"Which has got hold of us rather, I should say, by the way it is carrying us along," answered the captain, smiling.  No one knew Jerry so well as he did, though he often pretended not to understand at what he was driving.  "You ask a question to which it is rather difficult to reply in a brief way.  Take a piece of paper; draw a circle on it; now, draw three parallel belts across it--one in the centre, and one on each side of the centre.  Write on the centre belt, `Equatorial Calms;' on the upper, `Calms of Cancer;' on the lower, `Calms of Capricorn.'  The circle represents the globe; the ends of a line drawn at right angles to the belts where it reaches the circle, mark the poles.  The globe moves from west to east.  Now, suppose a mass of air sent off from the north pole towards the equator in a straight line, it not partaking of the diurnal motion of the earth would appear as if it came from the north-east.  Another mass starting from the equator towards the pole in consequence of the impetus given it, would be going faster towards the east than the earth, and would, consequently, appear as if it came from the south-west.  This actually takes place, but in the upper regions of the air.  The same exchange takes place between the south pole and the equator.  Now, let us see what becomes of these masses.  That which started from the north pole meets in the air at about the parallel of 30 degrees; the mass which started from the equator meeting with equal force, they balance each other, and produce a calm and an accumulation of atmosphere pressing downward, and ejecting from below two surface-currents--one towards the equator, which are the north-east trade-winds; the other towards the pole, called the south-west passage-winds.  This moving mass of air, which constitutes the north-east trade-wind, meets near the equator with another mass which has been moving on as the south-east trade--meeting with equal force, they form a calm; and then, warmed by the heat of the sun, they ascend, one-half streaming off high up towards the south-east--that is, counter to the surface-current--till it reaches the southern calm belt; another mass coming from the south-west, where it descends, and rushes as a north-west surface-wind towards the south pole.  We have traced the mass which started from the north pole.  Reaching the southern regions, it is whirled round till, at the pole itself, a perfect calm is produced, when it ascends and starts off as an upper current towards the equator; but meeting another current near the tropic of Capricorn, then descends, one-half flowing out at the surface, as I have before described, as the south-east trade, the other towards the south pole.  This is the most beautiful and regular system of atmospheric circulation kept up around our globe.  It has not been ascertained exactly why the masses I have spoken of take certain directions, but we know the directions they do take.  The red dust we found off the Cape de Verds assists us in certain degrees.  We know some of the agents--the diurnal motion of the earth, and the sun's heating rays.  There are certain counteracting or disturbing causes from which the surface-winds deviate from the courses I have described.  Some lands are covered with forests, others with marshes, others with sand.  All these may be disturbing causes--so are lofty mountains.  From these causes, and the more powerful effect of the sun's rays in one place than in another, hurricanes and typhoons occur, and the monsoons are made to blow--the harmattan on the west coast of Africa; the simoon, with its deadly breath, in Arabia; the oppressive sirocco in the Mediterranean.  What I have said will explain that beautiful passage in Ecclesiastes, 1st chapter, 6th verse, which shows the exactness of the sacred writers whenever they do introduce scientific subjects: `The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits.'  Who gave Solomon this information?  I doubt if any of his sages possessed that scientific knowledge which has only been attained by philosophers of late years.  Perhaps I may still more clearly explain to you the cause of the circulation of the atmosphere.  I told you that there were two agents at work--diurnal motion, and the heat of the sun; but to these may be added the cold of the poles, which contracts the air.  Suppose the globe at rest, and covered with one uniform stagnant mass of atmosphere; suddenly heat, cold, and the diurnal motion commence their operations.  The air about the equator would expand, that about the poles contract.  Thus two systems of winds would commence to blow--one above, from the equator towards the poles; and as thus a vacuum would be left below, a current would come from the poles to supply its place.  The diurnal motion prevents these currents running in straight lines.  That coming from the poles will appear to have easting in them, and those going towards the poles westing.  Not only, however, is the level of the atmosphere changed by the heating rays of the sun, but its specific gravity.  Thus the heated current moves more easily and rapidly than the colder; and the latter, consequently, turns back a portion of what was going towards the poles, and adjusts the equilibrium of the atmosphere.  I have already shown you the great importance of the circulation of the air in the economy of nature; and how, among the many offices of the atmosphere, it distributes moisture over the surface of the earth, making the barren places fruitful, and tempering the climates of different latitudes, fitting them as the abode of civilised man.  But I will not pursue the subject further just now.  You must do that for yourselves.  Try and remember what I have said, and think about it whenever you have an opportunity."  Jerry and I thanked the captain for what he had told us, and I, as before, at once dotted it down as well as I could in my note-book.