From William Jones
London Jany. 22nd. 1787
On account of the hurry of previous business, I have been prevented from finishing your Perspective Machine as soon as I wished, but herewith you receive it, and in a State which I presume will not be unacceptable. I have improved it, by adding the few requisites for a Drawing Board, which make it a complete Instrument. The use of the several parts of the Machine I presume will be obvious to you, viz. the T. and Bevil Square for drawing Parallel lines in all Directions. The box scale frame, for confining down the paper (which should be rather damp when first put down) on the board. The scale serving as a guide to the distance of the parallel lines. The Steel Pin at the corner of the Box is to be put in a hole in one of the joints to keep the brass frame upright when necessary. There is a groove in the brass frame, to contain a Pane of Glass mentioned by Ferguson. I made the frame square for an obvious reason viz. to admit the whole of the board, as the drawing of a Machine &c. when placed near the Instrument may require a larger space, than the Arches admit of. A Friend has informed me Sir that Monsr. Guyot, has lately published a very pleasant Work, entitled Récréations Physiques et Mathematiques, in 3 Vos. Octavo, containing a selection of many very curious and entertaining experiments [in] Electricity, Magnetism &c. I expect very shortly some copies of his book from him having ordered them 3 Months ago. If you think it worth while to call on him, as he is [a] curious man I have written underneath the Direction of his Address.
I am Sr. Your Obliged Humble Servt.,
Monsr. Guyot, directeur des Postes, rue francois Pres la rue mauconseil No. 12.