VOL. XXX.] PHILOSOPHICAL TRANSACTIONS. 345 



But the greatest impetus on an equal superficies, is the motion of a weight of 

 ^1 part of a grain, that runs over 1 inch; or the motion of a weight of l-f 

 grain, that runs over -j-i-5- part of an inch each second of time. 



Cor. 3. — The impetus of the air, impressed upon the lungs in a mean expi- 

 ration, is equal to the motion of a column of water, which runs over the space 

 of 1 inch in a second of time, whose base is the internal superficies of the 

 lungs, and height the ^^\ ^^ part of an inch. And the height of the column 

 is ^^ Q part of an inch in the strongest expiration. 



Cor, 4. The impetus incumbent on a superficies equal to the greatest circle 

 of a globule of blood, in a gentle expiration, is -±^ part of the weight of that 

 globule; but in a strong expiration -^ part of the same weight, which runs over 

 the space of an inch in a second of time. 



I shall here show the manner of measuring the diameters of the globules of 

 blood, since it may be of use to determine the magnitudes of other minute 

 objects. I wound a fine and pretty long hair several times round a small needle, 

 in such a manner, that all the circumvolutions touched each other exactly, as 

 was discovered by the microscope. Then taking with a pair of compasses the 

 distance between the extreme circumvolutions on each hand, I applied it to a 

 diagonal scale, and divided the space thus found on the scale by the number of 

 convolutions, whence is found the breadth of one convolution, or the diameter 

 of the hair. Afterwards cutting the air into very small shreds or pieces, I 

 strewed them on the plane of a microscope, on which I spread a little blood in 

 such a manner, that the globules could be distinctly seen. On viewing these 

 shreds with the microscope, I found them in some places disposed so conve- 

 niently, that I could reckon how many globules answered to the diameter of a 

 shred. But the shreds were of unequal diameters, because the hair was more 

 slender towards the extremity than nearer the root; so that sometimes 7 or 8, 

 at other times 12 or 13 globules answered to a transverse section of the hair; 

 and having repeated both experiments several times, I at length estimated the 

 mean diameter of the hair to be the 324th part of an inch, and the diameter of 

 a globule of blood the 10th part of the diameter of the hair, or the 3240th 

 part of an inch. 



Cor. 5. — The impetus, which the internal superficies of the lungs sustains 

 by the expiration of the air, is less than the motion of the gentlest dew 

 that falls. 



Scholium. — In the solution of the two preceding problems, I have not con- 

 sidered the obstacle to the egress of the air out of the lungs from the friction 

 of the sides of the trachea and its branches, since that is very inconsiderable; 

 nor does it seem, that it can be accurately enough estimated by any experiment. 



VOL. VI. Y y 



