1862.] on Meteorological Telegraphy, 451 



central station in London, from naany distant stations ; their respective 

 tension 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 from 

 the Atlantic — whether moving direct towards the south-westward 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 otherwise 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 &o\\i\\-w ester ly quarter is 

 accelerated — sensibly to our perception — by part of the eastward 

 constant (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, 

 complicated as they may appear, are familiar. Another important 

 consideration 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 gravitation ; — 

 Westward they cannot generally go when there is collision or meeting, 

 because the momentum, elasticity, and extent of the tropical "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 — eastwardly (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 contrary 

 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 dift'erent temperatures and 



* Sir John Herschel's excellent term. 



