§ 1. For all practical purposes, the fevers termed intermittent and remittent may be held to have their origin in one cause. Thus, whether on the marshy coasts of Essex and Kent, or the more dreadful banks of the Gambia and Niger, it is not improbable that the fever so destructive to European life is of one character--mild in Essex; fatal in Sierra Leone. But the fact is not to be overlooked, that when fever assumes an intermittent character, however it may conduce to the inefficiency of the population, it does not greatly swell the bills of mortality; on the other hand, the remittent form of fever constitutes that grand and hitherto insurmountable obstacle which Nature seems to have placed to the extension of the white man over the earth, excluding him, seemingly for ever, from the tropical regions of the world. A favourite theory with medical men was, that the evil influence which causes fever, whether in Essex or on the Gambia, by the Scheld or the Niger, was a certain miasma produced by marshes more or less remote from human abodes; sometimes it was maintained that to produce the miasma these marshes must be in a great measure dried up, or in the process of being so; at other times an opposite opinion was held. These hypotheses were refuted, or at least much shaken, by Major Tulloch, in his invaluable “Statistical Report on the Sickness, Mortality, and Invaliding among Troops on the Western Coast of Africa” (p. 26). “So long as the fever continued to make its appearance during the rainy season, excessive moisture was deemed one of the principal causes, but that theory has been abandoned since it has, on three or four occasions, appeared and raged with equal violence in the middle of the dry season. If we attempt to connect it with temperature, the range of the thermometer offers equally contradictory results, the disease having originated and prevailed nearly as often when that was at the minimum as when at the maximum. Variations in atmospheric pressure afford no clue whatever to the solution of the difficulty, for here, as in all tropical climates, the fluctuations of the barometer are exceedingly slight. No definite connexion has ever been traced between the prevalence of any particular wind and the outbreak of the disease; the breeze blows over the same district in the healthy as in the unhealthy season. Besides, it seems entirely to negative the supposition that any of these can be more, perhaps, than mere accessories, when we find, from 1830 to 1836, the colony of Sierra Leone remarkably free from fever, without any perceptible change in these respects. It does not appear that the composition of the atmosphere during the prevalence of yellow fever in this command has ever been examined, to ascertain if it differed from what has usually been observed at periods comparatively healthy; but this test has been applied without any satisfactory result in other countries. Unless some