nds to induce quiescence; as is seen in
hysteric cases, and chlorosis, and the other diseases of sedentary people.

4. The velocity of the particles of the blood in certain circumstances is
increased by venesection, which, by removing a part of it, diminishes the
resistance to the motion of the other part, and hence the momentum of the
particles of it is increased. This may be easily understood by considering
it in the extreme, since, if the resistance was greatly increased, so as to
overcome the propelling power, there could be no velocity, and in
consequence no momentum at all. From this circumstance arises that curious
phaenomenon, the truth of which I have been more than once witness to, that
venesection will often instantaneously relieve those nervous pains, which
attend the cold periods of hysteric, asthmatic, or epileptic diseases; and
that even where large doses of opium have been in vain exhibited. In these
cases the pulse becomes stronger after the bleeding, and the extremities
regain their natural warmth; and an opiate then given acts with much more
certain effect.

VI. There is another cause, which seems occasionally to induce quiescence
into some part of our system, I mean the influence of the sun and moon; the
attraction of these luminaries, by decreasing the gravity of the particles
of the blood, cannot affect their momentum, as their vis inertiae remains
the same; but it may nevertheless produce some chemical change in them,
because whatever affects the general attractions of the particles of matter
may be supposed from analogy to affect their specific attractions or
affinities: and thus the stimulus of the particles of blood may be
diminished, though not their momentum. As the tides of the sea obey the
southing and northing of the moon (allowing for the time necessary for
their motion, and the obstructions of the shores), it is probable, that
there are also atmospheric tides on both sides of the earth, which to the
inhabitants of another planet might so deflect the light as to resemble the
ring of Saturn. Now as these tides of water, or of air, are raised by the
diminution of their gravity, it follows, that their pressure on the surface
of the earth is no greater than the pressure of the other parts of the
ocean, or of the atmosphere, where no such tides exist; and therefore that
they cannot affect the mercury in the barometer. In the same manner, the
gravity of all other terrestrial bodies is diminished