alive, from having one of his legs shorter than the other. On steering the ship towards the object, it was found to be a piece of floating wreck. A story referable to the same principle is related by Dr. Ferriar-A gentleman travelling in the Highlands of Scotland was conducted to a bedroom which was reported to be haunted by the spirit of a man who had there committed suicide. In the night he awoke under the influence of a frightful dream, and found himself sitting up in bed with a pistol grasped in his right-hand. On looking round the room he now discovered, by the moonlight, a corpse dressed in a shroud reared against the wall, close by the window; the features of the body, and every part of the funeral apparel being perceived distinctly. On recovering from the first impulse of terror, so far as to investigate the source of the phantom, it was found to be produced by the moonbeams forming a long bright image through the broken window. Two esteemed friends of mine, while travelling in the Highlands, had occasion to sleep in separate beds in one apartment. One of them, having awoke in the night, saw by the moonlight a skeleton hanging from the head of his friend's bed,-every part of it being perceived in the most distinct manner. He instantly got up to investigate the source of the illusion, and found it to be produced by the moonbeams falling upon the drapery of the bed, which had been thrown back, in some unusual manner, on account of the heat of the weather. He returned to bed and soon fell asleep. But having awoke again some time after, the skeleton was still so distinctly before him, that he could not sleep without again getting up to trace the origin of the phantom. Determined not to be disturbed a third time, he now brought down the curtain into its usual state, and the skeleton appeared no more.

PART IV.

APPLICATION OF THE RULES OF PHILOSOPHICAL INVESTIGATION TO MEDICAL SCIENCE.

THERE has been much difference of opinion among philosophers in regard to the place which medicine is entitled to hold among the physical sciences; for while one has maintained that it "rests upon an eternal basis, and has within it the power of rising to perfection," it has been distinctly asserted by another, that "almost the only resource of medicine is the art of conjecturing." "The following apologue," says D'Alembert, "made by a physician, a man of wit and of philosophy, represents very well the state of that science. Nature," says he, "is fighting with disease; a blind man armed with a club, that is, the physician, comes to settle the difference. He first tries to make peace; when he cannot accomplish this, he lifts his club and strikes at random; if he strikes the disease, he kills the disease; if he strikes nature, he kills nature." "An eminent physician," says the same writer," renouncing a practice which he had exercised for thirty years, said, 'I am wearied of guessing.'"

The uncertainty of medicine, which is thus a theme both for the philosopher and the humourist, is deeply felt by the practical physician in the daily exercise of his art. It becomes, therefore, an inquiry of the utmost importance,-what the sources of this uncertainty are,--where that point is in our researches at which its influence begins,-and, when

we arrive at this point, what the means are by which it may be diminished.

The perfect uniformity of all the phemonena of nature we have seen to be the foundation of the certainty of results in physical science. For when the order and relations of these phenomena have once been ascertained, we calculate with confidence that they will continue to observe the same order. Proceeding upon this confidence, in relations which have been observed regarding the heavenly bodies, the astronomer foretels their positions even at very distant periods. In the same manner, the chymist, having ascertained the actions which take place when certain substances are brought into contact, and the new combinations which follow, decides with confidence that, in every instance in which these agents are brought together, the same actions will take place, and will be followed by the same combinations. This confidence, which lies at the foundation of all science, we have seen to be an original or instinctive principle, and not the result of experience; but it is the province of experience to ascertain the particular sequences to which it may be applied; in other words, to distinguish casual relations and sequences from those which we are entitled to consider as uniform.

The uncertainty of medicine resolves itself chiefly into an apparent want of that uniformity of phenomena, which is so remarkable in other branches of physical science. There are, in particular, two departments of our inquiries, in which we feel continually the effect of this want of uniformity,-the characters and the progress of disease, and the action of external agents upon the body.

Since medicine was first cultivated as a science, a leading object of attention has ever been to ascertain the characters or symptoms by which particular internal diseases are indicated, and by which

they are distinguished from other diseases which resemble them. But, with the accumulated experience of ages bearing upon this important subject, our extended observation has only served to convince us how deficient we are in this department, and how often, even in the first step in our progress, we are left to conjecture. A writer of high eminence has even hazarded the assertion that those persons are most confident in regard to the characters of disease whose knowledge is most limited, and that more extended observation generally leads to doubt. After showing the uncertainty of the symptoms which are usually supposed to indicate effusion in the thorax, Morgagni adds the remarkable assertion to which I here allude: "qui enim plura corpora inspexerunt, hi saltem, cum illi nil dubitant, epsi dubitare didicerunt." If such uncertainty hangs over our knowledge of the characters of disease, it will not be denied that at least an equal degree of uncertainty attends its progress. We have learned, for example, the various modes by which internal inflammation terminates, as resolution, suppuration, gangrene, adhesion, and effusion; but, in regard to a particular case of inflammation which is before us, how little notion can we form of what will be its progress, or how it will terminate.

An equal or even a more remarkable uncertainty attends all our researches on the second head to which I have referred, namely, the action of external agents upon the body. These engage our attention in two respects,-as causes of disease, and as remedies; and in both these views the action of them is fraught with the highest degree of uncertainty. In regard to the action of external agents as causes of disease, we may take a single example in the effects of cold. Of six individuals who have been exposed to cold in the same degree, and, so far as we can judge, under the same circumstances, one may be seized with inflammation of the lungs,

one with diarrhea, and one with rheumatism, while three may escape without any injury. No less remarkable is the uncertainty in regard to the action of remedies. One case appears to yield with readiness to the remedies that are employed; on another, which we have every reason to believe to be of the same nature, no effect is produced in arresting its fatal progress; while a third, which threatened to be equally formidable, appears to cease without the operation of any remedy at all.

With these complicated sources of uncertainty, which meet us at every step in our medical inquiries, it is almost unnecessary to contrast the perfect uniformity of phenomena, on a confidence in which we proceed in other departments of science. When we mix together pieces of zinc, sulphuric acid, and water, we pronounce with perfect confidence that the water will be decomposed, hydrogen evolved, the metal oxidated, the oxyd dissolved, and sulphate of zinc produced; we pronounce with equal confidence on all the properties, mechanical and chymical, of the new compound which is thus to be formed; and in no case have we the smallest doubt of the exact occurrence of every step in this complicated process. With what different feelings we contemplate, in its commencement, a case of dangerous internal disease,-its probable progress and termination, and the effect which our remedies are likely to produce in arresting it,—those best can tell who have most experienced them.

The certainty of a science, as was formerly stated, depends upon two circumstances; namely, the facility with which we ascertain the true relations and tendencies of things, or trace effects to their true causes, and causes to their true effects; and the confidence with which we rely on the actions, dependent on these relations, continuing to occur in all cases with perfect uniformity. This confi