The lieutenant and commander

by Basil Hall

Dr. Young and Hadley, the great authorities on the subject, are both wrong in their conclusions.[3] Where Hadley obtains his "experience" he does not tell; but certain it is, that no sailor who ever crossed the equinoctial line could possibly have furnished him with two of his principal statements. The Trades are not strongest near the equator, as he states, nor when they reach that district do they blow along it, or in a parallel direction, but almost always at right angles to it.

If the earth had no motion on its axis, but were surrounded as at present with an atmosphere, and if the sun moved round and round it exactly above the equator, without varying his declination, the following effects would ensue: That portion of the earth lying, say thirty degrees, on each side of the equator, being more exposed to the action of the sun than those further from it, would become much warmer; while the superincumbent air, being greatly heated by the contact, would expand, or become specifically lighter, and would consequently rise. The adjacent air, both on the north and south, being cooler, and, of course, heavier, would rush in to supply the place of the heated air. This air coming from the regions beyond the tropics would, in its turn, be heated, and rise on reaching the warmer equatorial regions, giving place to a fresh supply, which, it is easy to see, must be furnished by the descent of that portion of air formerly heated at the equator, raised into the cold regions of the sky, and forced into a regular circuit by fresh elevations of heated air. All these and many other interesting results are clearly developed in Daniell's Meteorological Essays, a book which every one at all interested in such inquiries will find it advantageous to study. The first edition of this work was published in 1823, some years after these speculations had been forced upon my notice by a long course of service between the tropics.

It will be understood, that, as long as we imagine the globe at rest while this circulation is going on, the course of the lower air along the surface would be directly towards the equator, from due north in one hemisphere, and from due south in the other; while in the upper regions the currents would follow the opposite directions, and stream towards the poles. But the instant we conceive the earth put into rotatory motion from west to east, a change would take place in the course of these aërial currents, both above and below. It must be recollected that a volume of air, when once put in motion, will move on, like any other body, by the mere force of its own momentum, till that motion is destroyed by its friction against the substances along or through which it is impelled. Any one who has observed the ring of smoke sometimes projected from the mouth of a cannon will be sensible that this is true.

It may likewise be of use, before going further, to consider, that, if the globe, instead of being unequally heated; were equally heated at all parts, from pole to pole, and being surrounded by an atmosphere, were then made to revolve on its axis, it would carry the atmosphere round with it exactly at the rate at which it was itself going. That portion of the air in contact with the equator would move about 1000 miles in one hour, while that in latitude 90° would be as motionless as the poles themselves.

From this it will be seen, that, while the equator moves at the rate of 1000 miles an hour, the district about the latitude 30° moves only 860, or 140 miles slower. The average whirling velocity of the earth's easterly motion, in the space between the equator and latitude 30°, may be stated at 950 miles an hour; and that of the belt lying between 30° and 40°, at about 800 miles.

In the hypothetical case, above suggested, of the whole surface being equally heated, and consequently the whole atmosphere at the same temperature, there would be a universal calm, whatever might be the rotatory motion impressed upon the earth. If, however, we next suppose, what really is the case, that the air over the tropical region is more heated than that which is farther from the equator, this rarefied air will instantly ascend, and occupy a place above the colder and denser air, which will flow in from the belts lying beyond the tropics.

When the comparatively slow-moving air of the temperate zone, lying beyond the tropics, first comes in contact with those quicker-moving parts of the earth forming the tropical edges of the torrid zone, the apparent motion of the air from the east, caused by the relative difference of the rotatory velocity between the air and earth, is great, compared to the other motion of the air, caused by its being drawn directly towards the equator, to supply the place of the heated portions raised into the sky. Consequently, at the tropical borders of both Trades the wind is found to blow very nearly from the east point.

Since the cool air of the temperate and comparatively slow-moving zones beyond the tropics is thus drawn towards the equator, and comes successively in contact with parallels of latitude moving faster and faster towards the east, there must be gradually imparted to it, by the increased friction, a considerable degree of the increased rotatory velocity belonging to the low latitudes it has now reached; that is to say, there will be less and less difference of velocity between the easterly motion of this temperate air and the easterly motion of the earth; and, consequently, the wind, as it approaches the equator, will appear to blow less and less directly from the eastward. But, while the earth's rotation within the tropics is thus acting on the slower-moving air which has travelled to it from beyond the tropics, with increased friction at every successive moment, there has been no such powerful counteracting influence in operation to diminish the meridional motion impressed on the air in question; for, although in proceeding from the tropics towards the equator, the wind might, at first sight, be supposed to have its speed somewhat lessened by friction along the earth's surface, the retardation due to this cause, if there be any at all, must be inconsiderable, compared to that which affects the motion caused by the difference in the rotatory velocity of the earth at the different parallels. It must be recollected, also, that there is a constant demand for fresh air from the north and south, to occupy the place of the heated and rarefied air which is raised up in the torrid zone; and this demand being pretty equal, the motion it produces on the air in the direction of the meridian must likewise be uniform.

If it be admitted that all the easterly character of the Trade-winds is due to the difference of velocity between the rotation of the torrid zone of the earth from west to east, and that of the air impressed only with the slower rotatory motion to the east of the temperate zone, it will follow, that, if this difference of velocities between the earth and the air in contact with it be diminished or annihilated, the easterly character of these winds will be diminished or annihilated likewise. At the same time, there is no cause in operation, that I can discover, to alter the direction of the meridional motion, as it may be called, of the Trade-winds, or that by which they are impelled directly towards the equator.

At first starting from the temperate zone, on its voyage to the equator, the cold air of that slow-moving region is impressed with a rotatory velocity of only 800 miles per hour to the eastward, but it soon comes over parts of the earth moving more than 100 miles per hour faster to the eastward than itself. The difference of velocity in the earth's rotation between latitudes 30° and 20° is 74 miles an hour, while between 20° and 10° it is only 45 miles, and in the next ten degrees the difference in rate per hour is reduced to 15 miles.