CHAPTER XIV. MISCELLANEOUS MEMOIRS. We propose, in the present chapter, to call the attention of our readers to some of the more remarkable Memoirs, or Philosophical Essays, of Dr. Young, which have not elsewhere been noticed; selecting those which are distinguished by the importance of the subjects of which they treat, or of the conclusions to which they lead, or which are otherwise calculated to show the extraordinary capacity which he possessed of solving the most difficult problems in the applications of mathematics to natural philosophy, by processes apparently the most inadequate to the purpose. He never confined himself to the beaten track of a systematic investigation. We find in his writings no symmetrical formulæ or analytical refinements. There is no seeking after generalities, when the particular question which he has in hand does not require them; whilst every expedient is freely resorted to, however irregular and unusual, if it serves the purpose which he has in view. Important and difficult steps are passed over as manifest, terms are neglected as insignificant, analogies take the place of proofs, and we are surprised to find ourselves at the end of an investigation, even within the limits of which would commonly be deemed hardly sufficient to master the difficulties which meet us at the beginning. But his rare space sagacity hardly ever deserts him; and though he has occasionally been led to hasty and premature conclusions, or committed mistakes in numerical calculations, from the brevity and rapidity of his processes, yet nothing can be more surprising than the general soundness of his views of mechanical principles and their applications, and the correctness both of his philosophical and numerical results. We shall consider these Memoirs generally in the chronological order in which they were written. A Memoir on Hydraulics, printed in the Philosophical Transactions for 1808, was introductory to another in the same Collection for the following year, on the Functions of the Heart and Arteries. The connection between these subjects was considered by him, as we have before observed," sufficiently close to give them both a professional character, and thus to exempt them from the restriction which he had imposed upon the class of publications which alone should be allowed to appear under his own name.. The motion of water in rivers and canals, both straight and crooked, and in tubes with different diameters and under various circumstances, had been made a special subject of investigation by a long succession of eminent Italian, and by several French, writers, but by very few of our own. The questions which it involves are extremely interesting from their important practical bearings; but, like all those connected with the motions of fluids, the conclusions to which they lead are extremely unsatisfactory, being expressed generally by empirical formulæ only, constructed chiefly with a view of representing the results of experiment, and with little reference to physical Supra, p. 212. LIFE. 2 E grounds, which were too difficult to investigate, and too little understood to afford a secure basis upon which they could be founded. The important element of friction alone, which enters so largely into all these questions, has never yet been, and probably never will be, brought completely within the control of an accurate analysis. It was the formulæ proposed by Du Buat, one of the most laborious and most successful of the French writers on Hydraulics, which were tested by Dr. Young, when preparing his Lectures for the Royal Institution, by an extensive series of experiments, and which received at his hands several amendments of their form, in order to adjust them more nearly to the results which those experiments gave. He was at that time ignorant of a nearly contemporary work * on this subject by Prony, who had availed himself of a happy application which another French engineer, Gerard, had made of Coulomb's theory of friction to the estimation of the resistance of water in pipes and canals, and who had been enabled, by means of the assistance which it gave him, to construct a formula not only much more simple than those of Du Buat or Young, but also much more conformable to experiment. This formula failed, however, in correctly giving the discharge of water through very narrowand much less through elastic-tubes; and it was therefore not available for the physiological researches for which Young's investigations were chiefly designed to be subservient. As a basis for these ulterior inquiries, Young makes a series of hypotheses with respect to the arterial and "Recherches Physico-Mathématiques sur la Théorie des Eaux Courantes." Paris, 1804. venous systems of the human body, and the quantity of blood which they contain, which, though they assume a regularity of distribution as a foundation for calculation which anatomical examination would not altogether justify, is sufficiently accordant with truth to enable us to reason with tolerable correctness with respect to the general effects which would follow from it. The aorta being supposed to be three-fourths of an inch in diameter, and the arterial circulation to be continued through a succession of twenty-nine classes of bifurcating branches, each pair of which are about three-fourths the diameter of those from which they issue, they will end in extremely minute capillaries, not exceeding one eleven-hundredth of an inch in diameter, but sufficiently wide to transmit freely two or three particles of blood side by side, according to the estimate of their magnitudes which he had made by the aid of his Eriometer. If we further assume the length of the aorta to be nine inches, and that of each successive class of arteries to be five-sixths of. that preceding it,-making that of the last of the capillaries about one-twentieth of an inch only,-the weight of the whole blood required to fill such an arterial system would be about ten pounds, or nearly onefourth part of that in the whole frame. If we should further assume the heart to pulsate seventy-five times in a minute, and at each pulsation to throw out an ounce and a half of blood, the velocity of its transmission in passing from the first to the last of this arterial series would diminish from about eight inches to one-ninetieth of an inch in a second. It was the celebrated Dr. Hales who made the bold and singular attempt to measure the hydrostatic pres sure propelling the blood in the arteries. He found it to be more than nine feet in the arteries of a horse, and five in those of a dog; and it was assumed, upon very probable grounds, to be seven and a half feet in those of an ordinary man, being reduced when it reaches the veins to about six inches, leaving a balance of seven feet in the former to continue the circulation. Dr. Young, assuming these facts as the basis of his reasonings, then proceeds to consider how far the results to which they lead may possibly be modified by the antagonistic effects of the resistance from friction and the muscular contractions of the coats of the arteries; but arrives at the conclusion that neither of them are so considerable as they have commonly been supposed to be, and that they may be neglected, in our physiological reasonings upon the phenomena of the arterial and venous circulation, without the danger of any very material error. If it be the business of the mathematician, aided by the researches of the anatomist, to explain the action of the machinery by which the circulating system of the human frame is carried on, it is that of the physician and physiologist to trace out the various causes which may derange it-to point out, in fact, the influences exercised by exposure to heat and to cold, by fevers and inflammations, by bleeding from arteries or from veins, and by various diseases and their treatment. The deviations, which may thus be produced, from the healthy state of the circulation, are followed out with great minuteness of detail, in connection with the medical agents and other applications which may be best calculated to counteract them. Few persons can be found-and I readily confess that I am not one of their number-with a union of acquirements so remote from |