Most navigators will, at some time or another, have noticed the effect of changes in humidity on the dimensions of paper maps or charts pinned up for display purposes. A display chart, or any other large sheet of paper, neatly pinned to a notice board in a warm dry room will tend to sag in an untidy manner if the atmosphere is allowed to become damp. Similarly a chart pinned up in a moist atmosphere is liable to contract and tear away from its anchoring pins if the weather becomes warm and dry. In the same way, the size of a paper chart in use in the air will vary appreciably with changes in atmospheric conditions; and it follows that if the size changes, so does the true scale. Such changes in the scale are of little consequence so long as dividers are used for measuring distances, since it can be assumed that the distance graduations on the chart all expand and contract in proportion to the change in size of the sheet as a whole. But if a graduated rule or straight-edge is used (and this is practicable only in the case of maps or charts based on some projection giving near-constancy of scale) then, quite apart from the scale errors of the projection, changes in the size of the chart will give rise to errors.

The procedures of dead reckoning require a number of different steps, amongst which are distance measurement, angle measurement, construction of straight-line tracks, the solution of the vector triangle and the process of multiplication and division. The navigator's plotting and dead reckoning instruments are designed to meet these needs. The ruler serves both as a distance-measuring scale and as a means of drawing a straight line track; the protractor measures angles; the dividers measure distances and the Dalton computer serves both as a method of solving the vector triangle and as a circular slide rule for multiplication and division.

Since the publication in 1951 of a paper read to the Institute on bird navigation, a number of significant advances have been made in this field and a brief review of such progress will not be out of place. Most of this recent work has been concerned with the ability of birds to home to an area previously known to them. A useful summary, in English, has been published on the experiments done in Germany on the simple, inherited, one-direction movements of young birds on their first migration. The use of the Sun as a compass, in at least many cases, appears to be well established.

At depths of, generally, between 20 and 250 fathoms in the oceans, sonic and ultrasonic transmissions are frequently scattered by a layer which can be detected on the echo sounding trace, sometimes so strongly as to suggest a sea-bed echo. The cause of this layer, of which the depth has been observed to rise at sunset and sink at sunrise, is not precisely known, though it is generally thought to be biological. Investigations into the deep scattering layer (DSL), as it is called, are being conducted in many countries, principally in the United States, France and Great Britain, and a recent paper by Tchernia has set out the present state of knowledge on the subject.

The use of radar for preventing collisions at sea seems to have raised four main problems in applying the International Regulations for Preventing Collisions at Sea. These are: (I) In what, if any, measure radar contact covers ‘sighting’ within the meaning of Article 28; (2) whether the position of another vessel can be ascertained by radar in the precise sense intended in para. 2 of Article 16; (3) how, if at all, the use of radar should be considered to modify the ‘existing circumstances and conditions’ which, under Article 16, govern the use of moderate speed in poor visibility; (4) the application of the Steering Rules (Articles 17–27) when radar is in use.

This paper describes a new method for the continuous development of astronomical position lines, relative to a moving D.R. position. At present the method is restricted to observations of the stars, but it might later be extended to other bodies. A basic requirement is a precomputed ‘star lattice’, which here takes the form of a series of graduated intercept curves drawn so as to be always at right angles to curves of constant altitude. The star lattice may be prepared and printed in advance, or may be constructed by the navigator himself, immediately before observation, from the data in the new tables, Sight Reduction Tables for Air Navigation (Vol. i, Selected Stars).

This paper will discuss various methods of defining the vertical in aircraft. It is unrealistic to examine the vertical in navigation only in terms of providing a reference for sextant altitude measurement and the paper will therefore discuss more general problems of aircraft stability, which can themselves to some extent be studied by means of the sextant.

IN the course of a systematic search for tables or graphs using the principles appealed to in the Experimental Astronomical Navigation Tables (see p. 333), I examined a copy of the above tables, whose full title follows:

‘Margetts's Horary Tables for shewing by Inspection the Apparent Diurnal Motion of the Sun, Moon and Stars, the Latitude of a Ship and the Azimuth, Time or Altitude corresponding with any Celestial Object.

‘London. Printed for and sold by the Author. “Published as the Act directs 1st July 1790.”’

These tables (or diagrams) do not appear to be well known and they seem to possess sufficient interest to warrant notice. The author's own description is so precise that I can do no better than to repeat it verbatim with a few added notes of explanation. The accompanying photograph, reproduced from the copy in the collection of navigational books and tables in the Admiralty Library by kind permission of the Admiralty Archivist, shows a typical opening of the tables.

I have been unable, in the course of an admittedly hurried and incomplete search, to find any biographical information about Margetts. Perhaps some reader of the Journal can supply the deficiency.

The influence of the tides on navigation requires no emphasis to those who have had the responsibility of navigating ships in tidal waters, but there will be few who will have entirely escaped some of the difficulties which tidal problems may bring, especially in unfamiliar waters. Most navigators will therefore have some sympathy for the experience of Alexander and his armies on the way to the mouth of the Indus. Their experience in the Mediterranean had not prepared them for what they had to suffer when they passed into tidal waters.

When the maritime history of the eighteenth century is under discussion it is usual to stress the limitations imposed by the inability of navigators to determine their longitude. It is generally assumed that since the advent of the Davis Quadrant or back-staff the latitude could be observed with considerable accuracy, and that with the compass in a wooden hull and with the more accurate graduation of the log-line as shown by Norwood, the dead reckoning should, over short periods of time, be reasonably reliable.

The loss of Admiral Sir Clowdisley Shovel with several ships on the Bishop and Clerks on 23 October 1707, and the subsequent attribution of the disaster by Captain Sir William Jumper of the Lenox, 70 guns, to defective compasses, led to the compiling of reports on these instruments by the Commissioners at Chatham and Portsmouth, by order of the Navy Board. Captain George St. Lo, the Commissioner at Chatham, was at this time about fifty years of age. We first hear of him being appointed a Lieutenant of the Phoenix, 42, in 1678; he was made Post into the Dartmouth, 30, in 1682. With the three years qualifying service necessary before he obtained his first commission he must have had fourteen years almost continuously at sea before his career was cut short by his being severely wounded and taken prisoner when he had to surrender the sorely battered Portsmouth, 46, to superior French force in 1689. While a prisoner of war he took the opportunity to study the dockyard methods of his captors and this, with his reputation for steadfast opposition to all corruption, made him particularly suited to fill the post of commissioner in a dockyard. In 1707 he had already been five years at Chatham, following eight in Plymouth dockyard.

It has hitherto been generally presumed that the division of the horizon into thirty-two points was a development of the late medieval period. Such a division, it has been said, was impossible in the pre-compass era. ‘It is questionable whether even so many as sixteen directions could have been picked out and followed at sea so long as Sun and star, however intimately known, were the only guides’, one eminent authority has declared; ‘Even the sailors in the north-western waters had only four names until a comparatively late date.’ Chaucer's reference in his Treatise on the Astrolabe to the thirty-two ‘partiez’ of the ‘orisonte’ has for long been quoted as the earliest evidence on the subject. The Konungs Skuggsjà, a thirteenth-century Norwegian work, however, refers to the Sun revolving through eight œttir; and the fourteenthcentury Icelandic Rímbegla talks of sixteen points or directions. An important discovery by the distinguished Danish archaeologist, Dr. C. L. Vebæk, in the summer of 1951, brings a new light to the whole problem and makes the earlier held view scarcely tenable. Vebæk was then working on the site of the Benedictine nunnery (mentioned by Îvar Bárdarson in the mid-fourteenth century) which stands on the site of a still older Norse homestead on the Siglufjörd, in southern Greenland. Buried in a heap of rubbish under the floor in one of the living-rooms, together with a number of broken tools of wood and iron (some of them with the owner's name inscribed on them in runes) was a remarkable fragment of carved oak which evidently once formed part of a bearingdial. This was a damaged oaken disk which, according to the archaeologists, dates back to about the year 1200.

In order to gain first-hand experience of the conditions encountered by the growing number of vessels plying the Hudson Bay route, Mr. R. E. Perry, an engineer of the Decca Radar Company recently made a passage to Port Churchill in the S.S. North Anglia.

During the course of this passage a complete radar log was kept by Mr. Perry. This log is noteworthy not only for the many reports of the radar detection of various types of ice but also for the detail and completeness of the information recorded.

In all forms of navigation a distinction can be made between the two processes of finding out what the present position is and finding out what direction to go in to reach a given destination. On land the former process becomes the more important of the two because the constraints of natural ground features or of artificial obstacles often make a preset course impossible, as a motor drive along English country lanes shows so well. The destination will often be reached from an entirely unforeseen direction.

The usual method of conducting ships’ speed trials is for the ship to make runs over a measured mile. Recently, however, sea trials have been carried out using other methods, which seem to offer certain advantages. The trials of the S.S. United States, for example, were carried out using Raydist and Decca was used in the speed trials of S.S. Manchester Spinner off the Isle of Man. This paper discusses various methods of determining the ship's speed, the diameter of the turning circle and other data normally measured during sea trials, and makes an assessment of their relative convenience and accuracy.

During the course of recent hydrographic and oceanographical investigations in the Pacific, H.M.S. Challenger made considerable use of the extensive Loran coverage now provided by the United States. This marine radio position-fixing aid was particularly useful in the west Pacific in the area between Japan and New Guinea where the coverage of ground-waves is considerable. In this article an attempt is made to describe some of the work for which Loran was used, the principles on which it was used, and the accuracy expected and errors obtained. No attempt has been made to describe the operation of the system, which has been adequately covered in this Journal and in numerous other publications.

A list of existing and projected shore radar installations to aid merchant shipping was given as an Appendix to Mr. Le Page's paper on the requirements for port radar (Vol. V, p. 295), where unfortunately First Narrows Bridge was described as spanning the Fraser River. In making this correction, Commander Lancaster, the Harbour Master at Vancouver, has sent the following description of the installation, which provides a useful supplement to Mr. Le Page's paper.