plainly perceive something which prevents its action. It is not fair reasoning to say that the action of the cause is prevented by something unknown, for we might as well say that this unknown something is the cause originally, and acts only upon certain occasions, of when it thinks proper. Now, if the filth of the canal of Cairo be the cause of the plague in that city, it ought to recur annually at the season when that filth exists in greatest quantity, and in the most putrid slate. Nevertheless we have the express testimony of Mr. Eyles Irwin, that at the time he was in Cairo there had not been any plague for seven years. The account he gives of it is a kind of contrast to that above quoted from Dr. Mead. “_Misir al Kaira_, says he, or the _City of Anguish_, so called from the frequent visits which it has received from the plague, but commonly called _Grand Cairo_ by us, is situated in lat. 30 degrees 3 minutes N. on an artificial branch of the Nile. Old Cairo nearly faces the river; but the new city is removed above a mile from it, and approaches to the range of mountains which runs through Upper Egypt, and abruptly breaks off here. It is undoubtedly one of the finest cities in the east; which, from the present style of architecture that reigns among the orientals, is but a faint commendation. The houses are in general built of stone, and, being elevated to several stories, would make a grand appearance, notwithstanding the inelegance of their structure, were not the effect destroyed by the excessive narrowness of the streets. This is one of the causes to which the ingenious Dr. Mead ascribes the birth of the plague in this capital; but experience evinces that it arises from _foreign and adventitious_ causes. _There has not been a plague here these seven years_; which is rendered more remarkable by the commencement of the Russian war at the date of its cessation. No one can account for this; though a year seldom passed by before without a visit from it.”[80] [80] Irwin’s Voyage up the Red Sea p. 335. From this it plainly appears, that, however these putrid effluvia may concur with other circumstances in producing the plague, they are by no means the _only_ cause; otherwise not a single year could have passed in Cairo without a pestilence; and the very same thing we shall find to hold good in every other, let us choose for a cause what we will. In order to investigate this matter fully, we must now consider what causes have been assigned by physicians for other epidemical diseases; and here, to avoid prolixity, we shall chiefly confine ourselves to those enumerated by Dr. Fordyce as the causes of fever; a gentleman whose very extensive experience must give the greatest weight to his testimony.[81] The principal causes assigned by him are,