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with that of known stars); and by any other remarks which may be suggested by their appearance.

7. Many opportunities will occur of observing the zodiacal light; more especially when the observer is near the equator, where probably it can be seen at all seasons, before sunrise and after sunset; or, if in northern latitudes, after sunset in February and March, and before sunrise in September and October; if in southern latitudes, before sunrise in March and April, and after sunset in August and September. The zodiacal light consists of a pyramid of faint light, whose base is somewhere near the place of the sun, and whose point is at a distance of perhaps 30° from the sun; the axis of the pyramid being usually inclined to the horizon, following nearly the direction of the ecliptic. Although it presents to the eye a considerable body of light, yet the light of any portion of it is so feeble, and the definition of its outline is so imperfect, that it cannot be observed with a telescope. The observer, therefore, should only attempt to observe it with the naked eye when the sky is very clear, and when the sun is so far below the horizon that no twilight is visible. He should then endeavour, with the assistance of a chart of the stars, to define as accurately as possible its boundary with reference to the stars; remarking especially the place of the point of the pyramid, the width where it rises from the horizon, whether its sides are curved, and in what parts the light is brightest. It will be found that these observations are made most accurately by occasionally turning the eye a little obliquely from the zodiacal light. In registering the observation, in addition to the particulars to be recorded as prescribed above, there should be a statement of the latitude of the ship, the day, the time at the ship (or the Greenwich time and the longitude of the ship), the state of clearness of the sky, and the state of the weather for the day preceding the observation. (See also Meteorology.)

*

* A valuable paper by Professor C. P. Smyth on the Zodiacal Light, containing observations made by him near the Cape of Good Hope in the years 1843-5, will be found in the Transactions of the Royal

Improvement of Nautical Astronomy.

8. So much attention has been given to every detail of Nautical Astronomy, that it is very difficult to fix upon any part of it to which the attention of navigators should be specially directed with a view to its improvement. Perhaps the principal deficiency at the present time is in the want of well-understood methods of observing (with the sextant) the altitudes of stars at night, and of observing the altitudes of the sun and moon when the horizon is ill-defined. Every endeavour ought to be made to become familiar with the use of Becher's horizon, or some equivalent instrument, and to acquire a correct estimate of the degree of confidence which can be placed in the use of it.

9. It is likewise desirable that efforts should be made to facilitate the observation of Occultations of stars by the moon, and the observation of Eclipses of Jupiter's satellites at sea. Occultations occur rarely, but the result which they give for longitude is usually so much more accurate than that given by lunar distances, that, in long voyages where little dependence can be placed on the chronometer, the observa tion of an occultation must be extremely valuable. The eclipses of Jupiter's satellites afford less accurate determinations of longitude, but they occur very much more frequently, and may be very useful where chronometers cannot

be trusted.*

Society of Edinburgh, vol. xx. The principal merit of this memoir consists in the actual observations of the vertex of the cone of light by means of a rough instrument made for the purpose. A far more elaborate series of observations of the general boundaries of the light is that made by the Rev. George Jones, between the years 1853 and 1855, while engaged with the United States Japan Expedition in the steam frigate Mississippi. The results of the observations are given in a series of Charts drawn on Mercator's Projection, and form the 3rd volume of the Results of the United States Japan Expedition.'[R. M.]

* Attempts may laudably be made to devise some available mode of suspending a chair, so as to afford a steady seat to the observer. Hitherto such attempts have failed of practical success, from setting out with the principle of perfectly free suspension, a principle which

Astronomical Geography.

10. The intelligent navigator, on arriving at any port which has not before been visited, or whose position is not very well settled, ought to consider it his first duty to determine with all the accuracy in his power the latitude and longitude of the port. Supposing him to have determined by the usual nautical methods the approximate latitude, longitude, and error of chronometer, the best method of determining the latitude will be to find the chronometertime at which the sun or any bright stars of the Nautical Almanac list will pass the meridian, and to observe the double altitude of any such object by reflexion in a mercurial horizon, several times, as near as possible to the time of the meridian passage. If the place is in the northern hemisphere, the observation of the double altitude of the pole-star may be made at any time when it is visible: convenient tables for the reduction are given in the Nautical Almanac. For these and other observations the navigator ought to be provided with a proper trough and a store of mercury. For determining the longitude, there is probably no method superior to that of lunar distances (the exactness of which will be increased if the sextant or reflecting circle be mounted on a stand), unless the stay at the port is so long that transits of the moon can be observed. In any case, if there be a transit-instrument in the ship, it ought to be mounted on shore as soon as possible. The instrument ought, on the first evening, to be got very nearly into a meridional position, and then a mark should be set up, and the instrument should always be adjusted to that same mark (even though it be not exactly in the meridian), and

tends to prolong and perpetuate oscillations once impressed. It remains to be seen what stiff suspension, as for example by a rigid rope or cable, or by a hook's joint, purposely made to work stiffly (and that more or less at pleasure), by tightening collars-as also deadening and shortening oscillations, by lateral cords passing through rings to create friction-and other similar contrivances may do. In the suspension of a cot, at least, I have found this principle signally available.— (ED. SIR J. H.)

should always be levelled, before commencing a series of observations. One or two stars at least, as near the pole as possible, should be observed every night, in addition to the Nautical Almanac stars necessary for chronometer-error, and the moon-culminating stars which are observed with the moon. The instrument should be reversed on alternate nights; and, if possible, as many transits of the moon should be taken after the full moon as before the full moon.

In the register of all these observations, the same rule should be followed which is laid down under the first suggestion; that every observation should be recorded unreduced, exactly in the state in which it is read from the sextant or chronometer; and that the unreduced observations should be accompanied with the elements of reduction of whatever kind; and that (if the navigator has had leisure to reduce them) the reduced results should also be given.

G. B. AIRY.

APPENDIX No. 1.

BY SIR J. F. W. HERSCHEL.

(A.)

A List of the most conspicuous Variable or Periodic Stars of which observations would be desirable, with their periods of Variation (so far as known) and changes of magnitude.*

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List of Fixed Stars in either hemisphere, approximately arranged in order of brightness, down to the fourth magnitude, for the purpose of mutual comparison under favourable circumstances of altitude, and especially in equatorial and tropical voyages, or land stations, with a view to bringing the nomenclature and scale of magnitudes in the two hemispheres to agreement, and to the improvement of this branch of astronomical knowledge. The comparisons to be made by the naked eye among the stars of both lists not differing much (at the time of observation) in altitude, and in the absence of the moon and twilight,

* Much has been done by astronomers since the year 1851 in the observation of known periodic stars and the detection of new ones; for which see the Notices of the Royal Astronomical Society.—R. M.]

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