*5 
of the Heart and Arteries. 
twentieth order, the dilatation does not exceed yi-- of the 
diameter, which is itself the 140th part only of an inch : so 
that it is not surprising, that Haller should have been unable 
to discover any dilatation in vessels of these dimensions, even 
with the assistance of a powerful microscope. If we estimated 
the magnitude of the pulse in the aorta, from the excess of the 
temporary above the mean velocity, which would perhaps be 
justifiable, that magnitude would become still less considerable. 
These calculations agree extremely well with each other, 
and with experiment, as far as they relate to the power of the 
heart, and the affections of the smaller arteries. But there is 
reason to think that the velocity of the pulse in the larger 
vessels is much more considerable than has been here stated ; 
and their dilatation is also less conspicuous, when they are 
exposed to view, than it would probably be, if it were as great 
as is inferred from the velocity here assigned. I have de- 
monstrated, in the hydraulic investigations which I lately laid 
before the Royal Society, that the velocity of an impulse 
passing through a tube, consisting of perfectly elastic materials, 
is half as great as that of a body supposed to have fallen from 
the given point to the base of the modular column of the 
tube : and that the height of this column is such that the tube 
would be extended without limit by its pressure ; consequently 
it must be greater than the height of a column equivalent to 
the pressure by which the tube is burst. Now it has been 
ascertained by Dr. Hales, that the pressure, required for 
bursting one of the carotids of a dog, is equal to that of a 
column of water one hundred and ninety feet high ; nor does 
he remark that the artery was very materially dilated ; and 
deducting from this height the five feet which express the 
