mountains near the equator. During the latter part of the century, Kirwan, an English chemist, calculated the temperature for various parallels of latitude, and in 1817 Alexander von Humboldt, after a voyage around the world, published his isothermal lines, or lines of equal temperature on the surface of the globe, by which he showed that the deviation from the normal, or calculated, temperature arose from the distribution of land and water, and from the geographical relief of the former. This work of von Humboldt formed the basis of all subsequent studies in comparative climatology. Meanwhile chemistry had kept pace with physics, and in 1774 the old theory, that air was one of the four elements from which all things originated, was rendered untenable by Priestley, who proved that oxygen gas, which he discovered, was a constituent part of air. The other constituent, nitrogen, formerly called azote from its destructiveness to life, was discovered soon afterwards, and its proportion in the air determined by the French chemist, Lavoisier.

In 1783 man became possessed of the long-sought-for means of rising freely in the air, and he speedily availed himself of it. The first balloons, filled with heated air, were called Montgolfières from the inventors, the brothers Montgolfier, living in Annonay, France. After animals had been sent up attached to one, Pilâtre de Rozier ventured to ascend in the aerostatic machine, first tethered captive but then set free, and before the close of the year a balloon, filled with hydrogen gas, or "inflammable air" as it was called, carried M. Charles 9000 feet above Paris. During more than a century the balloon has been the most important agent for the exploration of the atmosphere, and yet, notwithstanding the courage and devotion to science of the early aeronauts, their ascents with unsuitable instruments furnished much discordant and erroneous data. Some of the most remarkable balloon voyages and the modern methods of sounding and dredging the atmosphere, to borrow terms from the exploration of the ocean, will be described in two future chapters.

Perhaps the chief reason for the slow progress of meteorology to the status of a science is the variable character of its phenomena with the place of observation. In this respect it differs from astronomy, which was more easily cultivated in the restricted ancient world. Only after many years of observation at different places had contributed a foundation for climatology was it realized that man, in his relation to the atmosphere, resembled marine organisms confined to the bottom of the ocean, and that in order to discover the true conditions of the atmosphere it was necessary to observe them at considerable heights. In the last century the highest point at which physical observations had been made was the summit of Mont Blanc, less than 16,000 feet above the sea. The ascent of this mountain was first accomplished in 1787 by H. B. De Saussure and his guides with much difficulty and suffering, and the observations, abridged and rendered less accurate by the fatigue and sickness of De Saussure, were also influenced by the proximity of the mountain itself. In 1802 von Humboldt and Bonpland reached a height of about 18,000 feet in the Andes, where they made important observations. The ascent of man was rapid during the first years of the nineteenth century, for in 1804 Gay-Lussac rose in a balloon, without exertion or discomfort, to the height of 23,000 feet, and there made observations which were assumed to give the true atmospheric conditions. After an active campaign the conquest of the air by balloons was temporarily abandoned, and the field was left free to the mountaineer. But to-day supremacy rests with the aeronaut, for no one has succeeded in getting higher than 24,000 feet on a mountain, while the aeronaut has exceeded this altitude by a mile without great hardship, and lately has sent his unmanned balloons twice as high as the loftiest mountains.

Plate I., headed The Exploration of the Atmosphere, represents a vertical section of the lower portion of our atmosphere. On the right is a scale of miles above the sea, and on the left is a scale of barometric pressures corresponding to the height. The right-hand half of the diagram shows the eastern hemisphere with the Himalaya mountains, the left-hand half the western hemisphere with the Andes. There are seen the heights of the different kinds of clouds, measured at Blue Hill, as described in the next chapter; the highest meteorological stations, those on Mont Blanc and El Misti in Peru; the highest permanently inhabited place, which is a monastery in Thibet; and the greatest height to which man has climbed, namely, in the Andes. The heights at which observations have been made in balloons, carrying observers, or only recording instruments, may be compared with the height attained by the Blue Hill kites, to be described hereafter. Other altitudes can be noted, such as the height of the snow-line on various mountains, and as a thousand-foot measure, the Eiffel Tower in Paris, the tallest structure erected by man, may be used.

Plate I.—Comparative Altitudes.

The development of meteorological knowledge to the commencement of the present century has now been traced, but before beginning the consideration of the methods of exploring the atmosphere that form the subject of the book, let us, in order to understand this work better, review the general knowledge which we possess of our atmosphere as regards its origin, composition, extent, and conditions of heat and moisture. First, then, regarding the Origin of the Atmosphere, or vapour envelope which the name means. According to the nebular hypothesis of Laplace, our earth, like all existing suns and planets, was condensed from clouds of nebulous matter and became a highly-heated globular mass rotating, like every celestial body, from west to east. As the earth cooled, a crust was formed, and many of the substances that now exist in the earth were suspended as clouds in the cooler atmosphere surrounding it. Eventually, these substances were condensed upon the crust; the oxygen, especially, must have been diminished by combining with the rocks, while the lighter gases, such as hydrogen, may have escaped from the earth's atmosphere. No doubt, when vegetable and animal life began, the earth's atmosphere was denser than now and much richer in carbonic acid, which, during the carboniferous period, was absorbed by plants, and is now imprisoned in coal and limestone. Within historic times, however, there is no evidence of any change in the composition of our atmosphere, or the climatic conditions as a whole.

M. Jourdanet, a distinguished French physiologist, maintained that man appeared on the earth at the close of the tertiary period, when the barometric pressure at sea-level was, he supposed, about forty-three inches, or nearly a half more than it is to-day, and owing to the greater density of the air its temperature was also considerably higher. Under these circumstances he believed that man first occupied the high regions of Central Asia, and only emigrated to lower levels when the climatic conditions became ameliorated. In other words, M. Jourdanet believed in a literal "descent of man," but if this be true, many of the