402 Royal Institution : — 



stations, their respective tension and temperature, moisture or dry- 

 ness, 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 

 from the Atlantic, whether moving direct towards the south-west- 

 ward 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 speed direct being twenty to fifty or eighty miles an hour, 

 while its constant lateral or easterly tendency (like a ship's leeway 

 in a current), being only five miles an hour, 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 other- 

 wise be the velocity of the polar current. 



With the opposite principal current, the equatorial or south- 

 westerly, more briefly and correctly tropical, similar but opposite 

 results occur : the direct motion from a south- westerly quarter is 

 accelerated, sensibly to our perception, by part of the eastward con- 

 stant (about five miles hourly); and therefore a body of air approaches 

 us sooner (other things being equal) from the westward than it does 

 from the eastward. 



To seamen accustomed to navigate in ships making leeway, while 

 in currents setting variously over the ground, such movements, com- 

 plicated as they may appear, are familiar. Another important con- 

 sideration is the disposal or progress of bodies of air united, or mixed, 

 or contiguous to each other after their meeting, either directly 

 opposed or at an angle on the earth's (or ocean's) surface. They 

 do not vanish. They cannot go directly upwards — against gravita- 

 tion ; westward they cannot generally go when there is collision or 

 meeting, because the momentum, elasticity, and extent of the tro- 

 pical " anti-trade* " usually overpowers any direct polar current, or 

 rises over it, and more or less affects the subordinate below, by the 

 friction of its eastward pressure. Downward there is no exit ; east- 

 wardly (towards the east) the accumulating air must go ; and this 

 tendency, continued, causes the varieties of wind from the westward, 

 being more or less mixed, more or less purely polar or tropical as 

 either one prevails in combination. 



After a body of air has passed and gone to some distance south- 

 ward or northward, it may be stopped by an advancing and more 

 powerful mass of atmosphere which is moving in a direction con- 

 trary to, or diagonally across, its line of force. If their appulse be 

 gradual and gentle, only a check occurs, and the weaker body is 

 pushed back until its special qualities, respecting temperature and 

 moisture, are so masked by those of its opponent as to be almost 

 obliterated. But if these currents meet with energy at very differ- 

 ent temperatures and tension, rapid changes are noticed as the wind 

 shifts, and circuitous eddies, storms, or cyclones occur. 



Otherwise, when their meeting is, as first mentioned, gradual, 

 there is the return of a portion of either current (which previously 

 * Sir John Herschel's excellent term. 



