The purpose of the descent and approach is to move the aircraft from the cruise to final approach while keeping within the proper envelopes of speed, acceleration, rate of descent, adherence to A.T.C. clearances, avoidance of bad weather and clearance from terrain and obstructions and other visible aircraft. Man carries out this task, at least until the ILS glide slope is captured. The methods used are not as thorough as the programme an electronic computer would need for this task. Perhaps men prefer to use common sense, but perhaps a high level of safety can only be achieved with thorough rules.

The many references to Omega in your excellent discussions on Navigation and traffic control published in the January issue were particularly gratifying to one who has worked on Omega for the past decade. Especially memorable was Mr. Stringer's statement that ‘[Omega] satisfies the three R's—reliability, redundancy and range’ and Mr. Jones' observation ‘' [Omega] is already the most widely deployed ground-based navigation aid in the world by a very substantial margin’. However, many contributors used words such as ‘somewhat unreliable’ and ‘subject to real problems’ when discussing ambiguity resolution. Perhaps some further remarks on ambiguity resolution are in order.

In the Interim Report of the Institute's Collision Regulations Working Party (Journal, 23, 448), it was stated in the preliminary that Dr. Calvert was not able to subscribe entirely to propositions (1)to (19). Dr. Calvert here states his views.

My disagreement with the interim report is concerned only with the propositions and principles at the end. The body of the report I regard as masterly in the circumstances.

The two following incidents might be of interest as an example of situations which arise today, and which well illustrate the standard of seamanship and watchkeeping which are encountered.

Gaussian logarithms are designed to facilitate finding the common logarithm of a sum or difference of two numbers whose common logarithms are known. The object of a table of Gaussian logarithms, sometimes known as Addition and Subtraction Logarithms, is to give log (a±b) by single entry when log a and log b are known.

Mr. Anneveld's paper (24, 50) would have us believe that the effects of coriolis force on a ship may become appreciable as ship size increases. It is true that the drift angle does increase as the ship's size increases, because the coriolis force given by equation (1) increases as (length)3·5 and the resistive forces increase as (length)3 (on the assumption of geometrically similar ships and Froude scaling). However there is a fundamental flaw in his argument because equation (2) only applies to a vessel with zero forward speed. Moreover the effect of coriolis drift will also be to induce a yaw angle on the ship (because the centre of pressure is forward of the centre of gravity, where the coriolis force may be assumed to act) and this will automatically cause the helmsman or autopilot to apply starboard rudder in the northern hemisphere. This will immediately produce a force on the ship in the port direction, i.e. opposing coriolis force.

The following proposed system of rules was presented to the Collision Regulations Working Party of the Institute by Admiral García-Frías and is referred to in the January issue of the Journal (24, 56–9).

IT is evident that the revision of the Collision Regulations would only be effective with a system of Rules able to cope with all situations that may arise at sea. The system must include all that it is necessary and sufficient to solve the collision problem in the easiest and most effective way and consequently it must apply both in clear weather and in restricted visibility using radar; differences between these situations being defined, if necessary, in every Rule.

Since the appearance of the note by Sayers in 1968, followed by the publication of the table by Thompson in 1969, and having myself described the method which was the subject of Sayers' and Thompson's communications, in a work on seamanship in 1962, I have searched for the origin of the method whereby an observer may find his distance off a ship or floating mark by means of a sextant observation of the vertical angle between the visible or sea horizon and the waterline of the ship or mark whose distance off is required.

Lately there have been a number of articles and discussions in the Journal involving the use of computer programs to solve navigational problems. As one often involved in complex programming systems, I have some strong views on the subject. We are not an institution of programmers, and the few of us who have some programming knowledge, a very useful asset, are usually limited to one or possibly two languages. I would doubt if many people who could follow B. J. Moss's ALGOL could also follow R. J. Turner's FORTRAN. Thus it seems strange that authors should go to the trouble of publishing the routines by which they do their calculations. After all, most of us are only interested in the method of attack and the answers rather than computer techniques.