That much misapprehension and some prejudice prevail on this subject, my recent observation and experience have convinced me; and when I reflect on the pernicious effects, which, in Newcastle and its neighbourhood, are at this time taking place in consequence of them, and which they must continue to produce while they are permitted to exist, I feel that a collective detail of the evidence calculated to remove them is much needed, and that, being sensible of this, it becomes a duty incumbent upon myself to endeavour to supply so important a desideratum. In making this attempt, I have been desirous of avoiding any unnecessary delay, and have therefore, perhaps, been obliged to collect facts, and to deduce arguments from them, with a degree of haste, which, while it must have occasioned many imperfections in the execution of my design, will, I trust, be admitted as some apology for such defects: I am willing, however, to hope that they will not be found of sufficient magnitude materially to interfere with the useful tendency of the estimate. The works of those writers whom I have consulted, and whose authority I have quoted in support of the efficacy of Vaccination, are familiar to the Medical Profession, and, with scarcely an exception I believe, its members have drawn the same satisfactory conclusion from the facts which are detailed in them. But as these works, are, for the most part, strictly professional, they have not come before the public in general, who have not, in consequence, had equal opportunities of convincing themselves of the true value of Vaccination. It is, however, manifestly more important, in proportion as their relative number is greater, that the latter should be convinced of this, than the former only. The present estimate, therefore, is more particularly intended to satisfy the doubts, and to remove the apprehensions of the community at large; though I trust, should I in any degree have succeeded in the attempt, it may also be read by my professional brethren not without some portion of satisfaction and of approbation.