344 PHILOSOPHICAL TRANSACTIONS. [anNO 17 18. 



Cor. 5. — Given the motion and length, the bulk of the air will be directly as 

 the time. 



Cor. 6. — Given the motion and the time, the bulk of the air will be recipro- 

 cally as the length of the wind-pipe. 



Cor. 7. — The motion of the air is in a ratio compounded of the quadruplicate 

 ratio of any similar diameter of the animal, and the inverse ratio of the time 

 of expiration ; or in a ratio compounded of the ratio of the weight of the whole 

 animal, the subtriplicate ratio of the same weight, and the reciprocal ratio of 

 the time. For the weight of the animal, the cube of any similar diameter, 

 and the bulk of the expelled air, are all in the same ratio. And animal bodies 

 are supposed to be machines alike formed. 



Scholium. — We are to conceive the length here assumed, either as the length 

 of the wind-pipe, supposing all the branches of the trachasa equal in length; or 

 the mean length between the different lengths, if the branches are unequal. 



Prob. 1. — To determine the impetus or impression made on the internal sur- 

 face of the lungs by expiration. 



Since re-action is equal and contrary to action, it must necessarily happen, 

 that with what motion the air to be expired is urged by the internal superficies 

 of the lungs, with just so much is the superficies of the lungs mutually repelled 

 by the air; whence, by the preceding prob. the said impetus in a mean expira- 

 tion is = y and in the strongest expiration = — . a. e. i. 



Hence, by supposing the same things as before, the mean impetus of the air 

 on the lungs, is equal to the motion of about I4- drachm weight, which in a 

 second of time may run over 1 inch; or equal to the motion of a weight of 19 

 pounds, that runs over ttW part of an inch ; which is the velocity of the air 

 on the internal superficies of the lungs: supposing, with Dr. James Keill, that 

 the internal superficies of the lungs is equal to about 219OO square inches. But 

 the greatest impetus of the air on the lungs, is equal to the motion of a weight 

 of about 14^ ounce, that runs over 1 inch in a second of time; or equal to the 

 motion of a weight of 19 pounds, that runs over -1^-5- part of an inch in the 

 same time; which is the velocity of the air on the superficies of the lungs in a 

 strong expiration. 



Cor. 1. — From this proposition arise the corollaries subjoined to the preced- 

 ing prob. 



Cor. 2. — A mean impetus incumbent on a part of the superficies of the lungs, 

 equal to a section of the larynx, is the motion of a weight of xtW P^i't of a 

 grain, that runs over the space of 1 inch in a second of time; or the motion 

 of a weight of IJ- grain that runs over -rrrr P^rt of an inch in the same time. 



