Since government and industry have officially accepted the introduction of the International System (S.I.) Units for all forms of measurement it is relevant to examine the implications of this for marine navigation.

The present situation. Marine navigation is no different from many other fields in that a proliferation of units is used to measure the same dimension and, moreover, these units have no systematic relationship to each other. In this respect length is the worst offender: we measure heights in feet, depths of water in fathoms, distances in cables or nautical miles and moreover these nautical miles may be British standard, International standard or those precisely determined by latitude.

The publication of Admiral Ritchie's E. G. R. Taylor Lecture (Journal, 20, 1) brings into perspective so many names familiar to the practising navigator; indeed the achievements of these great men are surveyed and charted with great clarity. Captain Flinders receives mention only in passing and his initial surveys of the greater part of Australia do not appear on the summary diagram. Yet, in other contexts, his ship, his mentor and his kinsmen all obtain special mention.

COMMANDANT OUDET'S paper ‘The Black Flood’ is a masterly analysis of the circumstances surrounding the loss of the Torrey Canyon. We have come to expect contributions of this standard from Commandant Oudet, so much so that one wonders whether the sort of international tribunal which he advocates would be more effective in pointing to the lessons to be learned from casualties than Commandant Oudet's own work has been for many years past. In his discussion of the Torrey Canyon loss, Commandant Oudet raises the question of whether the size of tankers should be limited. This question has been voiced elsewhere with the implication that, economics apart, the trend towards larger and larger tankers is a bad thing. It deserves a more detailed study.

To simplify this study a hypothetical case is considered in which a company has to transport oil at a rate of 200,000 tons per week between two ports where the round voyage takes two weeks. In theory this could be done using one 400,000-ton tanker, two 2000,000-ton tankers, …sixteen 25,000-ton tankers, …forty 10,000-ton tankers…. Economic considerations favour the use of large tankers provided there is sufficient water at the two terminals and enroute, but is this morally justified in view of the possibility of repetitions of the Torrey Canyon incident?

F. C. Bell in his contribution raises most interesting philosophical questions. In commenting on them, I will refer to a recent translation (Navigation U.S.A., 14, 26) of Schuler's paper and use the same notation.

The impossibility of distinguishing by any physical measurement between a gravitational force and the inertial reaction force of an accelerating frame of reference is fundamental. In Fig. 1 our observer in an enclosed laboratory is observing the compression of a linear spring supporting a ‘proof mass’ m (he knows the natural uncompressed length of the spring).

THE following notes stem from Ronald Turner's somewhat controversial paper on the metrication of navigation.

The unique feature of the nautical or sea mile as a unit of distance is that it is directly related to a commonly-used unit of angular measure. To be more precise, the nautical mile may be defined as the length of a meridianal arc of the terrestrial sea-level surface, the geographical latitudes of the end points of which differ by one-sixtieth of a degree. In other words, the sea mile is a minute-mile, and the resulting convenience to mariners is one that is not too lightly to be sacrificed on the altar of the metric system.

The association of the seaman's unit of distance with the unit of angular measure and with the size of the spherical Earth, became a serious consideration soon after the initiation of the Age of Discovery by the Portuguese navigators sponsored by Prince Henry. Before this epoch, and indeed for some time after, seamen used landsmen's units of measure: units derived from homely, but certainly non-scientific, standards such as the length of a thumbjoint, forearm, foot and pace, and in no way related to the size of the Earth.

The inconveniences in trade and commerce caused by a wide variety of units of length and weight must always have prompted men to suggest standardizing these units.

In his note (Journal 21, 81) on this subject, Ronald Turner says ‘No longer will orbital periods of the rotation of the Earth on its axis be measures of time’. This is not so, either in general or in the particular, case of navigation.

Universal Time (U.T.), which is the generally accepted name for Greenwich Mean Time (G.M.T.), continues to be essential for all purposes (in astronomy, geodesy, surveying and navigation) for which are required astronomical observations related to the precise position of the observer on the Earth's surface. The Nautical Almanac must continue to tabulate the positions of the Sun, Moon, planets and stars with G.M.T. as the time-argument; and observations should be timed in a time-system related to U.T., such as the broadcast time-signals of Coordinated Universal Time (U.T.C:).

Mr. Sadler rightly emphasizes one aspect of time: that is to say time is one coordinate of a space-time set of coordinates that locate an event. The other aspect is that of time interval and it is this that the S.I. second defines. Thus if S.I. units are used the time interval between two events is measured in S.I. seconds; the orbital period of the Earth on its axis (referred to a specified frame of reference) is a certain number of S.I. seconds and, in this context, unless this happens to be 86,400 it is surely incorrect to call this interval a day.

Charles Cotter says my note is controversial. I do not think this is so. The controversy lay in the decision to adopt S.I. units. My purpose was to indicate the implications of this decision. The S.I. unit of plane angle is the radian and hence the argument for the retention of a unit of distance related to the minute of arc is not relevant. The problem involved in adapting rhumb-line sailing to S.I. units is not difficult and has already been dealt with with great clarity. The advantages of a single universal coherent system cannot be over-emphasized and it is to be hoped that difficulties will not be prolonged by isolated areas of resistance.

I Was interested to see Mr. Pepper's note in the July issue of the Journal. The intriguing question of Hariot's unexplained approach to a problem that appears to require a knowledge of the integral calculus and exponential functions had attracted my attention when Taylor and Sadler published their paper in 1953. In 1956 a note by me on this subject was published, with a comment by Mr. Sadler, which offers much the same explanation. I, too, was unaware that there was any evidence of Hariot's familiarity with the conformal property of the stereographic projection.

A horizontal angle between two charted objects provides a line of position and two angles between three or four objects will provide a fix or running fix except in the unique case of a ‘swinger’ or ‘revolver’. The method is faster than the conventional three-arm protractor.

A pelorus is frequently used to obtain two or more bearings for a position fix with generally sufficient accuracy. In rough weather the fix may not be particularly accurate, especially if the charted objects are a long distance away, such as lighthouses. If greater accuracy is desired, one solution is to use a sextant held horizontally to measure the two angles between three charted objects, plotting the fix by means of a three-arm protractor. The angles must be less than the 120-degree capability of the sextant and must be taken essentially simultaneously unless the snip's speed is slow. In some cases, it may be found that two objects lie to starboard and two to port, but the angles between the pairs exceed 120 degrees. The three-arm protractor cannot be used in this case. The method to be described will provide a fix or running fix without using a three-arm protractor.

The Forum comments about small craft lights by Bernard Hayman clearly shows up a shortcoming in the original argument put forward. In relegating the single white light to small craft, and then forgetting it, I have failed to develop this particular argument to its conclusion.

Although I cannot agree with Mr. Hayman's reasoning, the shortcoming must be rectified. To this end, may I propose that: ‘Vessels of less than 65 ft., propelled by any means (sailing vessels by definition are excluded from this), to show a single white light, visible all round the horizon for a distance of at least 5 miles. This light to be not less than 9 ft. above the gunwale. These vessels also to show sidelights prescribed in Rule 7(a(ii)), or if less than 40 ft. in length, may show in lieu the lantern prescribed in Rule 7(d).’

An interesting, easy and useful method of finding the observer's distance from a ship, buoy or isolated lighthouse, the height of which is unknown, is by measuring the angle between its waterline and the horizon. To this is added the angle of dip for the height of eye.

From the figure it can be seen that the angle so obtained is equal to the angle at the object between the observer's height of eye and his own waterline.

Douglas Lindsay's article in the Journal (20, 249) raises numerous interesting points and I agree with much of what he says, but he has orientated himself so strongly towards the problem of the big ship that he has recommended a change that would bring complete confusion at the smaller end of the scale.

The most useful points Mr. Lindsay raises are the need for increased ranges for ships' navigation lights, coupled with the dangers of extraneous lights. Both these points apply in their own way to small vessels. However, I disagree with Mr. Lindsay's article where he refers to the meaning of a single white light. He starts his argument by saying:

I Have been following the articles on this subject in the Journal 1 with some interest and hope that the personal views of one comparatively junior officer may be of some interest.

It is implicit in all these articles that, except in the close-quarter situation which the Rules are surely designed to prevent, the sound signals are unsatisfactory. What is not stated in so many words is that, international conferences notwithstanding, Rule 28 (a) is more observed in the breach than otherwise. I cannot of course be sure, but I suspect that this has been so since electric navigation lights superseded oil in general use, thus increasing the recognition range and allowing avoiding action to be taken at greater distances. There is also the natural human, if not legally commendable, desire to let sleeping captains lie.