Alternation of Air Currents.

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subordinate fluctuations, embracing in their whole extent and in different years the longer period referred to." *

It is usual to compare the weather to woman, or vice versa, woman to the weather; but perhaps if we take the trouble to inquire into the apparent fickleness and vagaries both of woman and the weather, we shall find adequate causes for the effects which we complain of, and come to the conclusion that if both of them do vary, they vary regularly, that is, according to certain principles, with which we should make ourselves acquainted. In the matter of the weather, it is the air or atmosphere that is concerned, and the following are the leading facts, as explained by Admiral Fitzroy in his "Weather Book,” which are the starting point of the inquiry :

"In our latitudes there is a continuous alternation of aircurrents, each specifically different, and denoting its approach by marked characteristics; and it has been proved by successive series of simultaneous statical observations over a wide range-embracing Scotland, Ireland, all England, and adjacent islands -that while these alternating or circuitously moving currents are thus incessantly passing, the whole body of atmosphere filling our temperate zone is moving gradually towards the east, at from two to eight miles, or an average rate of about five miles an hour. During strong westerly winds this eastward motion is greatly increased, and in easterly gales it is proportionately diminished, as measured by its passage along a horizontal surface of earth or ocean.

"The facts now weighed and measured mentally, in what may be correctly called forecasting weather, are the direction and force of wind, reported telegraphically to the central station in London from many distant stations-their respective barometric pressure and temperature, moisture or dryness, and their changes since former recent observations. These show whether any or either movement or change is on the increase or decrease; whether a polar current is moving laterally off, passing from our stations towards Europe, or approaching us

*This is the direction of the progress of the wave. That of the wind during the gales which accompany it is at right angles to that direction or from SW. to NE.

from the Atlantic; whether moving direct towards the southwestward with great velocity or with slow progress. If moving fast in the direction of its length, it will approach England more from the east, its direct speed being 20 to 50 or 80 miles an hour; while its constant lateral or easterly tendency (like a ship's leeway in a current) being only five miles an hour, as before stated, is then insensible to us, though clearly deducible from other facts ascertained, and is that much in alteration of actual direction, as well as of what would otherwise be the velocity of that polar current.

"When the polar current is succeeded or driven back by a tropical advancing from a southerly direction gradually, their action united becomes south-easterly (from the south-eastward); and as the one or other prevails, the wind blows more from one side of east or from the other. When the tropical is more from westward, their combination is a westerly wind."

With these preliminaries, which should be always borne in mind, the officials at the Meteorologic Office receive, about ten o'clock each morning (except Sundays), telegrams from about eighteen places round our own coasts, from a few French ports, and from Heligoland. These telegrams report (in cipher for brevity) the state of the atmosphere, including pressure, temperature, with direction and force of the wind, degree of dryness, rainfall, state of sky and sea, at each station. The observations thus telegraphed are immediately reduced, or corrected, for scale-errors, elevation above the sea-level, and temperature, and written out in prepared forms. The first copy, with all the telegrams, is passed to the chief of the department, or the person appointed by him, to be studied for that day's

forecasts.

At eleven o'clock, copies of the report, together with forecasts, are sent out to the Times (for second edition), to the Shipping Gazette, and to the principal evening papers. Copies of the forecasts, but only so far as they relate to weather expected in the Channel and on the French coasts, are telegraphed to Paris (by special request) for the Ministry of Marine,

Basis of the Forecasts.

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The whole of this work is finished by about halfpast eleven, when every one in the department is free to turn his attention to other duties.

Late in the afternoon telegrams are again received from a very few selected stations. Should it appear necessary (which is but seldom), in consequence of this later information, the morning's forecasts are more or less modified, and copies of the consequent report are sent out for the next morning's early papers.

Besides this daily service, occasional storm-warnings, or cautions, are sent to our own coasts and to Paris; and, when it appears advisable, also to Hamburg, Hanover, and Oldenburg, by the request and at the expense of the Government of those States.

The basis upon which the forecasts and cautions (which are merely forecasts symbolized) are founded may be very briefly stated. They are not prophecies or predictions. The term forecast means no more than an opinion resulting from scientific combination and calculation, liable to be occasionally, though rarely, marred by an unexpected "downrush" of southerly wind, or by a rapid electrical action not yet sufficiently indicated to our extremely limited perception and feeling. Doubtless we shall know more by degrees.

The forecasts, then, are mere opinions, but probably the best opinions that can be formed. For it is manifest, from the various telegrams received, that if we know what is and has been occurring around an area several hundred miles in diameter, we are in a position to form an opinion respecting the probable weather in a particular district.

Bearing in mind the preliminary principles which we have given, and considering with Dove, that there are two constant principal wind-currents-NORTHEAST and SOUTH-WEST-of which the characteristics, especially with regard to temperature and degree of moisture or dryness, are totally distinct; all varieties of

wind and weather in these latitudes may be traced to the operation of these two main currents singly, in combination or in antagonism; at times running in parallel lines but in opposite directions, frequently superposed, and occasionally meeting at various angles of incidence. Upon the relative prevalence or failure of either or both of these currents all conditions of weather appear to depend.

The direction of wind is from the region of high barometer towards that of low barometer.

The districts are the Northern Scotland; western Ireland, Wales, and adjacencies; Southern = English Channel and Bay of Biscay; Eastern East

Force of the Wind.

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;

map of the stations named in the Meteorological Reports in the papers:-1. Nairn; 2. Aberdeen 3. Leith; 4. Ardrossan; 5. Greencastle; 6. Cape Clear; 7. Liverpool; 8. Holyhead; 9. Penzance; 10. Brest; 11. L'Orient; 12. Rochefort; 13. Plymouth; 14. Weymouth; 15. Portsmouth; 16. London; 17. Yarmouth; 18. Scarborough; 19. Shields; 20. Helder; 21. Skuddesnoes; 22. Bay of Biscay.

To proceed. It is clear that changes must begin at some places earlier than others, and the observations telegraphed daily to the department from the outports afford the means of forming a very good opinion respecting the nature and probable course of such changes.

Take a recent example. On the 22nd of March, 1866, the following was the state of the area round about us in the meteorological point of view :*.

*We should perhaps state that the designations in the fourth column, marked "Force," show the state of nautical weather, referring to the number of knots that a ship can make per hour, or the extent of canvas which the wind permits her to carry. The meanings are as follows:

The last column, marked "Sea," is interpreted as follows: