As the attack of Small Pox in subjects who have undergone Vaccination, generally occurs after a long interval, it becomes a question whether this is owing merely to the chance of such subjects not having been exposed to variolous contagion, or to the effect of time in diminishing the antivariolous virtue of vaccination. The former is certainly conceivable; but when we consider the numberless severe proofs to which the recently vaccinated were experimentally exposed in the early part of this practice all over Europe, from which the most satisfactory evidence resulted; and when it is considered that, in the great majority of cases, Small Pox has not occurred till several years after vaccination, it seems by far most probable that the virtue of it is weakened by time. When parents, therefore, become anxious and apprehensive regarding the risk of Small Pox after a lapse of years, it seems quite reasonable that they should be indulged in having the operation repeated. Let all this be applied to the case of a community, in which the total eradication of Small Pox is quite hopeless. Let it be admitted that such occurrences as have been described do frequently occur: let it even be admitted, for argument’s sake, that every vaccinated case whatever must of necessity and unavoidably at some time or other in future life be affected with this mitigated species of Small Pox, would it not even under this great abatement be one of the greatest boons that could be conferred on humanity, being an instrument or remedy which would disarm Small Pox of its danger? Would not the next greatest benefit to the total extirpation of Small Pox, be the stripping it of its terrors by rendering it safe and harmless? It may be further remarked, that the benefit derivable from the different proportions of the persons vaccinated to the total population, advances in a considerably higher progression than the simple arithmetical. It is evident that the smaller the relative number of the vaccinated, the greater their chance of meeting with Small Pox infection, and that though the disease which they may catch is of a mitigated nature, it would nevertheless be desirable to avoid it on its own account, but still more on account of the prejudice it creates. This, in the eye of general benevolence, constitutes an additional, though secondary motive for extending the vaccine inoculation as widely as possible, even though the attainment of the _maximum_, that is, total extirpation, should be impracticable and hopeless.