The distance l)etveen two neighlioring isobars ia greater in proportion as the winds are less, and vice versa, the distance is small when the winds are strongest. The rate of change of pres- Mire in a unit of horizontal distance is called the gradient of pressure. This barometric gradient may be as large as 5 millimeters or 0.25 inch of barometric pressure per degree of a great circle when gales of wind prevail, and much greater in the narniw region around a sturm centre within which hurricane winds prevail. The barometric gradients attending storm winds must not bo considered as producing the winds. On the con- trar)-. the winds have a great influence in pro- ducing the gradients, and much steeper ones would occur if the resistances to the motion of the wind did not prevent. Slaps of isobars and gradients will be found in connection with the article on Meteorology.
I'SOBU'TANE. See Bi'tane and Isobitane.
ISOCHEIMONAX (i's.Vkl'm.Vniil) LINES.Lines that connect places having the same winter temperature. See Isotiieumal Lines.
ISOCHRONISM, l-sok'rAnism (from iso-
chruiKiiis, from Gk. iffdxpnvos, isorhro>w.i, eqiuil
in time, from ftros, isos, equal -f- xP'^^'os, chroiios,
time). A pendulum is isoehnmous when its vi-
brations are performed in equal times, whether
these vibrations be large or small; and it can
possess this property by being constrained to
move in a cj-doidal arc. (See Cycloid.) Iluy-
gens, who is believed to have first applied the
pendulum to clocks (q.v.), made his pendulum
isochronous by causing its string to wrap and
imwrap itself round two equal cycloidal cheeks,
the diameter of whose generating circle was e<]ual
to half the length of the pendulum. This device
is no longer used, however, and isochronism is
closely appro.ximatcd in practice by causing the
pendulum to describe a very small circular arc.
The term is also used in connection with the
balance of watches.
ISOCHRONOUS CURVE. See Cycloid.
ISOCLINAL, i'sA-klT'nal (from Gk. hos. isos,
equal + (cXiwix, Idiiicin, to incline), or I.SOCLINIC
Line. An imaginary line on the earth's surface,
Buch that at all points on it the 'dip' of a mag-
netic needle which is suspended free to turn in
anj' direction is the same. (See Magnetism,
Terrestrial, where will be found maps showing
isoclinals. ) These lines are, in a general sense,
parallel to the lines of equal latitude on the
earth, but this is, of course, only a rough rela-
tion. At two points in the earth's surface the
magnetic needle is supposed to point vertically
downward. The positions now assigned these
points, sometimes called the magnetic 'poles' of
the earth, are latitude 70° .5' X., longitiide 90°
43' W., and latitude 7.3° ,30' S., longitude 147°
30' W. The main agonie line (q.v.) passes
through these poles.
ISOC'RATES (Lat.. from Gk.'IffoKpirijj, Isotratcs) . Though one of the ten Attic orators of the .-Mexandrian Canon. Tsocrates was rather a publicist and a pamphleteer than an orator. His long life (n.c. 4.30.338), as Be Quincey interestingly expounds in his essay on i^ti/Ie. spans the century from Pericles to .Mexander. In youth
he was attached to the Socratic circle, and
Plato in the Phadnis commends him above other
orators for a certain touch of philosophy that
might lead to higher things. Elsewhere Plato
seems to allude to him with irony as a rival
teacher and the exponent of a competing ideal
of culture. His first school, opened at Chios,
was probably devoted to the professional rhet-
oric of the law courts, and he himself wrote
forensic speeches, a few of which have been
preserved, but which he was inclined to dis-
avow in later days when his school at Athens
came to represent what he regarded as the more
broad and liberal training in essay-writing
and epideictic (display) oratorj' on large po-
litical and Hellenic themes. From personal
participation in the combats of the law courts
or the asseniblj' he was shut out by a weak
voice and an invincible timidity. Isocrates
stands for three things: (1) The idea that the
Greeks should unite to conquer Persia. This is
.set forth in his most brilliant performance, the
I'uncgi/ricus, which cost him ten years, and is
supposed to have been published at 01ym))ia in
n.c. 380. Failing to influence Athens and Si)arta,
he appealed to individuals, Jason of Pher;p, the
tyrant Dionysius, Philip of Jlacedon. The legend
consecrated by Jlilton's sonnet that the "dis-
honest victory at Cha'ronca, fatal to liberty,
killed with report that old man eloquent," is suf-
ficiently refuted by the tone of the letter to
Philip. Indeed, the conquest of Asia by Alexander, though not accomplished by a union of Athens and Sparta, was in many ways a striking fulfillment of the prophecies of the Panegyricus. (2) Isocrates's ideal of culture as a faculty of elegant disquisition occupying the happy mean between the narrow utilitarianism of the advocate and the unprofitable subtleties of a Plato is to us, as exemplified in his writings, a ridiculous and platitudinous thing. But nevertheless his school did as much as the Academy of Plato to make the Athens of the fourth century the schoolmistress of Greece. Dissertations have been written about his pupils. From that school, says Cicero, as from the Trojan horse, a company of naught but chieftains issued forth. (3) Isocrates, though not himself a great writer, holds a great place in the evolutiim of European prose. He was a pupil of the brilliant Sophist Gorgias, who freely employed a< ornaments of prose jingling assiinaiice, alliteration, balanced antithesis of thought and expression, striking metaphor, and other rhetorical features of Greek poetri'. Isocrates tempered the excess of these 'Gorgian figures.' but retained them so far as consonant with the genius of ornate but not extravagant prose. He also practiced and taught the smooth organic structure of the long rhythmical period, the avoidance of hiatus, the conscious variation of phrase and selection of synonym. Those and many other traits of style employed by him in a me<'lianical and monotonous way were studied in him by the world's three great masters of prose, Plato, Demosthenes, and later. Cicero. And Cicero has made them the common property of all educated men in theory, if not in practice. Isocrates's twenty-one orations and ten possibly genuine letters fill two small volumes of the Teulmer texts. They were translated into Kngli-
