrument you map by the Latitude & departure of your Course and not by the Course and distance If your Latitude is north the angle of the Instrument is Upwards. If south downward the perpendicular of the Instrument is the Latitude and the the it has occurd to me there is a possibility I am describing what you well know the thought has stopt me your General Character assures me you will pardon this freedom
one word more. the Hypotheneuse of this Instrument is intended as a value as it is on the same scale as you rule the line you see If your distance is Correct
DLC: Papers of Thomas Jefferson.