A Declaration Concerning the newly invented Art of Double Writing was a pamphlet of 6 leaves, written by Sir William Petty (1623–1687) and first published in 1648.

Wherein are expressed the reasons of the Authors proceedings in procuring a Priviledge for the same : As also of the Time, Manner, and Price, of the discovery of the said Art, and of the Instruments belonging thereunto.

Whereunto is annexed a copie of an Ordinance of both Houses of Parliament, approving the feasibility and great use of the said invention, and allowing a Priviledge to the Inventor, for the sole benefit thereof for 14 years, upon the penalty of one hundred pounds.

Already in the Preface of his first printed publication, The Advice to Hartlib (first published 1647), Petty had written that there was "invented an Instrument of small Bulke and price, easily made, and very durable, whereby any Man, even at the first sight and handling, may write two resembling Copies of the same thing at once, as serviceably and as fast (allowing two lines upon each page for setting the Instruments) as by the ordinary way."

He then suggested that the returns of that invention might perhaps be enough to pay for the "Ergastula literaria" – the literary work-houses – that he wanted to establish.

The text of this broadside was in much the same terms as the Preface to Petty's The Advice to Hartlib of 1647/8; it had "an Extract of the Ordinance of Parliament dated 6.

Already early in 1603, a Bavarian Jesuit priest, Christoph Scheiner (1573–1650) had invented a pantograph, a mechanical instrument, in which a pencil was connected to a stylus by means of a parallelogram.

The Hartlib Papers hold an anonymous and undated copy of a text about "the nature and uses of the double writing instrument".

This partnership was to be confined to the development of Petty's inventions, "more particularly the double writing instrument, a machine for printing several columns at once, a scheme for making a great bridge without any support on the river over which it stands, and other undertakings of the same kind.

The failure of the reproduction of his unfinished invention had made it impossible for him to give to the world his own perfected instrument for multiplying copies of writing.