During 1943 there was produced, for trial purposes, a limited edition of the so-called Experimental Astronomical Navigation Tables (E.A.N.T.s). For various reasons these tables were not adopted for general service use in the Royal Air Force, and copies are no longer available. The tables possess some points of interest, which are described below for the first time.

In an earlier paper it was suggested that developments in air navigation had created a requirement for a new form of air plotting chart. The chart illustrating the present paper was produced experimentally by one of the airlines to fulfil this requirement. The main factors taken into account in designing the chart were as follows:

(i) The air navigator is always working against time. He therefore requires his plotting chart to show as much navigational information as possible, so that he need not spend time looking things up in books or transferring information from other charts.

(ii) The scale must be limited by the requirement for easy handling in the cockpit, especially when the chart is intended for use in aircraft that have no navigation table.

(iii) The design of a chart that has to show as much information as possible on a small scale is chiefly a problem of avoiding congestion. The items that might be shown therefore require careful consideration and only those that make a justifiable contribution to the chart's navigational purpose should be included.

(iv) When flying under visual flight rules (VFR) the navigator's problems are less acute than under instrument flight rules (IFR). The chart should therefore be designed for IFR; it will always be adequate for the easier case of VFR.

In view of the appeal for observations that has been issued by the Institute to assist in the determination of the accuracy of astronomical observations at sea, it may be of interest to navigators to recount some of the experiences of a hydrographic surveyor. Large regular surveys usually depend on geographical positions which are obtained on shore with accurate instruments. In cases where surveyors fix these positions with astrolabes, or occasionally theodolites, three observers will probably have taken observations of some thirty-two stars each. In the analysis, intercepts of the order of 4–6” are obtained, and the final position is probably accurate to about 1”. Some hydrographic surveying, such as a one-day search for a reported shoal out of sight of land, various minelaying and minesweeping operations out at sea, and the determination of oceanographical stations on long passages, however, demand ordinary sights taken in the normal way at sea. The value of the hydrographic operation may depend on the accuracy of these sights, and therefore the report on the operation generally attempts some assessment of their accuracy. Considerable care is taken to achieve accuracy and often the ship is stopped and the sights referred by range and bearing to a moored beacon, even in depths of 500 fathoms. Methods of determining the accuracy obtained may be by taut-wire runs from known points of land or taut-wire distances interconnecting several beacons. Failing, or in addition to, these methods, two to four observers' results may be compared, or Sun and star sights can be referred to the same beacon; even short tautwire or patent-log runs to well-defined and previously well-fixed fathom lines have been used.

Although the Astrograph has never been formally described in print, the basic principles of the instrument are sufficiently well known to require no further elaboration; so also are its limitations. Over many years, numerous proposals (but mainly unpublished) have been made for the extension of the Astrograph to cater for the Sun, Moon, and planets, as well as for a larger number of stars, but none have been really satisfactory. Without such improvements the Astrograph must have a very limited use, and accordingly a further study was made some time ago of its application to the Sun, Moon, and planets. Methods were developed on familiar lines for shifting (and rotating) the star curves for a standard declination to cater for neighbouring declinations, but these introduced complexities that were clearly unacceptable.

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.

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 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.

Equation (2) in the paper by J. Th. Verstelle on page 300 of the July number of the Journal should read The formula quoted on p. 301 for deriving the mean square error in the distance made good should read metres.

Refraction at Sea

The mean error quoted on line 8 of Captain Brett Hilder's paper (Vol. VI, p. 314) should read 1·2 miles, not 12 miles.