CIR 
degrees, numbered from 0 to 360 ; the ob- 
server may therefore take his angles as 
bearing from the north and south towards 
the east and west ; or, by that which is the 
most usual method, the whole circumfe- 
rence of a circle of 360 degrees, commenc- 
ing from the north point : a magnetic needle 
of the usual kind turns upon an iron point, 
fixed in the centre of the compass plate : a 
stop and trigger wire is applied to the com- 
pass box to throw the needle off its centre 
when not in use, in order to preserve the 
fineness of the centre point : a glass and 
brass spring ring covers the needle and 
closes the box: to the under side of the 
compass box, at the N. and S. points, is 
connected a bar about 15 inches long, from 
end to end, to each end of which is fixed a 
perpendicular brass sight about five inches 
long; each sight containing a long slit or 
perforation, and a sight line, so that the ob- 
server may take his line of sight, or obser- 
vation of the line, upon the station mark, 
at which end of the bar he pleases. 
CIRCUMSCRIBED, in geometry, is 
said of a figure which is drawn round an- 
other figure, so that all its sides or planes 
touch the inscribed figure. 
Circumscribed hyperbola, one of Sir 
Isaac Newton’s hyperbolas of the second 
order, that cuts its asymptotes, and con- 
tains the parts cut off within its own space. 
CIRCUMSCRIBING, in geometiy, de- 
notes the describing a polygonous figure 
about a circle, in such a manner that all its 
sides shall be tangents to the circumference. 
Sometimes the term is used for the describ- 
ing a circle about a polygon, so that each 
side is a chord ; but in this case it is more 
usual to say the polygon is inscribed than 
the circle is circumscribed. 
CIRCUMVALLATION, or line of cir- 
cumvallation, in the art of war, is a trench 
bordered with a parapet, thrown up quite 
round the besieger’s camp, by way of secu- 
rity against any army that may attempt to 
relieve the place, as well as to prevent de- 
sertion. See Fortification. 
CIRRUS, in botany, nclasper or tendril: 
that fine spiral string or fibre, put out from 
the foot-stalks, by which some plants, as the 
ivy and vine, fasten themselves to walls, 
pales, or trees, for support. It is ranked by 
Linnaeus among the fulcra, or parts of plants 
that serve for support, protection, and de- 
fence. Tendrils are sometimes placed op- 
posite to the leaves, as in the vine; some- 
times at the side of the foot-stalk of the 
leaf, as in the passion-flower; and some- 
CIS 
times, as in the winged-pea, they are emit- 
ted from the leaves tliemselves. 
CIRSOCELE, or hernia varicosa, in sur- 
gery, a preternatural distension or divarica- 
tion of the spermatic veins in the process of 
the peritonaeum. 
CISSAMPELOS, in botany, a genus of 
the Dioecia Monadelphia class and order. 
Natural order of Sarmentaceae. Menisper- 
ma, Jussieu. Essential character: male, 
calyx four-leaved ; corolla none ; nectary 
wheel-shaped ; stamina four, with cornate 
filaments. Female, calyx one-leafed, ligu- 
late, roundish ; corolla none ; styles three ; 
berry one-seeded. There are three species. 
CISSOID, in geometry, a curve of the 
second order, first invented by Diodes, 
whence it is called the cissoid of Diodes. 
Sir Isaac Newton, in his appendix “ De 
iEquationum Constructione lineari,” gives 
the following elegant description of this 
curve, and at the same time shews how', 
by means of it, to find two mean propor- 
tionals, and the roots of a cubic equation, 
without any previous reduction. Let A G 
(Plate III. Miscel. fig. 12) be tlie diameter, 
and F the centre of the circle belonging to 
the cissoid ; and from F draw F D, F P, at 
right angles to each other, and let F P be 
== A G ; then if the square P E D be so 
moved that one .side E P always passes 
through the point P, and the end D of the 
other side E D slides along the right line 
F D, the middle point C of the side E D 
will describe one leg G C of the cissoid ; 
and by continuing out FD on the other 
side F, and turning the square about by a 
like operation, tlie other leg may be de- 
scribed. 
This curve may likewise be generated by 
points in the following manner : 
Draw the indefinite right line B C 
(fig. 13) at right angles to A B the diameter 
of the semicircle A O B, and draw the right 
lines A H, A F, A C, &c. then if you take 
A M = L H, AO z= O F, Z C = A N, &c. 
the points M, O, Z, &c. will form the curve 
A M O Z of the cissoid. 
Cissoid, properties of the : it follows from 
genesis that drawing tlie right lines PM, 
K L, perpendicular to A B, the lines A K, 
P N, A P, P M, as also A P, P N, A K, K L, 
are continual proportionals, and therefore 
thatAKz=PB, and PN=:IK. After 
the same manner it appears that the cissoid 
AMO, bisects the semicircle A O B. Sir 
Isaac Newton, in his last letter to Mr. 
Leibnitz, has shewn how to find a right line 
equal to one of the legs of this curve by 
