The mariner is, on the whole, a cautious creature. When he takes action, he likes to know within fairly narrow limits how much is necessary and where that much will take him in relation to all potentially dangerous objects. He has as much respect for the fire as for the frying pan.

With the comparative absence of navigation aids in the earlier days of flying, up to about ten years ago, the navigator was constantly engaged in associating observations, calculations and chart work in what was essentially a high-class guessing game. The low speeds (just over 100 knots on flying boats and just under 200 knots on landplanes) provided enough time for this rather intriguing exercise to be carried out in a fairly relaxed atmosphere, conducive to an individualistic approach that could be justified by results. The navigator's performance was largely (at that stage of aviation) judged on the neatness of his plotting, the accuracy of E.T.A.'s and the absence of late course corrections before arriving at destination or a check point. If there is anything in the view that navigation is an art, this particular yardstick certainly induced considerable artifice in the navigator.

We have followed what has been written in this Journal about the accuracy of dead reckoning in the air and the determination and use of the most probable position. We wish to refer in particular to the article by J. B. Parker entitled ‘The navigational implications of Mr. Durst's paper’ (this Journal, 8, 113). We are not familiar with the routes of the North Atlantic or Central Africa; we have, however, eight years experience on the routes Lisbon–Luanda–Lourenço Marques, via Dakar, as well as those overflying the Sahara Desert via Oujda and via Agadir. The following considerations are in great part the result of that experience.

In his paper ‘Two Mariner's Astrolabes’ (Journal, 9, 338) Dr. D. J. Price stated that hitherto only seven mariner's astrolabes had been found and the details published. He then described two more mariner's astrolabes, the Dundee astrolabe of 1555, discovered some months previously, and the Kronborg astrolabe of 1600, discovered in 1928.

When plotting a series of radar bearings to determine the other vessel's course and speed, any alteration of own course or speed during the plotting interval greatly complicates the construction of a relative plot. A continuous alteration would make it very difficult to construct a plot at all. As far as the other vessel is concerned, should she also have radar, your echo on her PPI will not follow a straight line and she will be able to draw no conclusions regarding your course and speed. There is therefore no doubt that, when using radar for anticollision purposes, alterations of course and speed should, as far as circumstances permit, be as few as possible and each should be bold. Let us consider what is ‘bold’, treating alterations of course and speed separately, and remembering that it is necessary for the other vessel, if she is using radar, to appreciate that you have altered course as early as possible after your action.

In his Presidential Address, Captain A. M. A. Majendie emphasizes the value of the Institute in preserving the essential link between theory and practice. Particularly in aviation there is a risk that the ingenuity of the designer will lose value unless it is directly associated with the practical work of the operator. As airborne equipment becomes more complex, the operator cannot hope to grasp more than the fundamentals of much of the theory behind it, and his criticisms must perforce be tentative and somewhat uncertain.

In clear weather, many of us have used the curve of pursuit to avoid a vessel converging from well out on the starboard bow. We have said ‘Starboard’ and followed that order with ‘Head straight at her and report when you have regained course’. Convenient and safe though this technique may be in clear weather it can hardly be recommended for use with radar in fog. Apart from anything else, the other ship may have radar and may also take action as the distance between the two ships decreases. The slowly changing nature of our course would make it well nigh impossible for own ship's radar plotter to determine what action the other ship was taking, and might easily lead to disaster.

Some of the readers of the Institute's Journal may be amused at the following experiences in navigation, which I had fifteen years ago. In February 1942 I was crossing the Atlantic from Halifax N.S. to Liverpool in a convoy, and the passengers were of course never told anything about our position from day to day. I had no instruments, and not even a table of logarithms or of sines and cosines, but I did have much leisure and a good supply of blank paper, and I occupied myself by trying to find out where we were. On two occasions I told my estimates to the Chief Officer of the ship, who was not allowed to make any answer at the time, but on each occasion four or five days later he told me how far I had been from the true position.

In Dr. Derek Price's most interesting article ‘Two Mariner's Astrolabes’ he suggests that the heyday of this instrument was from 1530 to 1630. Some additional light is thrown on the closing date of the period of use of the astrolabe by Seller's Practical Navigation, a book which ran into many editions, the earlier ones being now somewhat rare. Successive editions of a book are usually of little value in giving a terminal date for the era of an instrument, for descriptions of it are only too often left in, unweeded long after the instrument is obsolete. Here, however, we have a description of the astrolabe inserted in the first edition of 1669 while it is omitted from the second edition of 1672. This later edition shows signs of careful rewriting and the fact that the author has expunged mention of some instruments, while retaining descriptions of others which never had much vogue, would seem to indicate that he thought that those which he had deleted would no longer be of value to the seaman.

A simple device to assist the graphical solution of problems involving relative motion is described, in the hope that it may contribute something to the interpretation of radar plotting.

Introduction. Basic requirements from a radar plot are:

• (a) the speed and course of the other ship;

• (b) an appreciation of the effect of a change of course or speed; and

• (c) a ready determination of the change of course or speed to produce a required effect.

Commander waters's review of Professor Morison's recent book on Columbus (Journal 10, 216) should be read in the light of the following facts, published in 1932 by the City of Genoa. They were supported by facsimiles of the documents from which they derive. The Great Discoverer was born in 1451, the son of a wool-weaver of Genoa. The family moved for a time to Savona, and were living there in 1472 when Christopher, in signing a deed, described himself as ‘a wool-worker of Genoa’. He went to Lisbon not before 1476, and while there visited Madeira to buy sugar for two Genoese merchants. He made a brief business visit to Genoa in 1479, when he said he must soon return to Lisbon. In all this there is no hint of a person who has knocked about the sea from the age often. There is no evidence that on the voyage to Guinea Columbus travelled as a sailor, while as regards the voyage to Iceland, he said it was made in February 1477, that the south of the island lay in lat. 73°N., not 63°N., and that he sailed 100 leagues (400 miles) beyond it and found that the sea was not frozen. The authenticity of this voyage may be left to the judgment of the reader.

It seems to me that the radar screen does not present collision avoiding information in the best form and that we should give some thought to the way in which we would like this information to be presented. Then we should ask the back-room boys to present it to us electronically. It must, of course, be remembered that a ship does not occupy a geometrical point and it will want to escape collision by more than the merest miss. Therefore we should build a margin of safety into the presentation.

In U.S. Naval Institute Proceedings for June 1957 (page 680), Captain P. V. H. Weems and Captain R. E. Jasperson outline a method for finding a position by observing one celestial body only. The principle of the method is to use the observed rate of change of altitude, together with the observed altitude itself, to solve directly for latitude and hour angle, and thus to obtain the position. The authors point out that the parallactic angle, C (the angle at the observed body), in the usual navigational spherical triangle is obtainable from:

sin C = sec (declination) × rate of change of altitude with hour angle.

The statistical analysis in the Institute Working Party's report on the accuracy of astronomical observations at sea (this Journal, 10, 223) is a fine piece of work for which we should all be most grateful to Mr. Sadler and the staff of H.M. Nautical Almanac Office. Indeed it is perhaps not out of place to recall the traditions by which the Royal Observatory supported by the Government of the day has always helped navigation, and to express gratitude to the Astronomer Royal for permitting and encouraging the work.