ALS in French, signed “Humboldt,” one page both sides, 7.75 x 9.5, 1823. Letter to mathematician and astronomer Louis Benjamin Francoeur, concerning a new artificial horizon. In part (translated): "Sir, having read this morning an interesting report that you made, Sir, to the Society of encouragement for a new artificial horizon attributed to Mr. Ducom…I thought you might be pleased to learn that this apparatus of 2 graduated tubes and placed in advance under the angle of reflection of…the sun is an invention of Mr. Köhler in Dresden…I described this apparatus very clearly in the introduction from my collection of obs. astronomical…book published for fifteen years with the tubes of Mr. Köhler…there is nothing new in what it seems in the apparatus of Mr. Ducom that the application of the canvas from Mr. Brosle…you may find the opportunity, Sir, to recall the invention of the astronomer of Dresden, who has been dead for a long time." Humboldt sketches the apparatus he describes in the left margin. In very good condition, with two old mounting remnants to the left edge, a strip of toning to the right edge and light irregular toning elsewhere, and a slice to the right edge passing through several words of text.Format: ALS