would nearly correspond with half the range of an ordinary spring tide, a term often written without any direct reference to the low water standard, and so ambiguous that it is to be hoped it will soon disappear from the face of our charts. With this standard, and the known daily height of the tide above mean water level, soundings taken at any hour may be prepared for comparison with the depths upon the chart by the simple formula

R+r. cosine (180°)

the low water standard to which the chart is

r = the height of tide for the day above mean water level. the duration of the tide.

D

t

=

Or, enter the traverse table with the time from the nearest high water as a course (allowing 5° of arc to every 10 minutes of time), and with r (half the range of tide for the day) as a distance; in the latitude column will stand a quantity which applied to the low water standard of the port + or -, according as the arc is less or greater than 90°, will give the reduction required. If the arc exceeds 90° take its supplement. But it is to be observed that all these corrections, although preferable to the old method of reducing soundings, are but approximations. In many places, especially in such as have great tides, it is necessary to distinguish between rising and falling.

If in a country subject to earthquakes, carefully watch the tide-pole during and after the shock, and if any undulations of the water are observed, note them, and the direction whence they proceed.

Be careful never to place the tide-pole at the mouth

of 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.

It

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. 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 spring-tides 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

*

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.

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.

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, shculd 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

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

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.

As regards coasts, and islands which are but little known, I have 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 the longitudes of 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

for ascertaining its true position and for giving a description as complete as a prudent regard to his own safety allowed."

See Art. 29.

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 31 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; the Agulhas Bank, and the sites of any volcanic islands which may have risen and disappeared.*

Sailing along a Coast.

28. When sailing along a coast or islands which may even be known and charted, it is advisable, as a general 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 longitude is observed. And as these observations may be made during several hours of the

See also Art. 4.