Knowing that improvements, which would do honour to the most elevated human understandings, are sometimes stumbled upon by men of more confined abilities; and that in medicine, as well as in every other circumstance in life, it is our duty to avail ourselves as much as possible of all discoveries tending to the common benefit, I embraced every _just opportunity_ of informing myself of facts, circumstances, and events, that either public fame, or more precise relations, brought to me. I use the term _just opportunity_, because, if I am not misinformed, endeavours have been used, inconsistent with equity and candour, to rob those who are intitled to our gratitude for assisting us in this important process, of that share of private emolument which is their due, let their title to the discovery be ever so paradoxical. To expose patients, even in the inoculated small-pox, to all weathers, was a thing unheard of. To permit them through the whole progress of the disease to go abroad, and follow their usual vocations, and that they should neither suffer any present evil, nor experience any disagreeable consequences, was still more surprizing; yet an infinite number of instances have confirmed all this; and some of these instances will appear in the sequel of this performance. The design of this treatise is to bring the practice still one step nearer to perfection, and lessen the ravages of a distemper, which is not a native of Britain, but, like the plague, has been imported from a foreign country, and demands the exertion of all the powers we are possessed of, either to exterminate it from amongst us, which perhaps is not practicable, or to render it less unsafe, if not wholly without difficulty or danger. The following directions for this purpose, are the result of an extensive practice: and if a strong persuasion of the truth of what he writes, founded on repeated trials and impartial observations, should have led the author to express himself in a very sanguine manner, the future experience of others, he trusts, will be his justification. Hertford, 1. Nov. 1766. Of the AGE, CONSTITUTION, and SEASON of the Year proper for INOCULATION. Before I proceed to describe the regimen and preparatives, it may not be improper to mention what has occurred to me in respect to the most suitable age and constitution for inoculation; and likewise what seasons seem to be more or less favourable for the practice.