Joseph Matkin was the ship's steward's assistant for the Challenger voyage. As such, he was a below-decks crewman and not expected to have much interaction with the officers or scientific staff. Since he was not an officer or one of the scientific staff, it is likely Matkin's role on Challenger would have been lost to history, had it not been for his prolific letter writing and the remarkable survival of the bulk of his letters, most of which are now in the possession of the Scripps Institution of Oceanography's archives.His duties focused primarily on the Challenger's stores of food and "household" goods, and the issuance of such to the ship's company from the tiny Steward's Issuing Room on the lowest deck in the bottom of the ship. He kept inventory and made purchases of fresh food throughout the course of the journey; hence the focus in his letters on the availability and price of foods, particularly fruit, at each landing.

Matkin wrote the lengthy and detailed letters, primarily to his mother, his three brothers, and his cousin, most including a travellog portion based upon the detailed journal he kept during the voyage, that eventually comprised several volumes. He mailed the journal volumes home as the books filled up, from various points throughout the journey, but none, as far as is known, have survived. Fortunately, he bases much of the letters on the detailed observations recorded in the journals, a fact which he states himself, and which is borne out by the lengthy identical passages of great detail which appear unvaried across several recipients' letters. Matkin's granddaughter donated the letters to Scripps Institution of Oceanography in 1985 where they joined the letterbook of his cousin J.T. Swann in the Institution's archives.