Dr. Young's Hydraulic Investigations. 185 
A similar mode of reasoning may be applied to other cases 
of the propagation of impulses, in particular to that of a con- 
traction moving along an elastic pipe. In this case, an in- 
crease of the diameter does not increase the velocity of the 
transmission of an impulse ; and when the velocity of the 
contraction approaches to the natural velocity of an impulse, 
the quantity of fluid protruded must, if possible, be still 
smaller than in an open canal ; that is, it must be absolutely 
inconsiderable, unless the contraction be very great in com- 
parison with the diameter of the pipe, even if its extent be 
such as to occasion a friction which may materially impede 
the retrograde motion of the fluid. The application of this 
theory to the motion of the blood in the arteries is very ob- 
vious, and I shall enlarge more on the subject when I have 
the honour of laying before the Society the Croonian Lecture 
for the present year. 
The resistance, opposed to the motion of a floating body, 
might in some cases be calculated in a similar manner : but 
the principal part of this resistance appears to be usually de- 
rived from a cause which is here neglected ; that is, the force 
required to produce the ascending, descending, or lateral mo- 
tions of the particles, which are turned aside to make way for 
the moving body ; while in this calculation their direct and 
retrograde motions only are considered. 
The same mode of considering the motion of a vertical 
lamina may also be employed for determining the velocity 
of a wave of finite magnitude. Let the depth of the 
fluid be a, and suppose the section of the wave to be an 
isosceles triangle, of which the height is b , and half the 
breadth c : then the force urging any thin vertical lamina 
MD CCCVIII. B b 
