Thus frustrated, Jenner’s ingenuity took another turn. It was the belief of farriers that if infected in dressing horses’ greasy heels, they too were secure from smallpox. The area of this conviction was narrower than that of the dairy-maids, farriers being neither so numerous, nor so observant of their beauty: but Jenner entertained their faith and converted it to his purpose. Horsegrease protected from smallpox, if cowpox did not. But might there not be one sort of cowpox that answered to the dairy-maids’ faith, if another sort did not? Happy thought! The defensive sort was derived from the horsegrease which protected the farriers: the non-defensive originated spontaneously on the cows. Men, fresh from handling horses’ greasy heels, milked cows and communicated to them the horses’ disease. Milkmaids, who in turn contracted from the cows that sort of pox, were like the farriers secure from smallpox, yea securer; whilst milkmaids who contracted pox spontaneously developed on cows were not secure. The milkmaids’ superstition was therefore justifiable: they were right and they were wrong--right when they got pox through the cow from the horse; wrong when they got pox from the cow simply. Why then, it may be asked, did not Jenner dismiss the cow from consideration? Why did he not base his prescription on the farriers’ experience, and use and recommend horsegrease exclusively for inoculation? The question is an obvious one, but it is not easy to make out Jenner’s answer with precision. His assertion was, that-- The virus from the horse is not to be relied upon as rendering the system secure from variolous infection, but the matter produced by it on the nipple of the cow is perfectly so-- which was to say that horsegrease attained its highest prophylaxy after transmission through the cow. Such was the doctrine of the _Inquiry_, “that master-piece of medical induction.” When the doctrine came to be reduced to practice, difficulties arose. Cowpox was considered wholesome and credible, whilst horsegrease was repulsive and incredible. Still fact was fact; and many were ready to accept horsegrease through the cow, or without the cow, if such indeed were the source of the new salvation. Cowpox proved to be a rare commodity, whilst horsegrease was common, and numerous attempts were made to produce cowpox by means of grease, but ineffectually. At some attempts, Jenner officiated. Marshall records that--