His third winter-sessions at Edinburgh having passed away, he was induced to go out and seek his fortune in Jamaica, and accordingly proceeded thither in a vessel commanded by one Captain Cunningham, who had previously been employed as master of a transport at the siege of Havannah.  It is far from improbable that it was from his conversations with this individual that Jackson derived those hints, of which at a future time he availed himself, respecting the transmission of troops by sea without injury to their health; but it is quite certain his conviction of the enormous value of cold-water affusions as a curative agent in the last stage of febrile affections, was imbibed from this source.

Arriving in Jamaica, he in 1774 became assistant to an eminent general practitioner at Savana-la-Mar, Dr King, who was also in medical charge of a detachment of the first battalion of the 60th regiment.  This latter he consigned to Jackson’s care; and well worthy of the trust did our young adventurer, though but twenty-four years of age, approve himself—­visiting three or four times a day the quarters of the troops to detect incipient disease, and studying with ardour and intelligent attention the varied phenomena of tropical maladies.  Four years thus passed profitably away, and they would have been as pleasant as profitable, but for one circumstance.  The existence of slavery and its concomitant horrors appears to have made a deep impression on Jackson’s mind, and, at last, to have produced in him such sentiments of disgust and abhorrence, that he resolved on quitting the island altogether, and, as the phrase is, trying his luck in North America, where the revolutionary war was then raging.  This resolution—­due perhaps, as much to his love of travel as to the motive assigned—­was not altogether unfortunate, for shortly after his departure, October 3, 1780, Savana-la-Mar was totally destroyed, and the surrounding country for a considerable distance desolated, by a terrible hurricane and sweeping inroad of the sea, in which Dr King, his family and partner, together with numbers of others, unhappily perished.

The law of Jamaica forbade any one to leave the island without having given previous notice of his intention, or having obtained the bond of some respectable person as security for such debts as he might have outstanding.  Jackson, when he embarked for America, had no debts whatever, and was, moreover, ignorant of the law, with whose requirements therefore he did not comply.  Nor did he become aware of his mistake until, when off the easternmost point of the island, the master of the vessel approached him and said:  ’We are now, sir, off Point-Morant; you will therefore have the goodness to favour me with your security-bond.  It is a mere legal form, but we are obliged to respect it.’  Finding this ‘legal form’ had not been complied with, the master then, in spite of Jackson’s protestations and entreaties, set him on shore, and the vessel continued on her voyage.