Inertia is that property of matter which opposes change of motion. Inertial navigation systems utilize this property for the detection and measurement of linear accelerations and rotations in order to arrive at velocity change and displacement relative to some known initial state in a particular frame of reference.

Most readers of this Journal will know a good deal about the long-established practice of setting adrift in the sea sealed bottles or other watertight vessels containing messages or questionnaire papers. There are numerous records stretching over a very long term of years of this having been done with the aim of conveying information—often in a jovial sort of way, but sometimes when it was feared that a ship seemed likely to be lost. In the latter case to which very few authentic records relate, mariners fearing the worst adopted the practice for obvious enough reasons. In the former case, records of the practice are legion and, a century or so ago, it was a common thing for floating bottles with papers inside to be flung overboard from outbound emigrant ships. The habit was still fairly common in much more recent times, and the writer once had quite a goodly number of missives recovered from stranded beer and wine bottles which, after the conclusion of a farewell party, had been thrown out from ships bound for America from the Elbe and other departure points. Generally speaking, such corked bottles were used empty (apart from the enclosed message), and so would perform journeys decided in large part by the winds. Where a serious intent lay behind the launching of a bottle designed to convey true news of distress, no very strong hopes could ever have been reposed in the glass messenger, since there could never have been any idea as to when it might go ashore and be found. It is probable that many bottles carrying messages of the sort have, over the years, been broken on rocky coasts or have stranded on unvisited shores perhaps to suffer burial in shifting sands.

This paper gives a description in very general terms of where and at what seasons may be found the strong winds which occur in the upper atmosphere above 15,000 ft. and discusses briefly the clear air turbulence which is sometimes found in and near these strong winds.

Jet streams of temperate latitudes. Navigators are now well aware of the comparatively narrow bands of strong winds, known as jet streams, which occur in the upper troposphere in temperate latitudes. These jet streams, which can stretch for over 2000 miles on occasions, are very variable in position and in strength. It is probably true to say that there is a jet stream with wind speeds exceeding 60 kt. somewhere between Jan Mayen (N. 71°) and Gibraltar (N. 36°) every day, except in quiet summer weather and exceptionally on quiet days in other seasons. Fig. 1, from Durst and Davis shows a typical jet stream; these winds are associated with fronts though the fronts are often of the dry type with very little associated weather. For example, the strong northerly jet stream over the British Isles on 14 November 1949, with speeds of over 150 kt., occurred over a surface anticyclone; it was in this jet stream that the Comet I, on an early flight, experienced very severe turbulence. The level of the maximum wind in these jet streams is about 30,000 ft. though the exact height varies from case to case. Since they are so variable in position and strength the navigator must take particular note of them and a close liaison with the forecaster is clearly essential.

In the early days of civil aviation, aircraft flew too low to encounter jetstreams. Later they were no doubt encountered, when aircraft were flying at medium heights of from 10,000–20,000 ft.; however, the infrequency of astro fixing in those days would generally disguise a high-velocity wind by meaning it with low-velocity winds, producing resultant winds of medium velocity. Occasionally, however, very high winds would be experienced and the unexplained displacement of position would be attributed to a navigational error or some other cause of that nature.

The pilot or navigator expects the forecaster to supply him with: (a) forecasts of wind and temperature; and (b) forecasts of meteorological hazards, e.g. turbulence, icing. The increase of operating height and range of modern aircraft make it increasingly difficult to maintain a high standard of accuracy of forecasts.

This paper, which gives a useful description of the procedures used by one transatlantic operator, was presented in Montreal at a meeting of the Canadian Institute of Navigation held on 7 December 1955. Many of the methods discussed are referred to in the preceding three papers.

1. Any discussion of today's long-range navigational practices and problems seems to lead inevitably to pressure-pattern and jet-stream flying. The reason might well be that during recent years the more important practical developments appear to have concerned techniques rather than aids or instruments.

The history of navigation and hydrography, as of other crafts or ‘mechanick arts’, has to be constructed from very imperfect materials. Instruments and charts, as the tools of practical men, were commonly discarded when they were worn out or superseded. The high mortality among original charts makes generalization in the comparative history of cartography hazardous. That no Portuguese charts before 1500 are now known does not allow us to conclude that none was made but only that none has survived. Many of the early charts that have been preserved are decorative examples drawn for royal or other patrons; those by which pilots conned their ships had a smaller expectation of life and are correspondingly rare.

Although no details of the proposed artificial satellites are yet available, the idea is by no means new, and it seems likely that any successful experiment of this kind must follow the plans already outlined by Professor S. F. Singer of Maryland University. At the Astronautical Conference at Zürich in 1953 he gave details of a small spherical satellite, weighing only 100 lb., which would revolve about the Earth in a period of 90 minutes at a height of 190 miles above the surface. In collaboration with Arthur C. Clarke and A. V. Cleaver of the British Interplanetary Society, Singer called this instrument MOUSE (Minimum Orbital Unmanned Satellite Earth) and proposed that it should be launched as a three-stage rocket with an initial all-up weight of about 35,000 lb., and a thrust of 65,000 lb., which is very little different from the performance of the V2 rocket. The first step would take the rocket vertically through the lower atmosphere and then turn over to begin the inclined part of its flight; the second stage would come into action when the first had burned out, and complete the climb to give the correct altitude and direction of motion. The third stage would merely give sufficient boost to attain the required speed, which is about 17,400 m.p.h. At the moment of cut-out (controlled from the ground) the nose tip would open and release the satellite. This would take the form of a sphere, about three feet in diameter, spinning so as to maintain a constant orientation, and packed with instruments. The spin axis, parallel to the Earth's surface and perpendicular to the direction of motion, would point to the Sun, so that one half of the sphere would be strongly heated by the Sun's rays, and the other hemisphere would be extremely cold. Power would be obtained from a new type of solar battery, and the readings of the instruments would be radioed continuously to Earth.

The first scientific instrument to be designed specifically for use at sea was the mariner's astrolabe. The cross-staff or Jacob's staff had been used by astronomers since the Middle Ages, and the magnetic compass used by navigators had long been familiar as an aid to land travel, a means of setting portable sundials, and in the east, a device for divination.

Lieutenant-Commander Bini's interesting paper on the use of bisectors shows the trap open to the believer in the 3-line fix and goes on to show the M.P.P. when four lines are used. However, as the ideal for the four lines is to have each at 90° apart, an error in either one or two lines provides the observer with an ellipse for the M.P.P. and is for practical purposes but a variation of the 3-line cocked hat circle of M.P.P. and little appears to have been gained by the working of the fourth line.

Air traffic controllers separate aircraft on the North Atlantic with reference to dead reckoning (D.R.) positions of aircraft. Two such D.R. positions are transmitted to a.t.c. regularly from each aircraft, one usually 10 to 30 minutes following a fix, the other one hour's run farther ahead. The accuracy of these positions is important, since it has a direct bearing on horizontal separation standards and optimum time between position reports.

The method described in Schütte's paper is very ingenious and evidently simple in its application; it depends on solving two pairs of equations of identical forms (in terms of different variables) by simple inspection of diagrams constructed in accordance with the first pair of equations. In his paper the author states, with reference to a figure and without proof, the two pairs of equations referred to. It is obvious that the fourth of Schiitte's equations (p. 310) is inconsistent with the figure and with the example he gives on the determination of altitude and ‘azimuth’ (pp. 312, 313). The trouble arises partly from misconception as to the use of the term azimuth. The method deserves a more detailed explanation of the derivation of the formulae and their application.

In his detailed and valuable account of the history of the Log (this Journal, 9, 70) Commander Waters is surely at fault as regards the part he ascribes to the Gresham Professor, Gunter. This eminent mathematician delighted in designing instruments with engraved scales, and at first issued the customary Description and Use in Latin, a sufficient indication that he wrote only for fellow scholars. Importuned probably by the instrument-makers, he later published English versions of the tracts (in 1623), and among the numerous ‘uses’ included a couple of chapters on those for navigational purposes.

Professor E. G. R. Taylor's review, in the April 1955 issue of this Journal, of my book A History of Marine Navigation has passed unnoticed by me during a long absence, and I beg leave to offer a few comments.

The reviewer's introductory remark is indisputably correct; the title is not an adequate one. The Swedish title, literally translated, is From Noah's Dove to the Gyro-compass: navigational art, ancient and modern. In the earlier chapters I have not followed a chronological sequence but rather proceeded in order of the degree of complication of the methods used. In the later part of the book, I admit that there are many lacunae. However, as the reader will find, there was no ‘final jump’ to electronic navigation; there were several stepping-stones, viz. wind- and current-charts, the gyro-compass, radio-location, and echosounding.