from the earth's atmosphere, neither light nor heat could be produced from coal, wood, or other combustible substances.

But the proportion, also, in which oxygen exists in the air is adjusted to the existing condition of things. Did the atmosphere consist of oxygen only, the lives of animals would be of most brief duration, and bodies once set on fire would burn so fast as to be absolutely beyond control. The oxygen is, therefore, mixed with a large proportion of nitrogen. This gas, not being poisonous, as carbonic acid is, harmlessly dilutes the too active oxygen. It weakens and prolongs its action on the system, as water dilutes wine or spirits and assuages their too fiery influence upon the animal frame.

Then, as to the carbonic acid

Every green leaf that waves on field or tree sucks in, during the sunshine, this gas from the air. It is as indispensable to the life of the plant as oxygen is to the life of the animal. Remove carbonic acid from the air, and all vegetable growth would cease.

But carbonic acid is poisonous to animals. It is for this reason that the proportion of this gas contained in the air is so very small. Were the proportion much greater than it is, animals, as they are now constituted, could not breathe the atmosphere without injury. On the other hand, that growing plants may be able to obtain a sufficiently large and rapid supply of carbonic acid from a gaseous mixture which contains so little, they are made to hang out their many waving leaves into the atmosphere. Over the surface of these leaves are sprinkled countless pores or mouths, which are continually employed in separating and drinking in carbonic acid gas. The millions of leaves which a single tree spreads out, and the constant renewal of the moving air in which they are suspended, enable the living plant to draw an abundant supply for all its wants.

This constant action of the leaves of plants is one of the natural agencies by which the proportion of carbonic acid in the lower regions of the atmosphere is rendered less than it is in the higher regions.

So, also, the watery vapour of the atmosphere is not less neces

sary to the maintenance of life. The living plant consists of water to the amount of nearly three-fourths of its whole weight; and from the surface of its leaves water is continually rising into the air, in the form of invisible vapour.

Were the air absolutely dry, it would cause this water to evaporate from the leaves more rapidly than it could be supplied to them by the soil and roots. Thus they would speedily become flaccid, and the whole plant would droop, wither, and die.

The living animal, in like manner, is made up for the most part of water. A man of 154 lb weight contains 116 Ib of water and only 38 b of dry matter. From his skin and from his lungs water is continually evaporating. Were the air around him perfectly dry, his skin would become parched and shrivelled, and thirst would oppress his fevered frame. The air which he breathes from his lungs is loaded with moisture. Were that which he draws in entirely free from watery vapour, he would soon breathe out the fluids which fill up his tissues, and would dry up into a withered and ghastly mummy. It is because the simoom and other hot winds of the desert approach to this state of dryness that they are so fatal to those who travel on the arid waste.

Thus the moisture which the atmosphere contains is also essential to the maintenance of the present condition both of animal and vegetable life; it pervades the leaves and pores of plants, and finds admission to the lungs and general system of animals.

There are, besides, other beautiful purposes which this moisture erves. When the summer sun has sunk beneath the horizon, and 300lness revisits the scorched plant and soil, the grateful dew descends along with it, and moistens alike the green leaf and the thirsty land; the invisible moisture of the air thickens into hazy mists, and settles in tiny pearls on every cool thing. How thankful for this nightly dew has nature everywhere and always appeared, and how have poets in every age sung of its beauty and beneficence!

THE PRESSURE OF THE ATMOSPHERE.

To persons who approach the subject for the first time, the pressure of the atmosphere seems to belong rather to the romance of science than to truth. "What! has the air any weight?" cries, perhaps, some astonished reader; and he becomes almost incredulous when gravely assured that it is pressing with a weight of fifteen pounds on every square inch of his body. A fish would find it difficult to believe that the element through which he is gliding has any weight, and yet we all know that water has weight, and exerts no small pressure on any vessel containing it. Thus it is with the atmospheric ocean. In both cases the reason why a body immersed does not experience this pressure is, that the fluid presses equally on all sides.

The common pump was invented 224 B.C., and soon afterwards it came into general use throughout the civilized world. The philosophers of the time explained its action by saying that when the piston was raised in pumping, a vacuum was formed over the water; but that "Nature abhorred a vacuum," and, consequently, filled it with water, as the most convenient material.

Some wells were very deep, and it was found that whenever the depth was more than 33 feet, the pumps were unable to raise the water. Centuries after the invention of the pump, some engineers asked Galileo why the water did not rise higher than 33 feet. He is said to have replied, that "Nature's abhorrence of a vacuum ceased at the height of 33 feet." Though the great Galileo did not know the true theory of the common pump, he certainly must have given such an answer rather in joke than in earnest.

It is supposed that he suggested to his pupil Torricelli that the weight of the air on the water surrounding the pump might force the water up into it when the pressure within was removed. Galileo died soon after, and the next year, 1643, Torricelli determined to ascertain if this was the true explanation. He thought that if the weight of the air was the cause, he could try the experiment of sustaining, by the pressure of the atmosphere,

a column of water 33 feet high in a tube closed at the upper end. This would have been a difficult experiment to perform; but, fortunately, he knew that the specific gravity of quicksilver was 13 times that of water. Of course a column of quicksilver 2 feet high would balance a column of water 13 times as high, or about 33 feet.

Torricelli took a glass tube more than 2 feet long, and filled it with quicksilver, and, after closing the upper end, inverted it, placing the open end below the surface of quicksilver in a cup before removing his thumb. As he expected, the quicksilver did not all run down into the cup, but stood at the height of 30 inches in the tube. Evidently the pressure of the atmosphere upon the quicksilver in the cup sustained the column in the tube; and as the tube was one inch in area, and the column of quicksilver weighed 15 pounds, not only was the pressure of the atmosphere on a square inch of surface ascertained, but the instrument called the barometer was invented-an instrument to show the pressure of the atmosphere at different times and in different places.

To afford further evidence that the weight of the atmosphere was the cause of the phenomenon, he afterwards carried the tube of mercury to the tops of buildings and of mountains, and found that it rose or fell exactly in proportion to the height at which he tried the experiment; and he also found that water-pumps, in different situations, varied as to sucking power according to the same law.

It was soon afterwards discovered, by careful observation of the mercurial barometer, that even when remaining in the same place, it did not always stand at the same elevation; in other words, that the weight of atmosphere over any particular part of the earth was constantly fluctuating; a truth which, without the barometer, could never have been suspected. The observation of the instrument being carried still further, it was found that in serene dry weather the mercury generally stood high, and that before and during storms and rain it fell; the instrument, therefore, it was soon perceived, might serve as a prophet of the weather, and become a precious monitor to the husbandman and the sailor.

When water which has been suspended in the atmosphere, and has formed a part of it, separates as rain, the weight and bulk of the mass are diminished; and the wind always blows when a sudden condensation of aeriform matter, in any situation, disturbs the equilibrium, for the air around rushes towards the situation of diminished pressure.

To the husbandman the barometer is of considerable use, by aiding and correcting the prognostics of the weather which he draws from local signs familiar to him; but it is still more useful to the mariner, who roams over the whole ocean, and is often under skies and in climates altogether new to him. The watchful captain of the present day, trusting to this extraordinary monitor, is frequently enabled to take in sail, and to make ready for the storm, where, in former times, the dreadful visitation would have fallen upon him unprepared.

The following incident will illustrate this:- "It was in a, southern latitude. The sun had just set with placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch was proceeding, when the captain's order came to prepare with all haste for a storm. The barometer had begun to fall with appalling rapidity. As yet, the oldest sailors had not perceived even the threatening in the sky, and were surprised at the extent and hurry of the preparations; but the required measures were not completed when a more awful hurricane burst upon them than the most experienced had ever braved. Nothing could withstand it. The sails, already furled and closely bound to the yards, were riven away in tatters; even the bare yards and masts were in great part disabled, and at one time the whole rigging had nearly fallen by the board.

Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and of the incessant peals of thunder, that no human voice could be heard; and, amidst the general consternation, even the trumpet sounded in vain. In that awful night, but for the little tube of mercury which had given warning, neither the strength of the noble ship nor the skill and energy of the commander could have saved one man to tell the tale. On