the sender and the examiner, when nothing can be made of a reeking waste of decaying weeds.

PHYSICS OF THE ATMOSPHERE.

By Prof. I. THORNTON OSMOND, State College, Pa., Meteorologist of the Board.

[An illustrated lecture delivered at the Bellefonte meeting.]

Size Constitution, Composition, Structure-Pressure-Solar Radiation, Nature, Absorption, Temperature-Movements, General, Cyclones,

Tornadoes-Meteors.

Most of you are familiar with discussions of the soil, its constituents, condition, cultivation, etc. It is in the soil that man works in the production of crops. All this toil of his, although a wise Providence makes it a necessary condition of bounteous reaping, is but an infinitesimal part of the work actually expended upon every crop between the germination of the seed and the maturing of the product. The kindly Power who puts forth this great sum of work, operates through his physical agencies, far more in the atmosphere than in the earth. He attends to all this work Himself, and has put none of it in our power to do, so we do not usually think much about it. I propose to talk about this atmospheric field wherein the processes of nature acting by the laws of a beneficent intelligence do a hundred million fold the work for making crops each year that man does with all his industry and pains.

Size.

The height of the atmosphere has not been accurately determined. The great mass of it is near the earth, three-fourths, or more, of it being below the tops of the loftiest mountains, on which the atmospheric pressure is small.

The force of gravity, which decreases as the square of the distance between the centers of the attracting masses increases, and the socalled centrifugal force of a revolving mass, which increases as the square of the velocity divided by the radius of rotation, will be in equilibrium about 25,000 miles from the center of the earth. Hence, at a distance of about 21,000 miles above the earth's surface, particles of air would be thrown off into space, like water from a rapidly moving carriage wheel. There can scarcely be an atmosphere belonging particularly to the earth at or beyond this limit. But this by no means proves that our atmosphere does extend to this height; satisfied that it cannot exceed this, we are to find what part of this space it actually fills, if we can.

Twilight Curve.-Opposite the sun, the conical shadow of the earth is continually thrown on or through the atmosphere. Just after sunset this shadow may be seen rising above the eastern horizon, rising higher as the sun is farther and farther below the western horizon. The boundary of this shadow and the illuminated air is sometimes quite distinct, especially in the tropics where the conditions are most. favorable

Let the sun's rays just pass the earth's surface at D (Fig. 1), then DA is the boundary of the shadow. An observer at O sees this ascending his eastern heavens. Knowing his own position, the angular

elevation of A at any moment and the position of D (place of sunsetting) at that moment, he can by simple methods of trigonometry, familiar to every surveyor or high school boy, find the distance AB. The solution makes AB about forty-five miles, after corrections for refraction, etc. Thus it is evident that to a height of about forty-five miles there is an atmosphere sufficiently dense to produce optical effects that affect the sense of vision.

Certain phenomena of lunar eclipses give evidence of an atmosphere capable of a slight refraction of light at a height of sixty-six miles; and the observation of meteorites and aurora indicate the existence of a very attenuated atmosphere at the height of two or three hundred miles, or even higher.

Possibly, we sometimes lose portions of our atmosphere; we quite certainly pick up gaseous masses, as well as solid, from the spaces through which we are ever flying.

Constitution.

Composition.-Air was considered an element for many ages. It is but little more than a century since its composition was discovered. Dr. Priestly (whose grave is in Northumberland county in this State), discovered the element oxygen sometime in 1774, and Lavoisier in November of that year discovered that one-fifth of our atmosphere was this gas, oxygen. This was not two years before the Declaration of Independence.

The constituents of the air have been frequently determined with great accuracy, and are found to be almost invariable in their proportions, slight variations occurring in large cities, and great variations near volcanoes and other special regions of peculiar phenomena. The constituents are, by volume: Nitrogen,

Oxygen,

Carbonic dioxide,

79.02

20.95

00.03

100.00

Aqueous vapor, a quite variable amount. Small and variable quantities of ammonia, nitric acid, hydrogen sulphide, ozone and sometimes other gases, are found.

The nitrogen, oxygen and carbonic acid of the air are not combined, and are of different density; but instead of being in layers, densest below, are always uniformly distributed in the above ratio in any space. In cases of liquid mixtures, we have two kinds of action. In one, as oil and water, the order of density is observed. In the other, as alcohol and water, each liquid spreads throughout the volume uniformly. In the case of gases, it is a law that all gases (which do not chemically combine) distribute themselves throughout the space occupied, each as if the others were not present. A denser gas will rise and mix through a lighter. This law of diffusion is of the highest importance to animal and vegetable life by the part it plays in arranging the constituents of the atmosphere.

Though carbonic acid forms but .03 parts in 100 of the air, its importance and the importance of its diffusion and presence everywhere are manifest; and likewise, the importance of the rather small constituent, aqueous vapor. But the immense importance of the part played in the physical phenomena of the heavens and the influence

on the life of the earth by some of the constituents named, of which there is never much more than a trace present, can be understood only by considerable study of the subject. (See absorption and temperature, under Solar Radiation, below.)

Structure.-We must consider not only the constituents of the atmosphere, but their structure, or the way they are put together and their relations and actions. Suppose a cubic inch of air in a tube, under a tight fitting piston. The piston, we find, is easily driven further down. Evidently there are spaces between the parts of the air, or the structure is one of particles and interspaces rather than continuous. This reduction of volume can be carried a great way, but demanding more and more pressure. What is the size of the particles, how far apart are they ordinarily, and how many have we in one cubic inch of air? As the result of a great deal of work and calculation, physicists have given approximate answers. In a cubic inch of air, at ordinary pressure and temperature, there are about 300,000,000,000,000,000,000 particles, each about 500000 inch in diameter, and from 1000 0000 to oooooo of an inch apart on the average. But each of these has a velocity of about 1,500 feet per second, so that it has about 8,000,000,000 encounters per second with other particles, changing its direction more or less each time. In all this, no two particles ever touch; but as they rush toward each other, before actual collision or contact occurs they are repelled with great force.

One cubic inch of air-No. 300,000,000,000,000,000,000 particles.
Size, 25000007 inch diameter.
Distance, 100000 T to o

Velocity, 1,500 feet per second.

inch.

Collisions, 8,000,000,000 per second, each particle.

Everyone has seen a swarm of small gnats, of a summer evening, stationary as a body, but each individual in rapid movement. If we can imagine each gnat diminished a million or two times in size, and the average distances between the gnats several million times, with a corresponding increase in the number of gnats, and a great increase in the velocity of their movements, we may get some conception of what is going on in a cubic inch of ordinary air. Suppose, now, that whenever any two come near each other they by a thrust of wings or legs hurl each other away with quite a force. Now, if we inclose our swarm and try to bring them into less space, since each gnat has a certain weight and moves with a certain velocity, there will be millions of blows during each second against each square inch of the inclosing surface. This will be, in effect, a pressure of a certain amount, increasing as the space becomes less and a greater number per second of gnats strike or are hurled against each square inch of the surface.

Pressure.

By means of a globe suspended to a fine balance, (Fig. 3) we find that a cubic foot of air at 60° F. and ordinary pressure weighs about eight one hundredths of a pound. Though possessing this considerable weight, the air does not fall all into a compact covering about the earth. This is due to the peculiar constitution and structure that we have just considered. All the particles are strongly drawn toward the earth, but the continual rapid vibratory movements and the elasticity with which they rebound, prevent very close approach of the particles to each other. Still, those portions which beside the attrac