298 



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 

 w^eks 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. 



Hydraulic Investigations, subservient to an intended Croonian Lecture 

 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 mi- 

 nutely and comprehensively the motion of fluids in pipes as affected 

 by friction ; the resistance occasioned by flexure, the laws of propa- 

 gation of impulses through fluids contained in elastic tubes, the mag- 

 nitudes 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 be- 

 stowed on Mr. Dubuat, by Professor Robison, and other late authors 

 on hydraulics, for his skill in adapting a formula to express the re- 

 sults 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 ac- 

 cords better with those of Gerstner, and extends also with equal ac- 

 curacy 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 appa- 

 rent 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, 



