Thomas Jefferson Papers

To Thomas Jefferson from Thomas Whitney, 2 July 1802

From Thomas Whitney

philadelphia July 2d. 1802


The brass work which I repaird no doubt you have received. the charge 1. D 50 C I have been paid. An Artificial horizon such as you mention cannot be had in this City. But a more simple kind have been in use some time which I believe answer every purpose of the Other ones full as well, can be obtained. they consist of a piece of parrallel glass, floated on Quiksilver in a Small Box, which without the Quiksilver (which is put away into a small Bottle when not in use) would not cost above 3. D 50 C. these kind of horizons can be used with a Sextant in Lat. 40. from the 28th of August to the 16th of Aprill and in more Southern or Northern Latitudes, proportionably a Shorter or Longer part of the year. the Observation is made by bringing the Image of the Sun seen by reflection in the Sextant into contact with the Sun seen by reflection in the artificial horizon. half the distance allowing for the Suns Diameter &c is the Altitude thereof. The size of this horizon is generally about three inches and Circular. the largest piece of parrallel glass I have is square and about 2¼. Inches by 1½ Inch which will answer the purpose being large enough to contain the Suns Image.

waiting your order I am with much respect yours

Thos. Whitney

RC (MHi); endorsed by TJ as received 3 July and so recorded in SJL.

The BRASS piece that Whitney repaired was from a Hadley’s reflecting quadrant, a device similar to a sextant. TJ had recently purchased the instrument (TJ to Robert Patterson, 22 Mch.).

The ARTIFICIAL HORIZON was to allow TJ to use the quadrant when the true horizon was not visible. SUCH AS YOU MENTION: see TJ to Whitney, 13 June.

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