and the same inference is drawn respecting the nature of the tail; for
if it were sufficiently dense to be visible by reflected light alone, at
the distance of 235 millions of miles, its opacity would entirely pre-
vent our seeing stars through it.
The length of the tail is computed to have been on the 18th of October upwards of nine millions of miles.
The resemblance of this comet to a nebula during the last ten weeks of its appearance excites a suspicion in Dr. Herschel's mind, that he may possibly have classed as nebulae other cometary bodies; but it would be a task of too many years' labour to revise his cata- logue of nebulae for the chance of discovering any deficiency of those formerly observed.
In the second part of this paper, Dr. Herschel informs us, that he has remarked a new irregularity in the apparent form of Saturn; for that in the month of June last, there was a visible protuberance of its south pole, which could not have been overlooked at the time of his former observations. This he ascribes to the refraction of light in its passage through the atmosphere of the ring, which was inter- posed between us and the southern hemisphere, but, passing behind the northern hemisphere, did not occasion a similar protuberance.
on the Motion of the Blood. By Thomas Young, M.D. For. Sec.
R.S. Read May 5, 1808. (Phil. Trans. 1808, p. 164.]In the present inquiry, Dr. Young undertakes to investigate minutely and comprehensively the motion of fluids in pipes as affected by friction; the resistance occasioned by flexure, the laws of propagation of impulses through fluids contained in elastic tubes, the magnitudes of pulsations in conical vessels, and the effect of progressive contraction along a canal ;—the physiological application of the results being reserved for a future opportunity.
In the first section the friction and discharge of fluids through pipes are considered; and the author assents to the encomiums bestowed on Mr. Dubuat, by Professor Robison, and other late authors on hydraulics, for his skill in adapting a formula to express the results of numerous experiments on this subject. But since the form of his expressions is not so convenient for practice as might be wished, and fails altogether in its application to extreme cases, Dr. Young has by approximation arrived at a formula, which appears to agree fully as well as Dubuat's with Dubuat's own experiments, which accords better with those of Gerstner, and extends also with equal accuracy to all extreme cases in which the former was erroneous.
In considering the velocities of water flowing through pipes, the friction appeared to consist of two parts, one of which is most apparent in small tubes, and varies as the velocity simply, and the other as the square of the velocity.
In order to show the agreement of Dr. Young’s formulae with the results of experiments, a table is formed containing forty experiments,
