The letter, as a transmission medium, has been diminished by its obsolescence in the modern period. Yet this should not inhibit our appreciation of its central place in the Enlightenment. Letters reached a pinnacle, before being superseded by the telegraph, within the wider infrastructure of correspondence networks. At no time before the eighteenth century were letters spread so widely or rapidly and at no time since has it so much mattered that they were. In the Republic of Letters correspondence disseminated personal information, political news, scientific priority and discoveries, the expansion of networks, mechanical directions, industrial secrets, and privileged entry into coveted sites of scientific innovation. The Delesserts were amateur scientists whose correspondence network incorporated these elements and reveals how practical science was executed from below.

The Delesserts' British-Franco-Swiss network represented a nexus for the dissemination of science in the late Enlightenment. Its existence, importance, and expansion resulted from the letter. However, letters also predicated the Delesserts' absence from the historical record. Their personal correspondence, l'Archive Delessert, remained in the private hands of their descendents until the twenty-first century. The emergence of these letters; as well as the scattered remnants of their correspondence in French, British, and American archives, has finally allowed their important network to be uncovered.

The family's expansive network was an extension of l'hôtel Delessert's salons and collections. Both were middling grounds where professional and amateur scientists mingled sharing information and materials. The network connected Protestant bourgeois men of science and industry. It was also inclusive, however, in terms of religion, gender, class, and nationality. In the nineteenth century the network became global. Benjamin Delessert (1773-1847) purchased from and sponsored voyages of discovery enlarging his massive herbarium. Two of his brothers pursued agricultural and banking in America in connection with the DuPont de Nemours family. The origins of the Delesserts' network, however, were far more remote.

The Delesserts were bankers by profession. Their participation in the Enlightenment, Franco-Swiss origin, and relocation to Paris insured their involvement in many ventures. Etienne Delessert (1735-1816) participated in the improvement of French agriculture and industry. His wife, Madeleine-Catherine Delessert (1747-1816), dedicated herself to her children's education. She was from a Swiss banking family and her marriage to Etienne in 1766, and their move from Lyon to Paris in 1776, forged an extensive correspondence-network.

Mme Delessert maintained an enduring correspondence with Jean-Jacques Rousseau (1712-78). It began during her youth when her family provided Rousseau refuge in the Swiss mountains and lasted until his final years in Paris. He remained an intimate friend and wrote Letters on Botany for Mme Delessert to instruct her daughter. The Delesserts' intimacy with Rousseau solidified their connections to French and Swiss acquaintances and enabled them to expand their network to Britain. Initially this included Huguenot descendents and Swiss émigrés. However, it grew to encompass British Francophiles and utilitarian industrial leaders James Watt (1736-1819) and Mathew Boulton (1728-1809) of Birmingham's Lunar Society. This connection facilitated the Delesserts' admittance to the mechanical wonders of Birmingham and the Industrial Revolution.