After the evacuation of San Domingo in 1798, our physician paid a visit to the United States, where he was received with signal distinction, his reputation having preceded him. The latter part of the year found him again at Stockton, publishing a work on contagious and endemic fevers, "more especially the contagious fever of ships, jails, and hospitals, vulgarly called the yellow-fever of the West Indies;" together with "an explanation of military discipline and economy, with a scheme for the medical arrangements of armies." He undertook, about this time, by desire of Count Woronzow, the Russian ambassador, the medical charge of seventeen hundred Russian soldiers, who were stationed in the Channel Islands in a sad state of disease and disorganization; and so admirably did he acquit himself, and so perfect were the hospital provisions he made, that (1800) the commander-in-chief nominated him physician and head of the army-hospital depôt at Chatham--as he says, "without any application or knowledge on his part." This appointment was the cause of his subsequent misfortunes.
At Chatham, with the warm approbation of Major-General Hewett, commanding the depôt, he introduced that system of hospital reform form which had elsewhere operated so successfully. The changes he effected, as soon as they were made, became known to the Medical Board, and were publicly approved of by one of its members. However, shortly afterwards, an epidemic broke out in the depôt (then removed to the Isle of Wight), arising from the fact, that the barracks were overcrowded with young recruits, but which the medical board ascribed to Jackson's innovations, and reported so to the Horse-Guards. The commander-in-chief directed an inquiry to take place before a medical board impannelled for the purpose, and the result of that inquiry may be guessed from a communication made by the War-Office to the commandant of the depôt. This states "the unanimous opinion of the board to have exculpated Dr. Jackson from all improper treatment of diseases in the sick," and the commander-in-chief's gratification, "than an opportunity has thus been given to that most zealous officer of proving his fitness for the important situation in which he is placed." The result of this wretched intrigue, however, was that Jackson, disgusted with the whole affair, requested to be placed on half-pay, to which request the Duke of York, with marked reluctance, at last (March 1803) acceded.