Harvey was as indefatigable a labourer as any we have named. He spent not less than eight long years of investigation and research before he published his views of the circulation of the blood. He repeated and verified his experiments again and again, probably anticipating the opposition he would have to encounter from the profession on making known his discovery. The tract in which he at length announced his views, was a most modest one,—but simple, perspicuous, and conclusive. It was nevertheless received with ridicule, as the utterance of a crack-brained impostor. For some time, he did not make a single convert, and gained nothing but contumely and abuse. He had called in question the revered authority of the ancients; and it was even averred that his views were calculated to subvert the authority of the Scriptures and undermine the very foundations of morality and religion. His little practice fell away, and he was left almost without a friend. This lasted for some years, until the great truth, held fast by Harvey amidst all his adversity, and which had dropped into many thoughtful minds, gradually ripened by further observation, and after a period of about twenty-five years, it became generally recognised as an established scientific truth. The difficulties encountered by Dr. Jenner in promulgating and establishing his discovery of vaccination as a preventive of small-pox, were even greater than those of Harvey. Many, before him, had witnessed the cow-pox, and had heard of the report current among the milkmaids in Gloucestershire, that whoever had taken that disease was secure against small-pox. It was a trifling, vulgar rumour, supposed to have no significance whatever; and no one had thought it worthy of investigation, until it was accidentally brought under the notice of Jenner. He was a youth, pursuing his studies at Sodbury, when his attention was arrested by the casual observation made by a country girl who came to his master’s shop for advice. The small-pox was mentioned, when the girl said, “I can’t take that disease, for I have had cow-pox.” The observation immediately riveted Jenner’s attention, and he forthwith set about inquiring and making observations on the subject. His professional friends, to whom he mentioned his views as to the prophylactic virtues of cow-pox, laughed at him, and even threatened to expel him from their society, if he persisted in harassing them with the subject. In London he was so fortunate as to study under John Hunter, to whom he communicated his views. The advice of the great anatomist was thoroughly characteristic: “Don’t think, but _try_; be patient, be accurate.” Jenner’s courage was supported by the advice, which conveyed to him the true art of philosophical investigation. He went back to the country to practise his profession and make observations and experiments, which he continued to pursue for a period of twenty years. His faith in his