Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 6, CHI-Crystallization

Circuit
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ter of a fecond j and that its motion was inftantaneous
II through a wire of 1319 feet. In 1747, Dr Watfon,
■ nc”^ 1QH' and other Engliih philofophers, after many experiments
of a fimilar kind, conveyed the ele6tric matter through
a circuit of four miles j and they concluded from this
and another trial,, that its velocity is inllantaneous.
CIRCULAR, in a general fenfe, any thing that is
defcribed, or moved in a round, as the circumference
of a circle, or furface of a globe.
CIRCULAR Numbers, called fp/ierica/ones, accord¬
ing to fome, are fuch whofe powers terminate in the
roots themfelves. Thus, for inftance, 5 and 6, all whofe
powers do end in 5 and 6, as the fquare of 5 is 2j j
the fquare of 6 is 36, &c.
CIRCULAR Sailing, is the method of failing by the
arch of a great circle. See Navigation.
CIRCULATION, the aft of moving round, or in
a circle $ thus we fay, the circulation of the blood,
&tc.
CIRCULATION of the Blood, the natural motion of
the blood in a living animal, whereby that fluid is
alternately carried from the heart into all parts of
the body, by the arteries, from whence it is brought
back to the heart again by the veins. See ANATOMY
Index.
In a foetus, the apparatus for the circulation of the
blood is fomewhat different from that in adults. The
feptum, which feparates the two auricles of the heart,
is pierced through with an aperture, called the/w-fl-
men ovale; and the trunk of the pulmonary artery, a
little after it has left the heart, fends out a tube into
the defcending aorta, called the communicating canal.
I he foetus being born, the foramen ovale clofes by de¬
grees, and the canal of communication dries up, and
becomes a Ample ligament.
As to the velocity of the circulating blood, and the
time wherein the circulation is completed, feveral
computations have been made. By Dr KeiPs account,
the blood is driven out of the heart into the aorta with
a velocity which would carry it twenty-five feet in a
minute j but this velocity is continually abated in the
progrefs of the bloody in the numerous feftions or
branches of the arteries ; fo that before it arrives at
the extremities of the body, its motion is greatly di-
miniftied. The fpace of time wherein the whole mafs
of blood ordinarily circulates is varioufly determined.
Some Rate it thus : Suppofing the heart to make two
thoufand pulfes in an hour, and that at every pulfe
there is expelled an ounce of blood ; as the whole mafs
of blood is hot ordinarily computed to exceed twenty-
four pounds, it muft be circulated feven or eight times
over in the fpace of an hour.
The curious, in microfcopic obfervations, have found
an eafy method of feeing the circulation of the blood
in the bodies of animals: for thefe inquiries it is ne-
ceffary to choofe fuch animals as are fmall, and eafily
manageable, and which are either wholly or in part
tranfparent. The obfervations made by this means
are preferable to any others we can have recourfe to,
fince* in diffeftions, the animal is in a ftate of pain,
or dying; whereas in animals fmall enough to be thus
viewed, all is left in its ufual courfe, and we fee what
nature does in her own undifturbed method. In thtfe
creatures alfo, after viewing, as long as we pleafe, the
natural ftate and current of the blood, we may, by
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preffure, and feveral other, ways, impede its courfe ; Circulatioa.
and by putting various mixtures into the creature’s y-
water, induce a morbid ftate, and finally fee the crea¬
ture die, either by means of this or by any other mea-
thod ; and we may thus accurately obferve all the
changes it undergoes, and fee what occafions the trem¬
bling pulfe, &c. of dying people.
The current of the blood in fmall animals, that is,
its pafling on through the veffels, either to or from
the heart, is very eafily feen by the microfcope ; but
its circulation, that is, its running to the extremities
of the parts, and thence returning, is more difficult j
becaufe the veffels where this fhould be feen are fo ex¬
tremely minute, as not eafily to come under obferva-
tion. .The larger arteries are eafily diftinguifhed from
the. veins by the motion of the blood through them,
which in the veins is always fmooth and regular^
but in the arteries, by feveral propulfions after the
manner of pulfation. But this difference is not to be
found in the more minute veffels, in all which, as well
arteries as veins, the motion of the blood is even and
regular.
The tranfparent membrane, or web between the
toes of a frog’s hinder foot, is a very proper objeft
to obferve the circulation of the blood in. The tails,
or fins of fifties are alfo very fine objefts 5 and when
the fifh is very fmall, thefe are manageable, and af¬
ford a view of a great number of veins and arteries,
with a very quick and beautiful fucceflion of blood-
through them. Lhe tail of a flounder may be very
conveniently placed before the double microfcope on
a plate of glafs j and its body being fupported by
fomething of equal height, the fifti will lie ftill, and
the circulation may be feen very agreeably. In the
minuteft veffels thus examined, the blood always ap¬
pears pale or colourlefs, but in the large ones it is
manifeftly red. The arteries ufually branch out ex¬
tremely before they join the veins to carry the blood
back to the heart y but this is not always the cafe y
for Mr Leuwenhoeck has obferved, that on each fide
of the little griftles which give a ftiffnefs to the tail
of a flounder, there may be feen a very open
communication of the veins and arteries; the blood
running towards the extremities through arteries, and
returning back again through veins, which were evi¬
dently, a continuation of thofe arteries, and of the
fame diameter with them. The whole fifti, on the tail
of which this, examination was made, was not more
than half an inch in length ; it is eafy to conceive,
therefore, how fmall the tail muft be ; and yet in it
there were 68 veffels which carried and returned the
blood; and yet thefe veffels were far from being
the moft minute of all. How inconceivably nume¬
rous then muft the circulations in the whole human
body be ? Mr Leuwenhoeck is of opinion, that a
thoufand different circulations are continually carried
on in every part of a man’s body in the breadth of a
finger nail.
T. he tail of a newt or water-lizard affords alfo a ve¬
ry entertaining profpeft of the circulation of the blood
through almoft numberlefs fmall veffels ; but no ob¬
jefts (hows it fo agreeably as one of thefe animals,
while fo young as not to be above an inch long ; for
then the whole body is fo very tranfparent, that the
circulation may be feen in every part of it, as well as.
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