To Thomas Jefferson from James Clarke, 27 May 1807

I have just received your letter of the 22d Instant respecting an instrument on my pheaton for measureing distances in traveling, and the pleasure I feel in complying with your request in having it coppied and introdused into publick use, will be greatly increased by your acceptance of it as a present. I would send it on to you by the first opportunity, or if you expect to return to Montecelo this summer, and can advise me of the time I will come up and bring it with me; as I wish to show you the manner of takeing it to peices and puting it together again: also the manner of connecting it with the carriage. The first I made had but one index which revolved once in ten miles, and had no bell, yet answered all the purposes of real use; but thinking it not so perfect as it could be, I made the present one which has two indexes and a bell, such are attached to house clocks. The one index points to the figure denoting the number of miles, the other to the fraction of a mile, which is decimally divided, and the bell (which may be heard several hundred yards) gives the alarm at the end of each mile: and the whole machinery (except the rod wh. receives the motion from the carriage wheel) is contained in a small brass case about the size of a common shaving box, and for simplicity, and accuracy I do not conceive there is room for improvement. This instrument was calculated for a wheel five feet one inch diameter, therefore it will be nessesary to have your hind wheels brought to that size.

I wish also to show you some other instruments which I have constructed, which as a lover of the arts and sciences I think you will be pleased with; particularly an instrument to determine the longitude by magnetick attraction and repulsion. I have just finished this instrument except, that I can’t get my needles properly magnetised here for want of a good magnet. I have not yet proved the truth of this instrument, in consequence of not having an opportunity (since it was compleated) of trying the effect in deferent places; but think the deference is very perceivable between this place and Richmond, which is only 30 miles—

Be pleased to accept my highest respect & esteem of Your Obedient Servant