only has the revival of its use been proposed; viz. for the accurate measurement of very small portions of time, by the flowing out of mercury from a small orifice in the bottom of a vessel, kept constantly full to a fixed height. The stream is intercepted at the moment of noting any event, and directed aside into a receiver, into which it continues to run, till the moment of noting any other event, when the intercepting cause is suddenly removed, the stream flows in its original course, and ceases to run into the receiver. The weight of mercury received, compared with the weight received in an interval of time observed by the clock, gives the interval between the events observed. This ingenious and simple method of resolving, with all possible precision, a problem which has of late been much agitated, is due to captain Kater.

(123.) The pendulum clock, however, and the balance watch, with those improvements and refinements in its structure which constitute it emphatically a chronometer*, are the instruments on which the astronomer depends for his knowledge of the lapse of time. These instruments are now brought to such perfection, that an irregularity in the rate of going, to the extent of a single second in twenty-four hours in two consecutive days, is not tolerated in one of good character; so that any interval of time less than twentyfour hours may be certainly ascertained within a few tenths of a second, by their use. In proportion as intervals are longer, the risk of error, as well as the amount of error risked, becomes greater, because the accidental errors of many days may accumulate; and causes producing a slow progressive change in the rate of going may subsist unperceived. It is not safe, therefore, to trust the determination of time to clocks, or watches, for many days in succession, without checking them, and ascertaining their errors by reference to natural events which we know to happen, day after day, at equal intervals. But if this be done, the Mirgu, to measure.

*

xooves, time

CHAP. II.

CLOCKS.CHRONOMETERS.

79

longest intervals may be fixed with the same precision as the shortest; since, in fact, it is then only the times intervening between the first and last moments of such long intervals, and such of those periodically recurring events adopted for our points of reckoning, as occur within twenty-four hours respectively of either, that we measure by artificial means. The whole days are counted out for us by nature; the fractional parts only, at either end, are measured by our clocks. Το keep the reckoning of the integer days correct, so that none shall be lost or counted twice, is the object of the calendar. Chronology marks out the order of succession of events, and refers them to their proper years and days; while chronometry, grounding its determinations on the precise observation of such regularly periodical events as can be conveniently and exactly subdivided, enables us to fix the moments in which phenomena occur, with the last degree of precision.

(124.) In the culmination, or transit, (i. e. the passage across the meridian of an observer,) of every star in the heavens, he is furnished with such a regularly periodical natural event as we allude to. Accordingly, it is to the transits of the brightest and most conveniently situated fixed stars that astronomers resort to ascertain their exact time, or, which comes to the same thing, to determine the exact amount of error of their clocks.

(125.) The instrument with which the culminations of celestial objects are observed is called a transit instrument. It consists of a telescope firmly fastened on a horizontal axis directed to the east and west points of the horizon, or at right angles to the plane of the meridian of the place of observation. The extremities of the axis are formed into cylindrical pivots of exactly equal diameters, which rest in notches formed in metallic supports, bedded (in the case of large instruments) on strong piers of stone, and susceptible of nice adjustment by screws, both in a vertical and horizontal direction. By the former adjustment,

the axis can be rendered precisely horizontal, by levelling it with a level made to rest on the pivots. By

the latter adjustment the axis is brought precisely into the east and west direction, the criterion of which is furnished by the observations themselves made with' the instrument, or by a well-defined object, called a meridian mark, originally determined by such observations, and then, for convenience of ready reference, permanently established, at a great distance, exactly in a meridian line passing through the central point of the whole instrument. It is evident, from this description, that, if the central line of the telescope (that which joins the centres of its object-glass and eye-glass, and which is called in astronomy its line of collimation) be once well adjusted at right angles to the axis of the transit, it will never quit the plane of the meridian, when the instrument is turned round on its axis.

(126.) In the focus of the eye-piece, and at right angles to the length of the telescope, is placed a system of one horizontal and five equidistant vertical threads or wires, as represented in the annexed figure, which

CHAP. II.

OF THE TRANSIT INSTRUMENT.

81

always appear in the field of view, when properly illuminated, by day by the light of the sky, by night by that of a lamp introduced by a contrivance not necessary here to explain. The place of this system of wires may be altered by adjusting screws, giving it a lateral (horizontal) motion; and it is by this means brought to such a position, that the middle one of the vertical wires shall intersect the line of collimation of the telescope, where it is arrested and permanently fastened. In this situation it is evident that the middle thread will be a visible representation of that portion of the celestial meridian to which the telescope is pointed; and when a star is seen to cross this wire in the telescope, it is in the act of culminating, or passing the celestial meridian. The instant of this event is noted by the clock or chronometer, which forms an indispensable accompaniment of the transit instrument. For greater precision, the moments of its crossing all the five vertical threads is noted, and a mean taken, which (since the threads are equidistant) would give exactly the same result, were all the observations perfect, and will, of course, tend to subdivide and destroy their errors in an average of the whole.

(127.) For the mode of executing the adjustments, and allowing for the errors unavoidable in the use of this simple and elegant instrument, the reader must consult works especially devoted to this department of practical astronomy.* We shall here only mention one important verification of its correctness, which consists in reversing the ends of the axis, or turning it east for west. If this be done, and it continue to give the same results, and intersect the same point on the meridian mark, we may be sure that the line of collimation of the telescope is truly at right angles to the axis, and describes strictly a plane, i. e. marks out in the heavens a great circle. In good transit observations, an error of two or three tenths of a second of time in the moment

See Dr. Pearson's Treatise on Practical Astronomy, Sopra lo Stromento de' Passagi. Ephem, di Milano, 1824. G

Also Bianchi

of a star's culmination is the utmost which need be apprehended, exclusive of the error of the clock: in other words, a clock may be compared with the earth's diurnal motion by a single observation, without risk of greater error. By multiplying observations, of course, a yet greater degree of precision may be obtained.

(128.) The angular intervals measured by means of the transit instrument and clock are arcs of the equinoctial, intercepted between circles of declination passing through the objects observed; and their measurement, in this case, is performed by no artificial graduation of circles, but by the help of the earth's diurnal motion, which carries equal arcs of the equinoctial across the meridian, in equal times, at the rate of 15° per sidereal hour. In all other cases, when we would measure angular intervals, it is necessary to have recourse to circles, or portions of circles, constructed of metal or other firm and durable material, and mechanically subdivided into equal parts, such as degrees, minutes, &c. Let ABCD be such a circle, divided into 360 degrees,

(numbered in order from any point 0° in the circumference, round to the same point again,) and connected with its centre by spokes or rays, x y z, firmly united to its circumference or limb. At the centre let a circular hole be pierced, in which shall move a pivot exactly fitting it, carrying a tube, whose axis, ab, is exactly parallel to the plane of the circle, or per