a river, and especially guard against having it within a bar, sand-bank, or any such impediment to the free action of the water.

The Bore.

25. If any place should be visited by that peculiar phenomenon, the bore, a wave which in some places comes rolling in with the first of the flood, with a crest foaming and rushing onward, threatening destruction to boats and even to shipping; note the time of the tide at which it begins, whether there be one wave only or more, the height to which it rises, and where it first appears with respect to its elevation above or below the mean water level of the ocean, and to any alteration in the feature of the river; and especially note the situation and extent of shoals at or below the spot. It seems essential to the formation of the bore that there should be first a great rise of tide; hence the reason why this phenomenon is said to occur at springtides only; and, secondly, that there should be an obstruction to the advance of the foot of the tide-wave, so that the crest of the wave is rapidly overtaking it.* It is desirable, therefore, that we should determine these points by observation on every occasion which offers, for which purpose there should be carefully noted the times of the passages of both portions of the wave between two places sufficiently far apart to insure accuracy, and between which the distance, if not known, must be determined; and with these observations should always be connected the rate of the stream soon after the passage of the bore; so that the observer may be able to write down the rate of the crest of the wave, the rate of the bore, and the rate of the stream. The rise of the water will further be essential to the satisfactory completion of the observation.

* The remark made in 'How to observe' (p. 35,) "that either rocks or shoals, or great depth of water, secure a river from the inconvenience of the bore," is not always correct; for the Severn is encumbered with shoals, and has a bore which has proved destructive to vessels grounded upon the sands.

Freshes.

26. Connected with the rise and fall of the water is that periodical elevation of the surface of rivers by "freshes," occasioned by heavy and continued rains in the interior of the country. These torrents not only raise the general level of the river, properly so called, but, where a bar exists, also raise the level there, so that vessels which cannot enter during the dry season are at such times able to pass over the bar. The time when the water begins to rise, when it attains its maximum, when it begins to subside, and regains its mean or ordinary level, should be carefully noted, and with it the elevation of the water, in feet, both in its ascent and descent.

Discovery of Land.

27. On the discovery of any unknown lands or dangers, the first endeavour, after the vessel is placed in safety, should be to fix the position of the place as accurately as the means of observation admit, and not to quit the spot until the danger is satisfactorily placed upon the chart.* Describe it as accurately as you can; determine its extent, height, and configuration; the adjacent soundings, and the quality of the ground; and give a sketch of its outline. If it be extensive, a running survey will be desirable.† If it be within sight of other land, its position must be fixed by bearings or angles between known points of the coast, and some conspicuous objects upon the land selected, which being brought in a line will lead ships clear of the danger. Do this for both sides, and give correct bearings of the transits, and, if possible, sketches of the objects.

* See Raper's Navigation,' 855, p. 328; and 856, p. 329. "No commander of a vessel," observes that talented officer, "who might meet unexpectedly any danger (before unknown), could be excused, except by urgent circumstances, from taking the necessary steps both for ascertaining its true position and for giving a description as complete as a prudent regard to his own safety allowed."

† See Running Survey, No. 29.

As regards coasts and islands which are but little known, there is given in the Appendix a list of such as are most deserving of attention, extracted from a return made by the able and indefatigable officer at the head of the Hydrographic department to an order of the House of Commons, 1848;† and all general directions for acquiring information which may have been already given must be considered to apply with double force to these countries. The limits of this paper do not permit of our entering into particulars as to the probable position of places which may be imperfectly determined, nor of the reported position of islands which are considered doubtful. In the Atlantic alone, for instance, there are islands reported continually. where none could possibly exist; and the islands of the Pacific have been multiplied by the errors of longitude made by persons visiting them; but wherever the charts place any islands as doubtful which you wish to seek (as it is always more probable that the latitude is correct than the longitude), the parallel of the supposed latitude should be gained at a meridian sufficiently distant from that given to exceed the probable limit of error in longitude, and a due east or west course pursued until a similarly distant meridian is gained on the other side; and if there should be any change in the colour of the water, sounding ought by all means to be tried; and especially we call attention to soundings upon the site near the equator marked as the seat of volcanic action from about 3° S. and 15° to 24° W., and also to the vicinity of the great bank S. and S.E. from the Falkland Islands, called Burdwood Bank, on which there has been found recently as little as 24 fathoms; also to the Agulhas Bank, and the sites of any volcanic islands which may have risen and disappeared.‡

Sailing along a Coast.

28. Even when sailing along a coast or islands which may be known and charted, it is advisable, as a general

* The late Rear-Admiral Sir Francis Beaufort, K.C.B. † Now brought down to the 1st January, 1859.

See also 4.

practice, to verify the position of the points and headlands as the ship sails along; and when the coast is new, or but indifferently explored, no opportunity should be omitted of determining as accurately as possible the position of every part within your power.

The position of places is determined from a ship with the least disadvantage, by being brought to bear east or west when the latitude is taken, and north or south when the longitude is observed. And as these observations may be made during several hours of the day,* much may be done in a single day's run, especially if patent log bases connect the stations, and astronomical bearings be employed. And upon all occasions the noting of transits, or the coming in a line of remarkable objects and of points of interest, should form a necessary portion of our duty, although we may believe them to be already satisfactorily determined, as they afford the most critical test of the accuracy of former surveys, and are especially useful in cases where longitudes of contiguous places may have been obtained by different observers.

If time admits of more than this being done (and in some of the countries which are but little explored it is extremely desirable that no opportunity should be lost of perfecting their outline), the heavy boats may be hoisted out and sent in-shore of the ship to run in the coast-line and the detail whilst the ship carries on a triangulation and continuation of bases in the distance, making what may be termed a running survey.

Running Survey.

29. Whenever this can be done, send the boats to a distance of 4, 5, or 6 miles at starting, and let them and the ship anchor, if possible, to measure a base by sound (19), and to get astronomical bearings and angles to the same points. Fix the ship's position by repeated observations for the latitude and by chronometer; then weigh and

4

*See Raper's Navigation,' 830 et seq., p. 320; also 834, p. 321; second edition.

put the patent log over and steer a steady course along the land (sounding, if the depth of water admit of it, without stopping). One boat now runs along the land from point to point, putting in the coast-line and its detail, getting astronomical bearings and angles as she proceeds, especially of all transits of points and headlands, and measuring her distance between them by patent log, and sounding, but without stopping. The other boat attends principally to the soundings, fixing herself as she requires, by angles and bearings between the points determined by the other boat and the ship.

At the end of a few miles' run, or at noon, or when necessary to renew the angles and bearings, a signal is to be shown, and the logs are then to be hauled in and read off, but not reset, fresh angles and bearings to be taken and a new base commenced, the distance between the ship and boats being again measured by sound. The log is then again put over, and the course of the vessel resumed. In this manner the day passes, the bearings and observations all being worked out at the moment-the outline run in, views taken, and every particular mapped and booked at the time, so as to leave nothing to memory. At the close of the day's operations anchor in position, measure a base by sound, and repeat operations as at starting, recall the boats, and in the grey of evening get the ship's position by stars and planets, which may at this time be observed with great accuracy before the horizon becomes too obscure. If the ship can remain at anchor, she will observe the set of the stream and the rise and fall of the water, however roughly it may be done.

As early as possible commit the triangulation to paper, that the ship may start in the morning with some points of land well fixed, so as to enable her to continue her triangulation throughout the day without the aid of the boat, although her co-operation as before should be renewed.

If there be no anchorage, the ship will maintain her position during the night under canvas, and in the grey of the morning picking up the place where she left off on the