VOL. XXX.] PHILOSOPHICAL TRANSACTIONS. 347 



nearly, the blood, being free of all resistance, will be carried through the 

 cava. 



Prob. 5. — ^To find the motion of the blood in the pulmonary vein near the 

 left auricle of the heart; or the motion of the whole mass of blood flowing 

 through the lungs. 



Besides the symbols made use of in prob. 3, let x be = the mean length of 

 the arteria venosa, whence, by theor. 3, case 4, the motion sought is found = 

 — ; that is, the motion of the blood, flowing through the lungs, is equal to the 

 motion of a mass of blood, thrown into the pulmonary artery in one systole, 

 with such a velocity, as to run over the length of the pulmonary veins and 

 arteries in the time between two pulsations, q. e. i. 



If in the human body x be supposed = 1-f foot, the motion of the blood in 

 the lungs will be equal to the motion of a weight of 3 lb. that runs over the 

 space of an inch in a second of time. 



Prob. 6. — ^To determine the absolute momentum of the blood in the pulmo- 

 nary vein. 



By the same argument, as that made use of in prob. 4, the motion sought 



is found = 2.5 X j. a. e. i. 



And by supposing the same things as above, the absolute motion of the blood 

 that flows through the lungs, is equal to the motion of a weight of 7-i- lb. that 

 in each second of time runs over the space of an inch. 



Scholium. — By Dr. Keill's experiment is determined the ratio, which the 

 natural velocity of the blood, through the aorta and its branches, has to that 

 resistance with which the blood would flow through the same, if the resistance 

 of the arteries and preceding blood was removed. We have transferred that 

 ratio to the blood flowing through the pulmonary artery ; because, by either 

 taking off^ or diminishing, according to any ratio, the resistance the blood 

 meets with in flowing through both arteries, the blood must necessarily be 

 equally accelerated in both ; for unless that happened, both ventricles of the 

 heart either will not be contracted in the same time, or will not discharge the 

 same quantity of blood; neither of which can possibly happen, without very 

 much disturbing and endangering the whole machine. 



Cor. To the three preceding problems. — Hence follow the corollaries, mu- 

 tatis mutandis, subjoined to the 5th prob. 



Scholium. To the four preceding problems. — It is to be observed, that 

 though the velocity of the blood, flowing both through the lungs, and also 

 through the rest of the body, be not really ecjuable, yet here it is supposed such^ 

 in order to find its mean motion. 



Y Y 2 



