isted at all, it has been calculated by Dr. Thiesen of Berlin that our sun would, by virtue of its enormous size-a million times that of our earth - and gravity, which is twenty-seven times greater, attach to itself a gaseous covering or atmosphere, which would be as dense as our own, far beyond the orbit of Venus. This, however, is known to be contrary to fact.

The sun's atmosphere is not more than about 500,000 miles deep, while that of the earth is certainly not more than 100 miles.

The height of our atmosphere has never been

measured as we measure distances on the earth's surface, for the very simple reason that we can never hope to reach the top. Indeed, we should find it very difficult to know where the top was, even if we were able to approach it, since the air would shade off so gradually into where it suddenly changed into the vacuum of space that we should with difficulty discover the place where we could say "thus far and no farther."

We can, however, arrive at some knowledge of the probable height to which the air exists in such quantity as to possess weight and resistance by calculation of the rate at which the pressure of the atmosphere diminishes as we ascend, and also by observation of the duration of twilight and the heights at which meteorites (or, as they are still popularly termed, falling stars) are visible.

Living as we do at the base of our ocean of air, like the flat-fish live at the bottom of the ocean of water, we are absurdly ignorant of the condition of the atmosphere a few miles overhead.

The highest ascent made by man up mountains is believed to be that of Zurbriggen on Aconcaqua, when he reached about 24,000 feet, or a little over 4 miles, while the highest in a balloon was that made by Dr. Berson of Berlin, who in 1894 ascended to a height of 30,000 feet.

Some years ago, in 1862, Glaisher and Coxwell made a memorable ascent over Wolverhampton, when they became unconscious at 29,000 feet, after which they were supposed to have ascended for a short time, to nearly 36,000 feet, but in Dr. Berson's case, by inhaling oxygen he was able to observe his instruments and carefully note the conditions around him.

His thermometer went down to 54 degrees below zero Fahr., while the mercury in his barometer sank from 30 to 9 inches. Six miles is probably the limit to which man will ever care to ascend into the atmosphere, since above this height he can only survive by the aid of artificial assistance. For permanent habitation it is found to be prejudicial to live at greater heights than 15,000 feet, so that it is only within a thin slice of our atmospheric blanket that human life is lived. Actually, the marvellous complexity of human thought and action, and the development of modern civilisation on this earth, has taken place, and will probably always remain confined within the vertical distance of a London shilling cab fare above the surface.

Apart from direct measurement, the pressure of the atmosphere gives us some clue to its height as well as to its weight. From the pressure observations alone, it ought to disappear somewhere about 38 miles, since at that height the mercury column of the barometer, which measures the weight of air above, would tend to disappear. Observations of meteorites, however, whose appearance depends upon their heating to incandescence by friction against a resisting medium, shew that some air exists at 100 miles, though at such great altitudes it is probably in a condition of extreme rarity. Observations of the duration of twilight, which is due to reflection from particles of dust and air, gave about 50 miles as the limit. Practically, therefore, we may take 50 miles to be about the limit up to which the atmosphere exists in a coherent form as we know it near the earth's surface.

CHAPTER II.

THE NATURE AND COMPOSITION OF THE

ATMOSPHERE.

To one of those superior beings who, we believe, inhabit the celestial regions, it must be infinitely pathetic to see the poor little human mites on this planet struggling for centuries through the mist of error and superstition, until they finally discovered one day the composition of the atmosphere in which they lived. By the Greeks.

the air was considered to be one of the four elements, and it was not until the middle of the last century that Priestley discovered that air was a

mixture of oxygen and nitrogen, and that its neutral character was due to the blending of a most active element, oxygen, with a most inactive element, nitrogen.

A slight difference in the proportion of either element would be fatal to life as we know it. With more oxygen in the air our lives, short enough as they are, would be still more brief, and though we might be more witty and brilliant, we should live in a state of such mental and physical intoxication that we should never be able to sit down quietly to do any solid work. In fact, the human race would be converted into a number of thoughtless, reckless, frivolous beings, who would probably end by destroying each other in a frenzy of over-excitement. On the other hand, too much nitrogen would reduce us to such a degree of dulness and inertia that our supposed national characteristics would be intensified and we should become like a row of statues or mummies, without action or passion, lifeless-in fact, matter without motion. The existing proportion therefore is decidedly adapted to our present requirements. The average proportion in which the two principal components of the atmosphere are found to occur is 21 of oxygen to 79 of nitrogen by volume, and 23 of oxygen to 77 of nitrogen by weight.

The proportion in which the remaining constituents enter is so small that it may be practically neglected when we consider the physical properties of the atmosphere, though it cannot be neglected when we regard its vital and chemical functions. The other constituents are carbonic acid, which occupies Tooths by volume, traces of ammonia, ozone, and the recently discovered argon.