Although meteorology probably has a greater influence on marine and air navigation than any other individual science, many users are not fully aware of the problems of present-day forecasting, nor do they make full use of the services available. This paper is intended to give some idea of the value of observers' reports, the importance of communications, and the problems of present-day forecasting. In particular, attention is drawn to the rapid increase in upper air soundings and the interdependence of surface and upper air observations in predicting the elements for all marine and aeronautical services. Arising from the application of upper air data to surface forecasting, the paper emphasizes the value of applying comprehensive analyses and of marine forecasts prepared by weather centrals ashore; in areas where facilities are not available to the marine user, however, knowledge and experience of single-observer forecasting are still needed.

A description is given of the basis and method of calculation of the new table of refraction included in the Air Almanac for 1943 onwards; the table caters for heights up to 55,000 ft. and all altitudes above the visible horizon.

As already announced the Air Almanac and the American Air Almanac will become a single publication, under the title of the Air Almanac, as from 1953. To cater for the greater heights at which aircraft now fly, and for the possible use of observations at very low altitudes, the refraction table has been completely redesigned. The explanation of the table in the Almanac is naturally restricted to instructions for and illustrations of its use, and, in view of the current interest in refraction, especially at low altitudes, it seems desirable to place on record a full description of the theoretical basis of the table and of the method by which it was calculated.

The theoretical determination of a fix from a number of simultaneous, or near simultaneous, positional data has attracted considerable attention during the last five years. A problem is presented whenever the amount of information obtained is more than sufficient to determine the fix, the most common case being when a set of three position lines is obtained. In this instance the size of the triangle of intersection can be used to estimate whether or not the position lines are likely to be consistent with each other; if they are, some central point inside the triangle is accepted as the fix. In doing this the influence of the position provided by the vessel's past history (the dead reckoning position) is ignored; once the fix is decided on, however, a comparison between the two estimates of position may be of value either in enabling a blunder to be detected or, possibly, in selecting some intermediate point as a position from which subsequent navigation should be based. Although the determination of a position from a number of data is, mathematically, a single problem whether these data include the dead reckoning estimates or not, it is usual navigational procedure to consider first the latest positional information. It is the study of this problem that is the object of this article.

H.M.S. Challenger is one of Her Majesty's surveying vessels. She is a ship of 1140 tons and carries a complement of 96 officers and men. Built in 1931, she is named after her illustrious predecessor in which the wellknown world oceanographic cruise was made in the years 1872–6. This expedition brought back a great wealth of chemical and physical data, and a huge collection of plants and animals from the oceans. These were checked and analysed and eventually, under the guidance of Sir John Murray, who had been a scientist on board, the results were published in fifty folio volumes known as the Challenger Reports. These reports still form, the starting point for the study of the oceans and marine life.

To find out the origins of the nautical mile it is necessary to go back to early times, for, like so many other things, it did not spring up in full bloom in a day, but evolved by slow degrees.

Early man, seeking units of measurement, logically looked about him for something in nature that he might use as a standard. For very short distances he used the width of his finger, the width of his hand, his span, the length of his foot, or the length of his forearm. As these varied between men, so the length of the derived units varied. The foot, for instance, varied from 11 to 14 inches. Even greater variation occurred in the length of the cubit, which originally was taken as the length of the forearm, from the elbow to the end of the middle finger. This unit was widely used, but varied from country to country, and was not even constant within various parts of the same country in some instances.

Men have always been curious to know the shape and size of the planet upon which they live. Most people have heard of the experiment by which Eratosthenes determined the circumference of the Earth, and the principle of his method underlies all subsequent determinations. What he did amounted to finding the difference of astronomical latitude between two places and measuring their distance apart in terms of a standard length (Fig. I). Astronomical latitude may be denned here as the complement of the angle between the vertical at the point in question and the direction of the Earth's axis.

Although the approximate position and strength of the main ocean currents have been known for upwards of two hundred years, it is only in the last few decades that oceanographers have attempted to explain them on the basis of sound physical theory. For many years the progress of oceanography was hampered by the lack of sufficient experimental data; today much new information is being gathered throughout the world, enabling existing theories to be examined critically and providing a basis for new work. It appears from the latest investigations that ocean currents are subject to quite large ‘synoptic’ changes superimposed on the general ‘climatic’ pattern in a similar manner to the movement of air masses in the atmosphere, but on a much reduced scale. This and other developments of theory during the century will be discussed in this paper.

In 1947 it was felt by the Ministry of Transport that although the radar to be installed by the Mersey Docks and Harbour Board at Liverpool would demonstrate the suitability of this type of equipment in meeting the requirements of the larger ports, it was probably beyond the resources of many small ports and indeed was too elaborate for their needs. It was therefore decided that the Ministry's Operational Research Group should survey representative small ports in the United Kingdom in order to estimate the advantages that could be expected if shore radar were to be fitted and to discover what limitations there were likely to be in such fittings. As a sequel to this inquiry, it was proposed to select one port as the site for a trial, to be sponsored by the Ministry, in which a simple radar installation would be employed. The vital importance of having direct radio communication between ship and radar station was fully recognized from the start, and it was assumed throughout the investigation that this facility would be ancillary to the radar, the pilots probably carrying portable transmitter-receivers.

The last term in the formula printed on line 28, page 104 of the April number should read (tilt°), not (tilt°)2.