MEASURES OF TIME.
Time is best defined as the succession of motion and phenomena, independent of relative human perceptions. Time is measured by man by the impressions of successive ideas, and these diminish in a ratio of their own increase ; conse quently, time appears less as men advance in age, or are variously employed. At ten, a year seems to be twice as long as at twenty; three times as at thirty; four times less at forty; five times at fifty ; and six times at sixty; circumstances of employment and position being the same. Hence a month employed in traveling seems equal to three of usual pursuits. Time being mentally measured hy the impressions of new ideas. Absolute time, independent of the feelings of individuals, is measured by certain regular motions, as the rotation of the earth, the swing of a pendulum, the fall of a body, the revolu tions of the moon round the earth, or the earth rOund the sun. The tropical year is 365 days 5 hours 48 minutes 51.6 seconds; but the sidereal year, or return to the sanne star, is 365 days 6 hours 9 minutes 11 seconds; but as the line of apsides or aphelion point advances 655 seconds, the orhit is completed in 365 days 6 hours 15 minutes 20 seconds, and this is called the anoma listic year. The Chaldeans made the sidereal year 365 days 6 hours 14 minutes, or one minute 49 seconds more than our pre,sent year, and the tropical year 365 days 5 hours 49 minutes 30 seconds, being 38 minutes 4 seconds more than ours. If Hypparchus was right in his measure of the tropical year, it is 11.2 seconds less than in his time. The lirahmins made it 1 minute 43 seconds more than now.
The preceesion of the equinoxes ia performed in 25,868 yeara; and the revolution of the line of apaidea is 20,931, or 1 degree, 43 minutea, 10 aeconds in a century. The precession of the equinoxea is 50 minutes 25. aeconda per annum, or 1 degree 52 minutes 45 seconda in a century, or the 360 degrees in 25,868 years+. Leap year ia the year which divides evenly hy four; but the year 1900 will not be leap year, to make up for the odd minutes gained between the aatronomical and computed year, aa 365 daya. The astronomical equinoxes are on the 21st of March and the 2let of September, and the sun ia in the tropics on the 2lat of December or June. Quarter daya in civil reckoning are March 25, June 24, September 29, and December 25, being feativala of the Catholic Church. The Synodical lunar month of her departure from the ann'a center to the return, or from fall to full, ia 29 day a 12 houre 44 minntea 12 aeconda, and was the universal month of the ancients, tvvelve being accounted a year. The difference between the solar and lunar year ia nearly eleven daye. The aun and moon return invariably to the aame relative positions' every 223 lunations ; according to the ancient Chaldeaua in 6585 daya 8 hours. Our modern tahles make it 17.29 seconda leaa. The Eaatern natIona, where the day varlea little, reckon the day from aunriae. The Romans reckoned as we do, from midnight. Christian nations assign thirty days to April, June, September, and November, thirty-one to other montha, and twenty-eight to February. making three hundred and eixty-five; but three hundred and aixty-six in Leap-year, when February ia twenty-nine. The Romana added the day in Leap-year on the aixth of the calenda of March, milking two sixths or his sentua, and hence the word Bisaextile. The astronomical day i6 the time which elapktea from the sun's being on the meridian of a place till hia return, divided into twenty-four hours of aixty minutes. And astrono mers begin the day at the departure of the sun from the meridian of the place, counting twelve hours till midnight, p.m. or after; and twelve hours from midnight till noon a.m. or before; their day after twelve at night being a day later than civil reckoning, which begins a new day at twelve at night, and reckons from twelve at night to twelve the next night. As the earth advances in its orbit 61 minutes 9.9 aeconds, when in ita perihelion, and only 57 minutes 10.7 seconda, when in its aphelion, while it returna the same meridian to the sun, that meridian arrives at the same fixed star in twenty-three hours 56 minutes 41 seconda or 3 minutes 5.59 seconda lese; others make it twenty three houra 56 minutes 34 aeconds or 3 minutes 56.6. aeconds leaa. Sidereal daya are always the same, and accord with a true clock; hut owing to the unequsl velocity, aa expresaed, of the earth in its orbit in the perihelion and_ aphelion, the sun's return to the in eridian variea; and also. aa the earth'a path ia inclined to the axis of rotation, the solar days vary. Owing to the first cause, the extreme difference ia minutea 39 fseconds on March 21, aud_ 6 minutea 5 aeconds on May 6; 0 on July 1, making the aun slower than the clock; but owing to the obliquity it ia 0 on March 21; 9 minutes 53 aeconsfa on May 6th, and 0, June 22, making the aun faster than the clock. The com bination of both causes producea the table of equation or time by which true clocks ought to he kept faster or alower than the meridian sun or a aun dial, called appar ent time, and the other true or mean time. Al3 the earth moves forward in ita orbit 59 minutea 8.3 aeconds, while it turne on its axis, any place arrivea at a fixed star 3 min utea 56.6 aeconde, before it arrives again at the center of' the aun, called twenty-fon r noura; hence the sidereal day ia but 24 houra 56 minutes 3.4 seconda, which is the cal period of revolution. Hence, in 365 solar days, the e rth turns 366 times on its axle, and by thia exact quau.i.y it gains' in ita orbit with reference to the sun and stars in every revolution, and hence the proceasion of the equi noxes. For sa the orbit la caused by the aun, and the equinoxea have reference to the sun and not to the stara, ao the equinoxea fall back 24,890 milea, or in space 50.30 seconda with reference to the atars. Every other hody like the earth turns once on its axis by going round a central body, and this, therefore, is the sole cauae of the equinoxial points. The Chat
desu e, Egyptians, and Jewe, began their cavil year from the autumnal equinox. The Persians, Gi eek.. Romans. and the English till 1752, began the year at the vernal equinox. Thence to make dates agree with those of other nationa, between January and Lady-day, writers uaed to put two 1708 datee, as Feb. 10, -- The bottom date being from 1709 January 1, and the upper that from the previous Lady-day. Sidereal daya are always 23 hours 56 minutea 3.4 seconds; hut aa the axis of the earth is inclined to the orbit. and the earth movea taster in the winter than in the aummer the clocka which meaaure equal time do not agree with the sun. A clock and a aun dial will, therefore, vary as follows:March 21, clock too faet 7 minutes 35 seconds; April 5. do. 2.55 eecouds; April 20, June 13, September 5, December 25, they are equal. When the sun is in Libra and Scorpio, or in Octoher and November, the difference is from ten minutea to 16 minutea 18 seconda, and the clock ought to be so much faster than the dial. The difference of velocity makes the dial faater than the clock, while the earth is moving from its perihe lion on January 1, to the aphelion July I.; and ou the contrary, the dial ia elower from July 1 to January 1. But the obliquity makea the dial faater from March 21 to June 22; and from September 22 to December 2t, and elower from June 22 to September 22, and from December 21 to March 21. The union of both producea the common equa. tion table, page 39, showing to the neareet minute how much a trne clock should be faster or slower than the sun. The pendulum for true and exact meaeuring of tlme was a suggeation of Galileo, in consequence of his observing the .oseillationa of a chandelier. Every oacillation, whe ther long or short, of the aame pendulum, is performed in the same time. A pendulum which vibrates aeconda London ought to he 39.-139 inchea nearly, and the length of pendulu ma for less or greater times' is as the square of tame. In India a day is divided into 60 Ghurriea. a Ghurry. into 60 Pula, a Pul into 60 Prana. and a Fran into )0 Tap,. in 2-5tha of a aecond. The beats iu an hour of a common second'e clock are 3.600, and 17.280 a common watch ; but second watchea heat 18,000 times, or 5 per aecoud. A lumi nous point to produce a vianal circle must go round aeven times in a second. Fewer than 30 vibrations in a second give no aound, and when the vibrations exceed 7,520 in a second the tonea ceaae to he diacriminat ed. 10 heats of a healthy pulae is equal to 9 seconds. The Roman luatra were periods of 5 years; and the Greek olympiada periode of' 4 years; and the flrat commenced in 776 B. C. The Metonick cycle Win 19 yeara, intended to be equivalent to, the Chaldean period. It was afterwaid adopted as the golden number for Easter. The Hegira, or Flight, took place July 16, 622, and la the Mahomedan Era. Their year la 12 lunar montha, or 354 days 8 houra 48 minutes: aud 11 days Using lost, a year must be allowvd every 33. to. reconrile their datea with ours. The periodical month of the Turks' and Araha, or aidereal period of the Moon ia 27 days 7 houra 43 minutes 48 seconds. The synodical month, or return to the conjunction of the Sun la 29 daya 12 hours 44 minutes 3 seconds 11 thirds. The Jewa began the year in March, and the months were Nina, Zif,, Sivan, Tammuz, Ab, Elnl, Tisri, Bul. Cisleu, Tiheth, She bat, Adar. The Sabbath, or seventh day is Saturday. The clays and nights, from sunrise to sunset, were divided into twelve equal parte or hours, 1, 2, 3, etc. The night watches were three hours each, from sunset to sunrise. 'I he months were lunar, or 30 days and 29 days, and they intro duced an etora month every two or three years. The day cummences and ends at sunset. The Jewish months were alternately 29 and 30 days, and their year of twelve lune tons 354 days. Their year commences with the vernal Asquinox. To recover tne four days they intercalcate a whole month after every two or three years, following their twelfth month, or Adar; and they call this extra month, ve-adar. The Jewish day commences at six in the evening, or sunset, and continuee till the same hour on the following evening. Their civil year commences with the new moon near the vernal equinox, in the month called Tisri, of 30 days, corresponding with part of September and part of October. The year 1829 was the Jewish year 5589, and ended September 27, beg,inniug September 9, 1828.
'The Mahometan year in 1829 was 1244, and ended July 2, beginning July 14. The Persians gave names to every day in the month, just as we give them to days of the week. Saturday, the 7th day, is, by the Arabs, called Subt. And Monday is called Jama, in all the Easternlaugnages. Law proceedinge preserve the Roman names of the days, ae 'Dies Solus, Lnnx, Martis, Mercurii, Jovis, Veneris, and Saturni, derived from the Roman deities. But in ordinary nee, the names are derived from the Saxon or Teutonic deities, the Snn, Moon,Tnesco.Woden,Thor,Friga,and Sat urn, The months,or lunar periods,are Roman ; and Septem ber, October, November, und December were so called when the Roman year began in March. April is so called -from Aperit, the epring. As the orbits o f the planets complete their seasons, their periods are taken to be their years; hence Mercury has nearly 4 years in ours ; Venus two-thirds; Mars is nearly 4 ttmes as long; Jupiter 12 times; Saturn 30; Herschel 83; and inore distant planets longer. September, etc., were the 7th, etc. month of the year of Romulus. From the spring to the summer Fel sticg le 92 days 21 hours 45 minutes. From the FI1M mer to the autumnal equinox, 93 days 13 hours 35 minutes, northern signs. From the autumnal to the winter solstice, 89 days 16 hours 47 minutes. From the winter to the spring equinox, 89 days 1 hour 42 minutes. South ern signs.