The 10-in Douglas protractor can be modified to reduce the time taken to plot astronomical sights in the following manner. Two narrow slots are cut parallel to the central E.–W. line, and ¾ in each side of it, from the central N.-S. line to the western margin of the graticule.

The writer has drawn the accuracy contour maps of a ship's position fixed by cross bearing, horizontal sextant angle method and Loran aid. The accuracy of the fixes has been evaluated in terms of the probability density.

Few aspects of geography are more satisfying than to follow the progress of geographical discovery across the oceans from the documents that remain to us, and the careful work of many scholars allows us to find some of the excitement of the early voyages without much effort to ourselves. But geographical exploration was only the first phase, and during the past 300 years much effort has been spent in finding out something about the oceans themselves: as early as 1698 Halley sailed as far as S. 52° on a purely scientific voyage, returning with useful information about magnetic variation and the trade winds. The promoters of the scientific voyage of H.M.S. Challenger argued that the vast oceans lie scientifically unexplored, and we say much the same today; but much has been done and in its way it provides a story as interesting as that of geographical discovery.

Two practical measures might go a long way at the moment towards solving some of the questions raised by Captain F. J. Wylie in his paper (Vol. VI, p. 271) and discussed in the last number of the Journal (p. 202). The first is the provision of a suitable plotting device. It should not be too difficult to evolve a mechanism whereby a strip of transparent tracing material is passed across the face of the radar screen by means of two rollers (similar in operation to the echo-sounder paper passing over the plate in the modern sounding machine) in a direction parallel to the ship's course and at a speed representing that of the vessel and proportional to the scale of the radar picture in use. Plotting would be done by marking the ship's position and all other objects on the screen at short intervals and joining the consecutive positions of each object with a line. There would then be a continuous and up-to-date true plot of the area, which would be no more cluttered up with plotting than the actual area was with ships and objects.

Captain G. C. Forrest's letter (this Journal, Vol. VII, p. 203) raises some interesting points well worth discussion:

‘(1) That radar-using ships, taking broad evasive action, shall alter course to starboard only, and not at all once they are within five miles of the other vessel, though they may reduce speed or stop.’

The following discussion, here printed in summary, took place on Mr. E. S. Calvert's paper ‘Visual Judgments in Motion’, which was presented at an Ordinary Meeting of the Institute on 21 May. The chairman was Air Chief Marshal the Hon. Sir Ralph Cochrane, G.B.E., K.C.B., A.F.C. The paper was printed in the last number of the Journal (July).

The Chairman: Am I right in thinking that I.C.A.O. have accepted the Crossbar system of lighting? Is it the normal pattern in international airports?

The Nauticator is a vectorial computer for the solution of spherical trigonometrical problems, designed by Captain Leiv Jernæs of the Royal Norwegian Navy, who is at present captain of the training ship M.S. Sjöveien, Oslo. The instrument has been designed particularly for the rapid reduction of astronomical sights, and the calculation of great-circle tracks, distances and vertices. It has been sent to the Institute to assess its value, and this note will discuss its value to the air navigator.

A total eclipse of the Sun provides an opportunity, rare though it may be, of obtaining an instantaneous fix from the Sun alone. Eclipses vary greatly in character, in position on the Earth, in the width of the path of totality, in the duration, and also in the direction of the path. However, the shadow of the Moon cast by the Sun is always a right circular cone which, in the case of a total eclipse, intersects the Earth's surface at some point before its vertex. Owing to the motion of the Moon in its orbit round the Earth the shadow moves at a speed of about 2000 m.p.h. from west to east (it varies considerably according to the distance of the Moon from the Earth).

This paper is concerned with measures which may be used to reduce the effect of echoes from the sea surface and from rain (sea and rain clutter) on radar displays so that targets which give a reflected signal power greater than the reflected clutter power can be detected. It should be noted that a conventional PPI display is fully brightened by signals of amplitude about three times the level of the inherent noise background, so that once the clutter has exceeded this limit it will appear on the display as a fully bright patch in which signals can be lost, even though their amplitude at the receiver input may be many times greater than the amplitude of the clutter.

In about 1946 work was begun both in the United States and in the United Kingdom on the development of radar beacons which could be used to make certain required points more conspicuous and to increase their detection range.

Two basic types of beacon were considered: ramarks, which are intended to give bearing information only and which appear on the PPI as a bright radial line passing through the position of the ramark, and racons, which give range as well as bearing information. The range information is given by die beacon transmitting one or more short pulses, the first of which appears on the PPI at the correct range and bearing of the beacon.