While here, & much confined to the house by my rheumatism, I have amused myself with calculating the hour lines of a horisontal dial for the latitude of this place which I find to be 37°–22′–26″. the calculations are for every 5. minutes of time, and are always exact to within less than half a second of a degree. as I do not know that any body here has taken this trouble before, I have supposed a copy would be acceptable to you. it may be a good exercise for master Cyrus to make you a dial by them. he will need nothing but a protractor, or a line of chords & dividers. a dial of size, say of from 12.I. to 2.f. square, is the cheapest & most accurate measure of time for general use, & would, I suppose, be more common if every one possessed the proper horary lines for his own latitude. Williamsburg being very nearly in the parallel of Poplar Forest, the calculations now sent would serve for all the counties in the line between that place & this, for your own place, New London, & Lynchburg in this neighborhood. slate, as being less affected by the sun, is preferable to wood or metal, & needs but a saw & plane to prepare it, and a knife point to mark the lines and figures. if worth the trouble, you will of course use the paper inclosed; if not, some of your neighbors may wish to do it, & the effort to be of some use to you will strengthen the assurances of my great esteem & respect.

Th: Jefferson