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Poplar Forest, August 23, 1811.

Dear Sir,—While here, and much confined to the house by my rheumatism, I have amused myself with calculating the hour lines of an horizontal dial for the latitude of this place, which I find to be 37° 22´ 26´´. The calculations are for every five 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 and dividers. A dial of size, say of from twelve inches to two feet square, is the cheapest and most accurate measure of time for general use, and 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 and this, for your own place, New London, and Lynchburg in this neighborhood. Slate, as being less affected by the sun, is preferable to wood or metal, and needs but a saw and 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 enclosed; if not, some of your neighbors may wish to do it, and the effect to be of some use to you will strengthen the assurances of my great esteem and respect.