greater than that necessary to the reconciliation of observation with theory, and the earth is shown to be more rigid than steel—a conclusion long since announced by Kelvin for other reasons.
Chandler afterwards made an important addition to the subject by showing that the motion was represented by the superposition of two harmonic terms, the first having a period of about 430 days, the other of one year. The result of this superposition is a seven-year period, which makes 6 periods of the 428–day term (428d × 6 = 2568d = 7 years, nearly), and 7 periods of the annual term. Near one phase of this combined period the two component motions nearly annul each other, so that the variation is then small, while at the opposite phase, 3 to 4 years later, the two motions are in the same direction and the range of variation is at its maximum. The coefficient of the 428–day term seems to be between 0.12″ and 0.16″; that of the annual term between 0.06″ and 0.11″. Recent observations give smaller values of both than those made between 1890 and 1900, and there is no reason to suppose either to be constant.
The present state of the theory may be summed up as follows:—
1. The fourteen-month term is an immediate result of the fact that the axes of rotation and figure of the earth do not strictly coincide, but make with each other a small angle of which the mean value is about 0.15″. If the earth remained invariable, without any motion of matter on its surface, the result of this non-coincidence would be the revolution of the one pole round the other in a circle of radius 0.15″, or about 15 ft., in a period of about 429 days. This revolution is called the Eulerian motion, after the mathematician who discovered it. But owing to meteorological causes the motion in question is subject to annual changes. These changes arise from two causes—the one statical, the other dynamical.
2. The statical causes are deposits of snow or ice slowly changing the position of the pole of figure of the earth. For example, a deposit of snow in Siberia would bring the equator of figure of the earth a little nearer to Siberia and throw the pole a little way from it, while a deposit on the American continent would have the opposite effect. Owing to the approximate symmetry of the American and Asiatic continents it does not seem likely that the inequality of snowfall would produce an appreciable effect.
3. The dynamical causes are atmospheric and oceanic currents. Were these currents invariable their only effect would be that the Eulerian motion would not take place exactly round the mean pole of figure, but round a point slightly separated from it. But, as a matter of fact, they are subject to an annual variation. Hence the motion of the pole of rotation is also subject to a similar variation. The annual term in the latitude is thus accounted for.
Besides Chandler, Albrecht of Berlin has investigated the motion of the pole P. The methods of the two astronomers are in some points different. Chandler has constructed empirical formulae representing the motion, with the results already given, while Albrecht has determined the motion of the pole from observation simply, without trying to represent it either by a formula or by theory. It is noteworthy that the difference between Albrecht’s numerical results and Chandler’s formulae is generally less than 0.05″.
When the fluctuation in the position of the pole was fully confirmed, its importance in astronomy and geodesy led the International Geodetic Association to establish a series of stations round the globe, as nearly as possible on the same parallel of latitude, for the purpose of observing the fluctuation with a greater degree of precision than could be attained by the miscellaneous observations before available. The same stars were to be observed from month to month at each station with zenith-telescopes of similar approved construction. This secures a double observation of each component of the polar motion, from which most of the systematic errors are eliminated. The principal stations are: Carloforte, Italy; Mizusawa, Japan; Gaithersburg, Maryland; and Ukiah, California, all nearly on the same parallel of latitude, 39° 8′.
The fluctuations derived from this international work during the last seven years deviate but slightly from Chandler’s formulae though they show a markedly smaller value of the annual term. In consequence, the change in the amplitude of the fluctuation through the seven-year period is not so well marked as before 1900.
Chandler’s investigations are found in a series of papers published in the Astronomical Journal, vols. xi. to xv. and xviii. Newcomb’s explanation of the lengthening of the Eulerian period is found in the Monthly Notices of the Royal Astronomical Society for March 1892. Later volumes of the Astronomical Journal contain discussions of the causes which may produce the annual fluctuation. An elaborate mathematical discussion of the theory is by Vito Volterra: “Sulla teoria dei movimenti del Polo terrestre” in the Astronomische Nachrichten, vol. 138; also, more fully in his memoir “Sur la théorie des variations des latitudes,” Acta Mathematica, vol. xxii. The results of the international observations are discussed from time to time by Albrecht in the publications of the International Geodetic Association, and in the Astronomische Nachrichten (see also Earth, Figure of). (S. N.)
LATIUM,[1] in ancient geography, the name given to the
portion of central Italy which was bounded on the N.W. by
Etruria, on the S.W. by the Tyrrhenian Sea, on the S.E. by
Campania, on the E. by Samnium and on the N.E. by the
mountainous district inhabited by the Sabini, Aequi and Marsi.
The name was, however, applied very differently at different
times. Latium originally means the land of the Latini, and in
this sense, which alone is in use historically, it was a tract of
limited extent; but after the overthrow of the Latin confederacy,
when the neighbouring tribes of the Rutuli, Hernici, Volsci and
Aurunci, as well as the Latini properly so called, were reduced
to the condition of subjects and citizens of Rome, the name of
Latium was extended to comprise them all. It thus denoted the
whole country from the Tiber to the mouth of the Savo, and just
included the Mons Massicus, though the boundary was not very
precisely fixed (see below). The change thus introduced, though
already manifest in the composition of the Latin league (see
below) was not formally established till the reign of Augustus,
who formed of this larger Latium and Campania taken together
the first region of Italy; but it is already recognized by Strabo
(v. 3. 2. p. 228), as well as by Pliny, who terms the additional
territory thus incorporated Latium Adjectum, while he designates
the original Latium, extending from the Tiber to Circeii, as
Latium Antiquum.
1. Latium Antiquum consisted principally of an extensive plain, now known as the Campagna di Roma, bounded towards the interior by the Apennines, which rise very abruptly from the plains to a height of between 4000 and 5000 ft. Several of the Latin cities, including Tibur and Praeneste, were situated on the terrace-like underfalls of these mountains,[2] while Cora, Norba and Setia were placed in like manner on the slopes of the Volscian mountains (Monti Lepini), a rugged and lofty limestone range, which runs parallel to the main mass of the Apennines, being separated from them, however, by the valley of the Trerus (Sacco), and forms a continuous barrier from there to Terracina. No volcanic eruptions are known to have taken place in these mountains within the historic period, though Livy sometimes speaks of it “raining stones in the Alban hills” (i. 31, xxxv. 9—on the latter occasion it even did so on the Aventine). It is asserted, too, that some of the earliest tombs of the necropolis of Alba Longa (q.v.) were found beneath a stratum of peperino. Earthquakes (not of a violent character within recent centuries, though the ruin of the Colosseum is probably to be ascribed to this cause) are not unknown even at the present day in Rome and in the Alban Hills, and a seismograph has been established at Rocca di Papa. The surface is by no means a uniform plain, but is a broad undulating tract, furrowed throughout by numerous depressions, with precipitous banks, serving as water-courses, though rarely traversed by any considerable stream. As the general level of the plain rises gradually, though almost imperceptibly, to the foot of the Apennines, these channels by degrees assume the character of ravines of a formidable description.
