Daily weather charts had made us familiar with the progressive movements of subsidiary, small, anti-cyclonic systems, and, theoretically, there was no reason why the larger and more persistent systems, which had shown themselves capable of resisting for weeks at a time all efforts to break them up, should not move off, rather than disperse, when the conditions became unfavourable. If it should prove that they had a progressive movement, it was natural that their persistence at the extreme ends of their swing should attract attention, for they would appear to linger there, in going and returning, for a longer period than anywhere else in their journey. All available weather charts from 1876 onwards were accordingly examined, and for the 11 years, 1881-91, the mean monthly positions of the cores, or centres, of the Siberian and Atlantic systems were plotted whenever sufficient information could be obtained. The conclusion arrived at was that there was a distinct progressive seasonal movement. In the case of the Siberian system this was to the west in the early months of the year, and to the east in the later months, and the persistence which had been observed to occur annually in winter for such a long time over Eastern Siberia was, as anticipated, in consequence of its reaching the extreme end of its swing whilst in that district. The Atlantic system showed a movement on to and off the land, according to the seasons. This fact will be found to be one of special interest to us when we come to consider similar movements off and on to the land in the southern hemisphere. Another important feature was disclosed by the examination. The tracks followed by the Siberian system were not the same each year, nor was the date of arriving at and leaving the extreme winter and summer positions the same. There were good grounds for thinking that the track was deflected north and south of a mean or average track till it reached a maximum, when it gradually returned to the mean again; and, further, that the date of reaching the extreme limits of its swing was sometimes before, and sometimes after a mean or average date. The whole of the movements seemed to be governed by some law of which we have no conception whatever, and in no case were the successive deviations from the mean abrupt. One principle seemed to govern not only the movements, but the formation of the various anti-cyclonic systems. Wherever strong contrasts of temperature were found to be set up from any cause, atmospheric conditions resulted which gave rise to anti-cyclonic systems and largely controlled their movements. It was not difficult to find numerous charts by which to put this to the test. In the " Synchronous Weather Charts of the North Atlantic" for the period August 1st, 1882, to September 3rd, 1883, we have a daily record of the isobars, the isotherms of air and water, and the resultant weather, and with the help of these an examination can be made under the most favourable conditions of anti-cyclones and their movements. Access was accordingly obtained, by leave of the Royal Society, to the original ships' records from which the charts were compiled. The difference between the shade temperature of the air and of the sea at noon was plotted, and a comparison made with the completed

synchronous charts of the isobars. A very decided result was obtained. Without exception the anti-cyclones lay over those parts of the Atlantic where the temperature of the sea was lower than that of the air, whereas the cyclonic systems were confined to those parts where the temperature of the sea was higher than the shade temperature of the air. Further, where the strongest contrasts of temperature prevailed, there would be located the core or centre of the anti-cyclonic system. Numerous instances were found in which the core of the system would move away from a locality, losing intensity as it did so, but as it returned to the same spot many days afterwards it would gradually increase to its original intensity, and resume its control over all the weather conditions in the neighbourhood. Applying these results to the progressive movements of anti-cyclones, generally, we have strong reasons for believing that anti-cyclones will by preference move to those localities where they will find the surface temperature of either sea or land colder than that of the air above it; and will follow more readily such a track, from the extreme summer to the extreme winter position, as affords a maximum of these congenial conditions. This is one of the reasons why the track must vary in different years, especially when it leads across mountain ranges on which the snow lies longer in some years than in others, or across districts to which rivers bring the rainfall from a distance, and in varying quantities.

At the Bristol meeting of the British Association in the summer of 1898, Mr. Douglas Archibald brought to notice a set of 20 types of weather systems which had been found to prevail over a great part of the northern hemisphere. They were the result of an exhaustive examination of synoptic charts by Professors Van Bebber and Köppen, and they excited considerable attention, because their authors were able to announce that not only do such types persist for 4 days on an average, but that they exhibit a tendency to succeed one another and to be associated in some cases with one another. In preparing the types, they had analysed charts extending over several years, tabulating the number of days each type had appeared, and the number of times and the manner in which the types had been associated amongst themselves. In this way it was proved that some of the types belonged to certain seasons, and would at some seasons precede, and at others follow, those with which they were most closely allied. On looking through the author's original work for an explanation of these points, I found that they had attempted none. They had made an analysis of very great value and had left any lessons to be drawn from it to others. Coming so shortly after the paper I had read before the Royal Meteorological Society in July of the same year on the progressive movements of anti-cyclonic systems, it had a special interest for me. It offered a chance of testing the theory in a conclusive manner. If the types which were found to precede those they were allied with at one season, proved on examination to return after a sufficient interval and follow the same types at a succeeding season, such a fact would be a strong confirmation of the existence of a regular succession of types controlled

by a persistent force. And if it should further prove that anticyclonic systems occupied a predominant place in the types, their progressive movements might be traceable in the charts. (Here followed 14 lantern slides illustrating the types, and showing that those found to be associated with one another supported the view that the anti-cyclones which controlled the distribution of pressure had a progressive movement).

Let us now consider the lessons which these types teach us regarding the distribution of pressure which we find prevails near South Africa. The principles governing the distribution are the same They may be summed up as follows:

in each case.

I.

The high pressure systems, which are known under the name of the permanent anti-cyclones, have a seasonal migration north and south with the sun.

2. There is a movement off the land on to the ocean in summer; and on to the land from the ocean in the winter, but the Atlantic system seldom entirely deserts its position. It is owing to this system, and to those like it which prevail over the oceans, that the migration north and south with the sun has come to be recognised.

3. At certain times during the winter a continuous belt of high pressure is formed round the globe, which is due to the merging together of all the systems. Each of them, nevertheless, has an individuality which it never loses, and the systems will finally separate and dominate the area to which they belong.

Whether the formation of an anti-cyclone gives rise to that of a cyclone, or vice versâ, and what the connection is between the two are still moot points, but it is certain that the two are concomitant, and that where an anti-cyclone prevails a cyclone cannot rage, nor exhibit its objectionable characteristics. It can, and does, however, show by various indications, especially in the sky and by changes of pressure, that it is in existence; and in the writer's opinion it is sometimes superposed above, or partially above, the anti-cyclone. In this way we may account for many of the anomalies which are being disclosed by observations with kites in the upper levels of the atmosphere.

4. Further, these types support the hypothesis that has been advanced, that the changes which are observed in the distribution of pressure are the result of a regular progressive movement of the permanent anti-cyclonic systems from their extreme winter to their extreme summer positions and back again. Secondary and minor. anti-cyclonic systems are formed which have irregular movements of their own, quite irrespective of the general progressive movement of the more permanent systems of which they are off-shoots.

SOUTH AFRICAN ANTICYCLONES.

South Africa has two well-defined anti-cyclonic systems near it; one to the west which, like the system over the Atlantic in the northern hemisphere, prevails very generally over the South Atlantic and seldom deserts it entirely, and the other to the east, which will be

The

referred to as the Australian system. Not having as yet any Marconigrams to report their movements at sea, we must turn for assistance to such charts of pressure as we possess, and to those of currents and hurricanes. In Buchan's excellent set of synoptic charts of atmospheric pressure, the mean positions of these systems appear for each month of the year. The charts were compiled from the best records which were available for the 15 years 1870-84. movements of anti-cyclonic systems which have been traced in daily synoptic charts of the northern hemisphere, help us to fill in the movements of the southern systems from one month to another, as follows:-The Australian system reaches its extreme easterly position off the west coast of that continent about February. From about November to February low pressure has prevailed, for the most part, both over Australia and South Africa. From March to May the Australian system travels westwards over the Indian Ocean, approximately within the parallels of 30° S. to 40° S., always migrating north, as a whole, with the sun. During the same months the South Atlantic system, which had previously been in a central position in mid-Atlantic, has moved eastwards and has extended across South Africa, trying to merge with the Australian system either in April or May. A complete belt of high pressure, similar to what exists during mid-winter in the northern hemisphere, is being firmly established across South Africa. In July the commencement of the return of the two systems to their summer positions begins to be traced on the charts. It is noticeable that just as we saw in the types high pressure systems identifying themselves in winter with certain regions of America and Europasia, so we find in the southern hemisphere certain similar high-pressure systems identifying themselves with regions of South Africa and Australia.

In addition to Buchan's charts, we have a series published by the authority of the Meteorological Council of Great Britain, which brings our knowledge of the movements of these anti-cyclonic systems to the year 1900. When the two are compared we find that in their main features the progressive movements are the same in both, but there is one difference which is of great importance in its bearing upon the second point in our hypothesis regarding the departure of the anti-cyclones from a mean path. In the chart for July the maximum pressure is found in recent years to be lying much further east than previously, and to be to the south-east of Mauritius. The Atlantic system is merged with it right across South Africa, without any centre of pressure or core in mid-Atlantic, as was the case in July up to the year 1894, at any rate. This supports the view which has been advanced, viz., that these systems not only depart from a mean track in going from and returning to their extreme winter and summer positions, but that they sometimes are early, and sometimes late, in leaving them.

I mentioned in explaining the types that in the northern hemisphere winds blow round an anti-cyclone centre in the same way as the hands move round the face of a clock, while in the southern they blow the reverse way, or anti-clockwise. Charts of ocean

currents help us in the following way. If the core leaves the ocean there will be no centre for the winds to blow round. The currents, which are the direct result of the persistent winds belonging to these permanent systems, will slacken if the core remains away a sufficient time, and the charts will record a change in velocity. If the system is far from its mean position we have no difficulty in believing such reports as we are hearing now, that a current like the Gulf Stream is changing its course. Reliable indications of the mean positions of the cores of the Atlantic anti-cyclonic systems in both hemispheres are given by the monthly charts of the ocean currents. There is no necessity, in order to understand the charts, to go beyond what these mighty weather controls the permanent anti-cyclonic systems of the globe tell us. In this connection I would bring to your notice the reversal of the Agulhas current, close in-shore along the south coast of Africa, which appears in these charts in certain winter months. As far as my investigation has at present gone, this fact is at once the proof that an anti-cyclone system prevails over South Africa during the months when the reversal takes place, and an indication of the position which that system occupies. I would urge upon the Observatories of Cape . Town and Durban to give their attention to this point, for if the connection between the two facts can be established, it would enable warnings to be issued beforehand, which may be the means of saving many a ship. Only recently there was a wreck which was held to be due to abnormal currents along the south coast.

PROGRESSIVE MOVEMENTS.

In the absence of daily synoptic charts to help in tracing the progressive movements of anti-cyclones, we must turn to all the data that have been collected regarding winds, rainfall, temperature, pressure, cloudiness, and humidity.

That there has been some change in the distribution of pressure over South Africa in recent years is shown by the continued drought which has prevailed over some parts, especially the Transvaal and the Orange River Colony. Moreover, we have in Mr. C. M. Stewart's important paper on South African Meteorology, contributed to the Association in 1903, strong proof of the same fact. He pointed out that an investigation of the tri-daily observations made at the Royal Observatory during the period 1896-1900 led him to very differen: conclusions respecting the prevalent winds, from those given by Dr. Buchan in the Challenger Report for the 18 years 1842-55, 1862-65.

A comparison of the two tables, month by month, in which the percentage frequency of the winds from the 8 principal points of the compass is given, indicates that whereas the Challenger results make the South to be the prevailing direction of the wind in every month of the year, Stewart's analysis shows decided traces of a "monsoon influence; in that during the winter months, June-August. the prevailing direction is north-westerly, while during the rest of the year it is southerly. Both tables indicate (1) that in every month the