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 50th 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 ardor 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 favor 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. What was to be done? Almost penniless, landed on a part of the coast where he knew not a soul, Jackson well-nigh gave himself up to despair. There was a vessel for New-York loading, it was true, at Lucea; but Lucea was 150 miles distant, on the westernmost side of the island, and not to be reached by sea, whilst our adventurer's purse would not suffer him to hire a horse. No choice was left him but to walk, and that in a country where the exigencies of the climate make pedestrianism perilous in the extreme to the white man. Having reached Kingston, which was in the neighborhood, in a boat, and obtained the necessary certificate, he started on his