Throughout a zone of a few degrees in breadth, which extends round the globe in the neighborhood of the equator, and the limits of which undergo a certain amount of variation, dependent on the sun’s passage of the equinox, the variation of temperature throughout the year is confined within very narrow limits, and the result is a general prevalence of calms--that is, of undisturbed atmosphere. Wind is air set in motion, mainly by the existence of different conditions of temperature between adjacent bodies of air--of colder and denser air pressing against warmer and lighter air, and taking the place which is left vacant by the latter, as it rises into the higher regions of the entire aerial sea. Between the heated air of the tropics in general, and the comparatively cooler air of the regions lying some distance north and south of the tropics, for example, there is a very manifest difference as to temperature, as well as in regard to other conditions; but for a few degrees in the immediate neighborhood of the equator there is no such obvious difference, and, consequently, nothing to occasion disturbance (temperature alone being considered) in the general equilibrium of the atmosphere. Hence the prevalence of calms in that region. Within the parallels of 8° or 10° on either side of the line, the angle at which the solar rays reach the earth is at no time more than a few degrees from the perpendicular, for the equator divides the total amount of angular difference which is involved in the entire yearly path of the sun. The average breadth of the calm latitudes--or the _Zone of Calms_, as it is the custom, in books and maps, to term it--may be stated at about six or seven degrees. The mid-line of this zone does not coincide with the equator, for the reason that the equator does not represent the line of the earth’s highest temperature, owing to the preponderance of land in the Northern Hemisphere. Hence the Zone of Calms is, for the most part, to the northward of the equator--extending, with varying seasonal limits, from about the first to the seventh or eighth parallel of north latitude. But the limits oscillate with the sun’s passage of the equinox and consequent place in the heavens vertically over either side of the equator. The calm latitudes are the dread of the mariner, whose ship is often delayed for weeks together within their limits. The wearisome and tantalizing nature of this delay can, perhaps, only be adequately appreciated by those who have experienced the monotony attendant on a calm in mid-ocean, when, with a still and glassy sea around, a glittering atmosphere, and a burning sun overhead, the sails hang idly by the yards, and the vessel makes no appreciable progress.