I. The _blood-vessels_ were affected more or less in every case of this fever. I have elsewhere said, that a fever is occasioned by a convulsion in the arterial system[16]. When the epidemic, which we are now considering, came on with a full, tense, and quick pulse, this convulsion was very perceptible; but it frequently came on with a weak pulse, often without any preternatural frequency or quickness, and sometimes so low as not to be perceived without pressing the artery at the wrists. In many cases the pulse intermitted after the fourth, in some after the fifth, and in others after the fourteenth stroke. These intermissions occurred in several persons who were infected, but who were not confined by the fever. They likewise continued in several of my patients for many days after their recovery. This was the case in particular in Mrs. Clymer, Mrs. Palmer's son William, and in a son of Mr. William Compton. In some, there was a preternatural slowness of the pulse. It beat 44 strokes in a minute in Mr. B. W. Morris, 48 in Mr. Thomas Wharton, jun. and 64 in Mr. William Sansom, at a time when they were in the most imminent danger. Dr. Physick informed me, that in one of his patients the pulse was reduced in frequency to 30 strokes in a minute. All these different states of the pulse have been taken notice of by authors who have described pestilential fevers[17]. They have been improperly ascribed to the absence of fever: I would rather suppose that they are occasioned by the stimulus of the remote cause acting upon the arteries with too much force to admit of their being excited into quick and convulsive motions. The remedy which removed it (to be mentioned hereafter) will render this explanation of its cause still more probable. Milton describes a darkness from an excess of light. In like manner we observe, in this small, intermitting, and slow pulse, a deficiency of strength from an excess of force applied to it. In nearly every case of it which came under my notice, it was likewise tense or chorded. This species of pulse occurred chiefly in the month of August, and in the first ten days in September. I had met with it formerly in a sporadic case of yellow fever. It was new to all my pupils. One of them, Mr. Washington, gave it the name of the "undescribable pulse." It aided in determining the character of this fever before the common bilious remittent disappeared in the city. For a while, I ascribed this peculiarity in the pulse, more especially its _slowness_, to an affection of the brain only, and suspected that it was produced by what I have taken the liberty elsewhere to call the _phrenicula_, or inflammatory state of the internal dropsy of the brain, and which I have remarked to be an occasional symptom and consequence of remitting fever. I was the more disposed to adopt this opinion, from perceiving this slow, chorded, and intermitting pulse more frequently in children than in adults. Impressed with this idea, I requested Mr.