In the last number of the Journal (14, 243) Hollingdale showed mathematically that anticollision manoeuvres for the case of two craft moving in a plane can be formulated on a rigorous logical basis; and that a simple set of manoeuvres could be devised to meet all the geometrical requirements for anti-collision. The convention adopted was that each craft should manoeuvre so that if the other craft stood-on, the sight-line (the line joining the two craft) would always rotate in an anti-clockwise direction. The analysis showed that, mathematically, this was the complete and only answer to the basic problem.

The author of the present paper compares systems, such as that represented by the Collision Regulations and the Rules of the Air, based on onus to avoid, with a system based on a convention as to the direction of rotation of the sight-line. The first type of system, he holds, is defective for visual sightings and almost useless for radar sightings. The system put forward in the paper can be applied to both visual and radar sightings whether unilateral or bilateral by orientating a simple diagram with own course. A means of inter-communication is desirable but even without it such a system can, the author believes, safely be employed in the greater proportion of encounters.

The collision problem as a whole is beset by operational factors which are entirely distinct from the geometry of the situation, and for that reason no purely mathematical approach can, in isolation, be entirely valid. Nevertheless, there can be no doubt that studies such as the present one, based on a proper understanding of the geometry, can contribute greatly to the solution of this most pressing problem of navigation.

This paper is concerned with a limited part of the marine radar field, namely the equipments intended primarily for the larger ships. In these sets the greatest refinement and versatility is aimed at, to cover the wide variety of circumstances met with by the ocean fliers, the great tankers and the deep-sea tramps seeking port in any corner of the world. The paper is a factual record of radar characteristics as we enter the sixties and the fifteenth year of commercial radar; it seeks to make no invidious comparisons, but it is intended mainly to provide food for thought.

Marine radar has reached a very interesting stage in its progress. From 1947 to 1952 there was a gradual development in the United Kingdom towards a common, rather strict, specification. In the United States there was a similar trend towards a somewhat different standard. Between 1953 and 1959 the American tendency was to move towards the high definition favoured by Britain while British manufacturers began to offer the large diameter displays, 15 or 16 in., which were always popular in the States. Other trends on the whole have been similar, that is towards high definition, operational refinement, general reliability and better control of rain and sea clutter.

This article reports the discovery of two documents of considerable interest in the early history of navigation in England. When Henry VIII was planning to send ships of the royal fleet to Harderwijk in Guelderland in 1539, no pilot's book or chart of this part of the coast of northern Europe could be found in England. Therefore two experienced shipmasters, John Aborough of Devon and Richard Couche of Dover, were sent in haste to the Low Countries to make a survey of the coast and chart the route the king's ships would have to follow. Working with speed and secrecy, they compiled and brought back to the king a rutter giving sailing directions for Zeegat van Texel and the Zuider Zee and also a rough chart showing in detail the channels through the Haaks Banks, the entry to Marsdiep and the channel from thence to Enkhuizen. These two documents are the earliest original examples of such navigational directions drawn up by Englishmen which have so far been discovered. Both are reproduced in full and discussed in detail in this study.

Among the Marquess of Salisbury's family archives at Hatfield House is a document of great interest in the early history of navigation in England. It is a seaman's rutter giving directions for the navigation of Zeegat van Texel and the Zuider Zee which was compiled by two English shipmasters in 1539 on direct orders from King Henry VIII. A narrow roll of manuscript fashioned by roughly sewing four strips of parchment end to end, being not quite 6 in. wide and nearly 3½. long when fully opened out, this appears to be the earliest original English rutter which can be found today. It is true that the well-known set of fifteenth-century ‘Sailing Directions’ published by the Hakluyt Society in 1889 were compiled at an earlier date. But these have only survived in a copy transcribed by a professional scribe, William Ebesham, among a number of treatises on heraldry, chivalry and similar matters contained in a volume called the Great Book, part of the library of a country gentleman of East Anglia, Sir John Paston. The parchment roll at Hatfield would appear, therefore, to be the earliest example of an original English rutter which has yet been discovered.

From an inspection of upper-air contour charts for 300 and 200 mb., the location of the axis of jet streams of 80 kt. or more has been assessed once daily over the two years 1957–8. Results are presented for each of the four seasons by means of isopleths of frequency (Fig. 1) and also by means of frequency-roses in areas of 5 degrees of latitude by 10 degrees of longitude (Figs. 2–5); each rose shows the number of occasions of direction of the axis from the eight compass points. The area covered extends from latitude 30° N. to 70° N., and from longitude 60° W. to 30° E. An Appendix contains some discussion of the technique of locating the axis of jet streams on contour charts.

The characteristics of jet streams were described in a paper by Chambers in this Journal for July/October 1959. The present paper goes a stage further by giving the number of occurrences of jet streams per season over a two-year period in an area from the North Atlantic to the Mediterranean. The number of occurrences depends very much on the chosen definition, about which there is no general agreement. Whilst a jet stream may be fairly described as a fast-moving stream of air in the upper troposphere with great extension in the direction of the wind and persistence of the order of days, for statistical purposes it is necessary to be more precise. In this paper a critical speed of 80 kt. is required at the 300- or 200-mb. level in order that the stream shall qualify as a jet stream. Further discussion of this point is given in the Appendix, but any definition is largely subjective and it does not matter a great deal what definition is used provided the reader is aware of the limits to which the statistics refer.

In December 1958 the Institute held a meeting at the Royal Geographical Society at the suggestion of the Hydrographer of the Navy, to sound sea-going opinion on the Admiralty Chart. An account of the meeting was given in Volume 12 of the Journal. The discussion printed below took place at a meeting of the Institute held on 17 February, again at the suggestion of the Hydrographer of the Navy, to take account of what steps had been taken as a result of the last meeting and to examine the situation further. The discussion was introduced by Rear-Admiral E. G. Irving, O.B.E., the Hydrographer of the Navy who had with him the heads of the various branches within the Hydrographic Department.

The Hydrographer referred to the many points which had been made by Masters at the last meeting and in correspondence resulting from it and an account of what has been done as a result of these suggestions is given as an appendix to this discussion.

The Hydrographer: Of the general topics raised at the last meeting I would like to mention a few because I wish to stimulate discussion on these points in order to obtain guidance for future policy.

A Signal from a marine light is the oldest and simplest of the means available to the navigator for determining his position: so old, indeed, that one may well wonder whether anything can still be added to our knowledge of it, and so simple that one may question, too, whether anything is to be gained by seeking to perfect it while such vast fields of inquiry still await imaginative scientific research. However, like all human activities, navigation is caught up in a rapid evolutionary process which calls for an adaptation of time-honoured methods to new conditions of operation. Moreover, whatever advantages modern methods of position-finding may offer, the marine light will always have one great advantage that cannot be replaced, that of being directly perceptible to the human eye. It is well worth our while, then, to have a look at any defects in the present system of marine lights and see how they may be put right.

The cocked hat problem is a difficult one and though Captain Cotter in a recent article, has gone a long way towards clarifying it, I believe that the fundamentals of the subject are more difficult than this article suggests. A brief treatment of these, together with an indication of how they may be applied in practice, may be worth while.

In the July Journal (14, 362) Klerk and Steensma describe a new and ingenious method of radar plotting. The following comment, sent in at the invitation of the editor of the Journal, follows closely my remarks on a similar article in the Dutch review De Zee (December 1960).