Before the House of Commons on March 4, 1774, prison reformer John Howard reportedly testified that he had visited 38 out of 42 county jails in the kingdom. Contemporary newspapers provided four apparently independently reported accounts of Howard testimony:

1. “Mr Howard was next called, who gave the Committee a full and distinct Account of all the Gaols he had been into, which he said were 38 out of 42 in this Kingdom, and he intended to see those four.” Public Advertiser, Mar. 6, 1774.
2. “Mr. Howard, Sheriff of Bedford, was called in, and gave the House a particular account of the impositions that were practiced in the different gaols of this kingdom; said he had traveled and seen 38 out of the 42, on purpose to make himself master of the subject, and explain the abuses to the House.” London Evening Post, Mar. 3 to Mar. 5, 1774.
3. “Mr. Howard, sheriff of Bedford, was called, and gave the House an account that he had seen thirty-eight out of forty-two gaols in the Lent circuit, besides others, as Bristol, Ely, Litchfield, &c. That those he had not seen, in a few days he would set out to visit.” Annual Register, 1774, Chronicle, p. 100.
4. “I had the honor of begin high sheriff for my county two years ago…. I traveled for that purpose [to learn the state of gaols] through the kingdom, and have been in thirty-eight out of the forty-two county gaols.” Middlesex Journal, and Evening Advertiser, Mar. 5 to Mar. 8, 1774.

The figure of 42 can be understood as the number of eighteenth-century English counties, with the inclusion of Monmouthshire (considered an English county for parliamentary purposes) and counting the North, East, and West Ridings of Yorkshire separately. Assuming one county gaol per county would explain Howard’s reference to a population of 42 gaols. According to data in The State of the Prisons in England and Wales (4th ed, 1792), Howard visited a total of 44 jails in 32 English countries prior to his testimony. How those figures relate to to Howard’s cited figure of 38 isn’t clear.

Prisons and jails/gaols apparently weren’t significantly distinguished in England. Between autumn, 1775 and January, 1777, Howard made about 350 visits to 230 prisons or jails (gaols).^

More data in dataset for John Howard’s first visits to county jails