"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.