Rationis_, which he determined to translate in Latin, but only
finished about a tenth part. Halley, extremely interested by the
subject, but with an entire ignorance of the Arabic language, resolved
to complete the imperfect version! Assisted only by the manuscript which
Bernard had left, it served him as a key for investigating the sense of
the original; he first made _a list of those words_ wherever they
occurred, with the _train of reasoning_ in which they were involved, to
decipher, by these very slow degrees, the import of the context; till at
last Halley succeeded in mastering the whole work, and in bringing the
translation, without the aid of any one, to the form in which he gave
it to the public; so that we have here a difficult work translated from
the Arabic, by one who was in no manner conversant with the language,
merely by the exertion of his sagacity!

I give the memorable account, as Boyle has delivered it, of the
circumstances which led Harvey to the discovery of the circulation of
the blood.

"I remember that when I asked our famous Harvey, in the only discourse I
had with him, which was but a little while before he died, what were the
things which induced him to think of a circulation of the blood, he
answered me, that when he took notice that the valves in the veins of so
many parts of the body were so placed that they gave free passage to the
blood towards the heart, but opposed the passage of the venal blood the
contrary way, he was invited to think that so provident a cause as
nature had not placed so many valves without design; and no design
seemed more probable than that, since the blood could not well, because
of the interposing valves, be sent by the veins to the limbs, it should
be sent through the arteries and return through the veins, whose valves
did not oppose its course that way."

The reason here ascribed to Harvey seems now so very natural and
obvious, that some have been disposed to question his claim to the high
rank commonly assigned to him among the improvers of science! Dr.
William Hunter has said that after the discovery of the valves in the
veins, which Harvey learned while in Italy from his master, Fabricius ab
Aquapendente, the remaining step might easily have been made by any
person of common abilities. "This discovery," he observes, "set Harvey
to work upon the _use_ of the heart and vascular system in animals; and
in the _course of some years_, he was so happy as to disco