' »«?»// -> • • 
SEM 
wherein the cubes of the ordinates are as 
the squares of the abscisses. Its equation 
is axx — y^. 
The area of the space A P M (Plate XIV. 
Miscel. fig, 1), is = ]| scy = -^ AP x 
PM, or A of the circumscribing rectangle. 
The content of the solid generated by the 
revolution of the space A P M about the 
axis A P, is 1 p .r = .7854 A P X P or 
a of the circumscribing cylinder. And a 
circle equal to the surface of that solid may 
be found from the quadrature of an hyper- 
bolic spaCjC. Also the length of any arc, 
AM, of the curve may be easily obtained 
from the quadrature of a space contained 
under part of the curve of the common 
parabola, two semi-ordinates to the axis, and 
the part of the axis contained between 
them. This curve may be described by a 
continued motion, viz. by fastening the 
angle of a square in the vertex of a com- 
mon parabola ; and then carrying the inter- 
section of one side of this square and a long 
ruler (which ruler always moves perpen- 
dicular to the axis of the parabola) along 
the curve of that parabola. For the inter- 
section of the ruler, and the other side of 
the square will describe a semicubical para- 
bola. Maclaurin performs this without a 
common parabola, in his Geometria Or- 
ganica. 
SEMIDIA METER, half the diameter, or 
a right line drawn from the centre of a cir- 
cle, or sphere, to its circumference j being 
the same with what is otherwise called the 
radius. The distances, diameters, &e. of 
the heavenly bodies, are usually estimated, 
by astronomers, in semidiameters of the 
earth ; and the distances of the secondary 
planets from their respective primary ones, 
by semidiameters of the body of the pri- 
mary planet. 
Suppose the semidiameter of the Earth 
to be unity, then the measures of the Sun 
and planets will be, as follow : The semi- 
diameter of 
The Earth 1 
The Sun 111-25 
The Moon 0.27 
Mercury 0.38 
Venus 1-15 
Mars...,,,.. 0.65 
Jupiter 11.81 
Saturn 9-77 
Herschel 4.32 
SEMIPARABOLA, in geometry, a curve 
defined by the- equation ax”' ~ ' =!/”’; 
as a =: y’, and a a’ = y'*. In semipara- 
SEN 
bolas, = ^ 
2 ™ — ^ ; or the powers of the semiordinates 
are, as the powers of the semiabscisses 
one degree lower ; for instance, in cubical 
semiparabolas the cubes of the ordinates 
are as the squares of the abscisses ; that is, 
y^ ; :: x^ 2 ^- 
SEMPERVIVUM, in botany, houseleek, 
a genus of tlie Dodecandria Polygynia class 
and order. Natural order of Succuleniae. 
Sempervivae, J nssieu. Essential character : 
calyx twelve-parted; petals twelve;, cap- 
sule twelve, many-seeded. There are four- 
teen species. 
SENECIO, in botany, groundsel, a ge- 
nus of the Syngenesis. Polygamia Super- 
flua class and order. Natural order of 
Composite Discoide®. Corymbifer®, Jus- 
sieu. Essential character: calyx cylindri- 
cal, calycled, with the scales mortified at 
the tip; down simple; receptacle naked. 
There are seventy-five species. 
SENSATION. The brain is a soft pulpy 
mass, of a whitish colour on the inside, oc- 
cupying all the cavity of the skull. Minute 
differences are observable in the substance 
of the brain in different parts of it, but it is 
unnecessary to enter upon a statement of 
them here. (See Anatomy.) The spinal 
marrow is the continuation of the lowest 
part of the brain, which passes through the 
great opening of the skull down the hollow 
of the back bone. The substance con- 
tained in the hollow of the back bone is 
the same with that in the cavities of the 
skull; and it is sometimes convenient to 
comprehend both the portions under the 
same general name of brain. From the 
brain proceeds the nerves, which at first 
are fine fibres of the same substance with 
the brain : these fibres meet and form soft 
white pulpy cords, which afterwards spread 
themselves over various parts of the body, 
by splitting into innumerable and exceed- 
ingly minute branches. Anatomists count 
forty pairs of nerves, (for they come off in 
pairs, though they afterwards separate), and 
of those nine or ten only come from the 
brain at the bottom of the skull, and the 
rest from the spinal marrow. Those from 
the brain are distributed to various parts 
of the head ; those from the spinal marrow 
are distributed over the trunk and extremi- 
ties. The external organs of sense, the 
nerves, and the brain, are the organs of 
sensation. All, as we are at present consti- 
tuted, are necessary to sensation. If the 
external organ is destroyed no sensation 
can be produced : where there areno nerve.s 
