The sailor will not be kept from the sea, even though empires fall and foreign invaders multiply. Yet we cannot expect to hear much of him in such troubled times. We know, however, that although European ships no longer sailed to India, yet, after the barbarian destruction of the Roman Empire, overseas trade did revive, and the foundations of such famous maritime states as Venice were laid. We know, too, that although the Arabs overran the whole length of the Mediterranean Sea, they were pushed back out of the islands by sea-borne expeditions from Italy and Catalan Spain, while when the Holy Places in Palestine were captured by the Turks (who were not ‘gentlemen’ like the Arabs), there were ships and sailors ready and able to carry crusading armies to the East, and to provision them while they were there.

This paper is concerned with two different problems. The first of these is to correct a magnetic compass so that the essential relationships for zero deviation are maintained even when the ship rolls and pitches. The Harvey-Raynes corrector is described and its advantages are explained. This corrector makes it possible to steady the compass at any time without affecting the accuracy of the existing deviation card.

The second problem considered is the automatic correction of deviation for transmitting magnetic compasses. A system of ‘point-by-point’ correction is developed, based upon the representation of the deviating force by a truncated Fourier series; it is shown that such a truncated series may be represented mathematically in a form immediately suitable for practical application, some such forms being described in detail. It is shown that by means of a small additional device the ‘point-by-point’ type of corrector may be made to correct both variation and deviation, so that for any given part of the Earth the transmitting magnetic compass may be made to point true north.

The paper is divided into two parts.

Part 1, under the heading of Non-standard Propagation, concerns variations in the apparent performance of marine navigational radar in its use for detecting shipping and land targets. The effects of changes in the density of the lower layers of the atmosphere on radar transmissions, their distortion of radar coverage for better or for worse, and resultant errors in the evaluation of echoes are here examined.

Part 2 describes the way in which weather echoes may influence and restrict radar-assisted navigation. The attenuation of field strength with various types and intensities of precipitation, and the effects of these factors on the navigational and collision-warning uses of radar are examined. The detection, tracking, and application of weather echoes as an aid to short-range weather forecasting are also covered.

The basic principles of Consol have been explained in a previous article; the factors affecting the range and accuracy of the system are here discussed. A summary is given of the results obtained practice, and the article concludes with a brief discussion of developments which will lead to improved performance.

The principles of radar-meteorology have been described by Lieutenant Satow in his paper. This note contributed by the Technical Secretary suggests a system for the collection of data from which a statistical evaluation of one aspect of the subject may be made. Bound copies of this paper with 25 log sheets may be obtained on application to the Institute, price 1s. 6d.—Editor.

The effect of instrument errors in the basic pressure pattern formula (I) has been discussed. Calculations have been based on a 150 n.m. distance interval between altitude determinations, and the permissible instrumental tolerances are wider if the length of this run is increased. A particular latitude of N. 50° has been assumed, but this involves no loss of generality since ageostrophic errors and those due to the latitude term in the formula are believed to be compensatory. In general, both ageostrophic errors and pressure altimeter errors increase together with altitude, and if instrumentation is adequate for 10,000 feet (the level for which the probable error of 3 knots holds good) it will be so at higher levels. It appears that satisfactory results may be obtained by using a Mk. VI radio altimeter together with a Mk. XIV Kolsmann pressure altimeter, and taking the mean of four observations of D at the beginning and end of the run.

For many years past great efforts have been made to devise methods of obviating the necessity for making navigational calculations by logarithms, on the grounds that the logarithmic process is too slow for modern navigational purposes. For some such purposes it undoubtedly is; but all these short methods suffer from some disadvantage or other, and when the preliminaries (which generally involve calculating the hour angle) common to both are completed, the difference in time between completing the sight by inspection and interpolation, and by logarithms, is not so great. For this reason it seems possible that logarithms would still be considered preferable for a wider range of purposes than at present, were it not that most of the volumes of logarithms still commonly used for navigation are designed for more accurate calculations than are necessary or justifiable for the practice of navigation, and hence are unnecessarily cumbrous and slow in use.

It is the present usage by the National Physical Laboratory and the Trade to give, on its Certificate, the ‘constant’ (graduation and centring) errors of a sextant only at stated points on the scale. If these errors are of a magnitude which is worth while applying as a correction, this usage is not quite what the user requires. The N.P.L. Class A Test, issued for instruments reading directly to 10″ (or o′·2 in the case of decimal subdivision) allows combined graduation and centring error up to 40″ or 4 scale divisions of the micrometer or vernier; the Class B Test for instruments graduated to 20″ permits such error up to 2′ or 6 scale divisions. Thus, even with instruments that pass these tests, it may be worth the marine navigator's while to apply the error as a correction. What he wants to know, however, are the limits of such errors as are worth his while to apply. In other words, he requires a Critical Table of Corrections, of the type used in the Air Almanac and in certain mathematical tables by L. J. Comrie, by whom, it is understood, the method was introduced.

When astronomical sights have been reduced by means of tables calculated for a fixed equinox, it is necessary to make a correction for the precession that has occurred since the date for which the tables were calculated. In the Astronomical Navigation Tables this correction is applied to each intercept separately. It is, however, possible to apply the correction once only to the fix obtained. This would probably be simpler, particularly with such tables as the Experimental Air Navigation Tables and H.O.249.