It hence becomes of the utmost importance, that those should themselves be convinced of the true value of Vaccination, who may possess the power, by whatever means (never omitting, however, when possible, to do it through the medium of _reason_) of extending the operative influence of their conviction to others. From the wealthy, the intelligent, and the educated parts of society then, I venture to hope that the following Estimate will meet with some serious consideration, and that, in whatever degree it may directly contribute to remove doubt, it may, at least, excite such a spirit of candid and deliberate enquiry into the subject of Vaccination, as may ultimately render its great value universally acknowledged, and its practice in every instance adopted. _Westgate Street, Newcastle, November 20th, 1824._ ------------------------------------------------------------------------ AN ESTIMATE, _&c. &c._ INTRODUCTION. What reliance may safely be placed on Vaccination as a means of exemption from Small Pox? It may be affirmed, without hazard of contradiction, that no question connected with the physical well being of mankind involves considerations of more serious interest, or consequences of more vital moment, than that which has just been proposed. During the last quarter of a century, it has engaged a degree of attention, both from medical practitioners and from society in general, proportionate to the great importance of the subject; and though among the former, with very few exceptions, one pretty uniform opinion may prevail respecting the true value of the Jennerian discovery, I have reason to know, that many doubts and apprehensions still linger in the minds of not a few of the latter. These have been cherished or revived by circumstances, of a nature, it must be admitted, well calculated to shake their confidence in Vaccination as an absolute preventive of Small Pox; which, unfortunately for the cause, on its first introduction into practice, it was generally believed and asserted to be; but which subsequent experience has proved can no longer be contended for. I trust, however, to be enabled to prove, to the satisfaction of every impartial enquirer, that the value of Vaccination is not essentially diminished on that account; that an imperfect knowledge of its effects can alone have given rise to the doubts and apprehensions referred to; and that nothing is required for their entire removal but a more intimate acquaintance with the subject. It is the purpose of the present Essay, then, to bring under review the principal facts connected with the history of Small Pox and of Vaccination which, in any way, bear upon the question proposed; to arrange them in such order, and to place them in such lights, as may best elucidate the subject, and enable those,