towards you, since cases will often arise when stations so situated will give very inaccurate results, as with the objects A HI at P in plate A.

20. While operations on shore are going forward, boats can be sounding out the harbour, and fixing the points of reefs, rocks, &c., bearing in mind that it will always be found more satisfactory to land upon every rock or point, &c., than to lie off in the boat, and fix them by estimated distances or by intersections, either from these or from other stations.

*

In sounding, fix the boat at starting by two sextant angles; note the direction in which it is intended to run out the line of soundings, and note any two objects distant from each other, that are in a line upon that bearing, or, if the port be not too extensive, make use of staves with flags -shifting them along the coast at the end of each line of soundings the exact distance it is intended to run them apart-the boat showing a signal when the flags are to move; then keep the marks on, and sound at regular intervals 6, 5, 4, 3, 2, or fewer casts in a minute according to the depth; and at given short intervals note the time and fix the position of the boat by two angles as before mentioned, as also whenever there is any material alteration in the depth, or whenever the number of casts alters in a given time. When arrived at the end of the line fix the boat's position, and alter the course, sounding all the time until far enough for running back the second line of soundings parallel with the first. Fix the boat's station here again, and take a new leading mark. If the eye cannot catch a leading object at the moment, drop the grapnel to maintain the spot, for much more time is lost by over-running the lines than by coolly waiting for a guide to direct the course. Proceed in this manner, running all the soundings in parallel lines or nearly so, until the anchorage is all sounded out.

* See the form in Appendix No. 4 for entering these angles and soundings.

Having mapped all that is intended to be comprised in the survey, protract the work carefully on board upon a sheet of drawing-paper. Draw in the coast-line, rocks, shoals, hills, &c., and every other feature from your rough, attending to the hydrographic method of delineation represented in plate A.

21. The soundings follow next, when reduced to the low-water standard of the port by the tide gauge. (See 24.) If there be no station-pointer on board, protract the angles upon a piece of transparent paper, and mark the stations with their proper numbers. If the soundings have been taken equally, divide the spaces between the triangles into as many parts as there are casts, and fill in the corrected soundings in the order in which they occur. All soundings which may have been taken when the tide was up and by reduction to low water are dry: draw a line under. In every chart-box in the service will be found the abbreviations adopted in Admiralty Charts; these are to be strictly followed, and in Appendix No. 11 are given a few symbols which will be found useful in taking angles, &c., and in other surveying operations; and in plate A are given the usual hydrographic delineations of banks, cliffs, shoals, &c.

Lastly, put a meridian line and scale to the plan. Insert the variation, geographical position, time of H. W. F. and C., the low-water standard, to which the soundings are reduced, and the range at springs and at neaps; note the duration of the ebb and flood, both by the shore and by the stream; draw leading marks, and put in views, heights of mountains, &c.

If time does not permit of a regular survey being executed, still a useful record may be made by an itinerant survey, or even an eye sketch, assisted by sextant angles, a few soundings judiciously taken, the true bearing of one object, and the measurement of a base by sound, or with a Rochon micrometer as before mentioned.

Sailing Directions.

22. Whenever a survey is executed, sailing directions should accompany it, and too much care cannot be bestowed upon this important part of a surveyor's duty.

They should contain a description of the coast (see 11); directions for making the land; for approaching, and sailing into or out of the port both by daylight and with the aid of marks, and also by night or in thick weather, when the lead and the lighthouse, if there be one, must be the seaman's principal guide. How a vessel is to proceed with a leading or a beating wind, and with or against the tide-how far she may stand on either tack-what water she may expect to find at low-water springs-and how she may ascertain the depth by calculation on any other day -within what limits a vessel may safely steer in bad weather and when no pilot is on board-where the best anchorage lies, the depth in which a vessel should anchor, and directions for bringing up;-with other particulars which have been mentioned under the heads of approaching a port, especially noting all beacons, buoys, lighthouses, and landmarks, &c. (see 11).

To these affix views of the land and sketches of the leading marks, the geographical position, the time of H. W. F. and C., rise at springs and neaps, the low-water standard of the port, &c., and the variation of the compass; point out the best watering-places, and let all bearings given be magnetic, and noted as such.

Port regulations and quarantine laws will not be misplaced at the end of these directions.

Tide Pole.

23. When a survey is determined upon, a tide-gauge should be set up, and from half an hour before to half an hour after every high and low water the place of the tide should be registered every ten minutes.* In addition to this, whilst the

* See Forms Nos. 6 and 7.

sounding of the port is in progress, the place of the water must be noted every half-hour to facilitate the reduction of the soundings to the low-water standard. The tide-gauge should be fixed in a well-sheltered spot, with its zero at such a depth as to ensure its being below the low water at springs. When the pole is properly secured and settled down, paint a mark in the rock corresponding with one of the divisions on the gauge, and note which in your book, in case the pole should be washed down. If you remain long enough in port, let your observations be continued at least through an entire lunar month. When you come away, mean the high and low water heights of each day, and take a mean of them again for the mean place of the water, and cut a mark in the rock corresponding with that mean level of the sea before you remove the pole. As this is. the true scientific level of reference in all matters relative to the tides, refer this level again to some mark in a contiguous building, that a reference may at any time be made to it, by persons who might not be able to find the rock.

Let the watch be always at mean time at the place. The high and low water observations should be continued night and day with equal carefulness in order to determine the amount of diurnal tide; and every observation should be recorded, although it may not seem to agree with the others.

If tides are taken at coral islands, or at stations within a belt of coral, it should always be noted in the journal whether the sea or land breeze be blowing, and with what strength, and also whether the surf be high upon the reefs and sending its water into the lagoon, filling it faster than it can escape.

In the Appendix will be found two forms, one of which (No. 7) is for registering the tides every half-hour, the other (No. 6) is for the high and low water only.

For further information upon the tides see the next article.

Soundings.

24. Before any soundings are inserted in the chart they

should be reduced to a standard obtained by meaning the three or four successive lowest waters of each spring-tide, and meaning them again for a general mean. This standard should be noted in a very conspicuous, and unmistakeable manner as being so many feet below the mean water level, and recorded as the low-water standard of the port. It is a quantity which 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

Where R

R+r. cosine (180°)

the low-water standard to which the chart is adapted. r = the height of tide for the day above mean water level, the duration of the tide.

Ꭰ =

t = the time from high water previous.

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 portor, 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.

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