1849 (iv). [502.] On Nov. 15, at sea, in the S. Atlantic, a comet was seen from the U.S. Ship "Maryland," with a nucleus as bright as Mars, and with a tail, curved and pointing to the S.W., nearly 1° long. From the notes of Captain Horner, Mr. Hind worked out the following position: at 9h 49m G.M.T., R.A. 20h 36.6m, decl. +4° 18'.-(Month. Not. R.A.S., x. 122 and 192.) 1854 (iii). [503.] On March 16 a bright nebulous object was seen by Brorsen. Its position at 8h 15m 348 Senftenburg M.T. was: R.A. 2h 30m 128, and decl. +1° 112.-(Astronomische Nachrichten, 897, vol. xxxviii. March 27, 1854.) [504.] 1856 (i). In January a comet was seen in the N.W. sky at Panama.(Letter in the Morning Herald.) 1856 (ii). [505.] On Aug. 7 an object, supposed to be a comet, was seen in Virgo by E. J. Lowe.-(Month. Not. R. A. S., xvii. 114.) A comet was also seen at Arequipa, in Peru, for a fortnight previous to Aug. 21 for 2 hours after sunset.-(Letter in the Times, Oct. 8, 1856.) 1859 (i). [506.] In Feb. a very faint comet was seen by Slater, in R. A. 11h 48m decl. +19° 49'. He saw it again on May 7 and 22, when it had become fainter, not being visible with any aperture below 11 inches. Its movement was very slow, and seemed to be in a northerly direction. -(Month. Not. R. A.S., xix. 291.) 1865 (ii). [507.] Encke's comet. This object was discovered by Tebbutt at Windsor, N. S. W., on June 24. It was very faint, and was seen only on that occasion and on June 29. Its observed place on the 24th is noted to have differed very much from that assigned by calculation.-(Ast. Nach., 1551. Oct. 6, 1865.) OBJECTS RECORDED AS NEBULÆ, BUT WHICH MAY POSSIBLY HAVE BEEN COMETS. 614 H. R.A. for 1860 2h 44m 68: decl. + 36° 55'7′: observed by Bessel. Looked for and not found by D'Arrest, who supposes it to have been a comet. Ee 2094 H. R.A. for 1860 10h 17m 5s: decl. + 27° 43′9′: observed by Sir J. Herschel. Looked for 6 times and not found by Lord Rosse. "This then was a comet or a lost nebula." 50 H III. On March 19, 1784, Sir W. Herschel observed an exceedingly faint nebula, 3m 158, following 45 Canum, and 4m south. Sir J. Herschel states that he has found a memorandum that this nebula is lost, and was probably a comet. 3550 H. R. A. for 1860: 13h 21m 139: decl. +6° 43'4': observed by D'Arrest, but "not found again on Feb. 19, 1863. clear. Perhaps a comet." Sky perfectly Hevelius, in his Prodromus Astronomia, states that he once saw in the head of Hercules, near a, a nebula. This was searched for unsuccessfully by Messier. The nearest object is 901 II II, but this would be quite beyond the reach of the telescopes used in the time of Hevelius, so it must have been a comet that he saw.-(Smyth, Cycle, ii. 385.) I have not succeeded in recovering this statement. Hind, in the Companion to the Almanac for 1860, refers, under the years A.D. 533, 577, 622, 660, 673, 674, and 742 (or 743), to objects which it is barely possible were comets: this is the utmost that can be said. In the same category stands an object dated by Pingré for 1174. BOOK V. CHRONOLOGICAL ASTRONOMY. CHAPTER I. What Time is.-The Sidereal Day.-Its length.-Difference between the Sidereal Day and the Mean Solar Day.-The Equation of Time.-The anomalistic Year.-Use of the Gnomon.-Length of the Solar Year according to different observers.— The Julian Calendar.-The Gregorian Calendar.-Old Style versus New Style.Romish miracles.-Table of differences of the Styles. TIME PIME is, strictly speaking, of infinite duration; we are, therefore, obliged to choose some arbitrary unit by means of which a measurement of time may be effected. For short intervals, the diurnal rotation on its axis of the globe we inhabit; for longer intervals, the annual revolution of the Earth around the Sun, are the standards of measurement we employ; but any event which takes place at equal intervals of time may serve the purpose of a chronometrical register. Thus, the ages of certain trees may be ascertained by counting the number of concentric rings in the trunk, one being formed annually; the ages of certain cattle, by the number of rings on the horns; the ages of horses may in like manner be ascertained by the successive disappearance of marks from their teeth; so also the pulsations of the heart, the flowing of a certain quantity of water from one vessel to another, the oscillations of a pendulum, may all be employed to measure time: but, in practice, the solar day is a natural interval of time, which the domestic habits of man force upon him; and accordingly we find that amongst all nations this unit of measurement is, under some form or other, the one adopted. The interval in which the Earth rotates on its axis is known to us as the Sidereal Day; it is determined by 2 consecutive passages of a star across the meridian of the place of observation, and is subdivided into 24 equal portions, called sidereal hours, which in turn are made up of 60 sidereal minutes, &c. The sidereal day may be otherwise defined to be the time occupied by the celestial sphere in making one complete revolution. The duration of this interval can be shewn by theory to be invariable, and the actual comparisons of observations made on numerous stars, in widely different ages of the world, most completely corroborate this conclusion. Here we have, then, a chronometric unit far surpassing in accuracy anything that can be artificially contrived. Laplace has ascertained, from a careful comparison of modern with ancient observations, that the length of the sidereal day cannot have altered so much as th of a second in upwards of 2000 years; it may, therefore, be regarded as possessing that indispensable qualification for a standard unit, invariability. The solar day is reckoned by the interval elapsing between 2 successive meridian passages of the Sun. The orbit of the Earth not being exactly circular, (and its axis being considerably inclined,) it follows that the daily velocity of our globe round the Sun, or, what for our purpose is the same, the daily motion of the Sun through the Zodiac, is not uniform a, and the length of the solar day, therefore, varies at different seasons of the year; this variable interval is called the apparent solar day, and time so reckoned is apparent time. In order to obviate the inconvenience which would attend such a method of reckoning time, astronomers have agreed to suppose the existence of an imaginary sun moving in the equator, with a velocity equal to the Sun's average velocity in the ecliptic. When this fictitious a The average daily motion is o° 59′ 8.2", but with the Sun in perigee, in January, it amounts to 1° 1' 9'9"; with the Sun in apogee, in July, it is only 0° 57′ 11.5'. or mean sun comes to the meridian, it is said to be mean noon; and when the true meridian passage of the Sun takes place, it is apparent noon: the interval between the two mean passages being called the mean solar day. If the Sun were, like a star, stationary in the heavens, then it is clear, from what I have said above, that the solar and sidereal days would be equal; but since the Sun passes from west to east, through a whole circlethat is to say, through 360° in 365.2422 days—it moves eastward about 59′ 8.2′′ daily. While, therefore, the Earth is revolving on its axis, the Sun is moving in the same direction; so that, when we have come round again to the meridian from which we started, we do not find the Sun there, but nearly 1° to the eastward; and the Earth must perform a part of her 2nd revolution before we can come under the Sun again. Thus it is that the mean solar day is longer than the sidereal day in the ratio of 100273791 to I the former being taken at exactly 24h om os, the latter, expressed in mean solar time, is 23h 56m 4'0913, or o‘99726957a. Clocks regulated to keep sidereal time are in general use in astronomical observatories-1 revolution of the hands of the clock through 360°, or 24h, thus representing I complete revolution of the heavens; but it is obvious that a sidereal hour is shorter than a solar hour, the difference being 9.8256. 24h of mean solar time are equal to 24h 3m 56.55% of sidereal time. In consequence of the sidereal day being 3m 55'91 shorter than the mean solar day, the stars all pass the meridian 3m 55'91o earlier every day. This gaining of the stars upon the Sun is called the acceleration of sidereal upon mean time; an obvious consequence of this acceleration is, that the aspect of the heavens varies at different times of the year, those stars which at one time are seen on the meridian at midnight, passing it at 9h in the evening after about 6 weeks. The clocks we have in common use are all regulated to mean time, and will therefore shew 12 o'clock sometimes before, and sometimes after, the true Sun has reached the meridian: this difference between mean time and apparent time is called the equation of time, and tables are constructed for the purpose of reducing the one to the other. Four times a year, the correction |