tion of the earth are subject also to the force with which large amounts of overlying air are urged toward the earth, are considerably compressed. Thus the air is densest at the surface of the earth and grows rarer quite rapidly as the vertical height increases.

The density is everywhere as the pressure, if the temperature is uniform. This is known as Boyle's law. It is easily proven with a bent tube, Fig. 3, with the shorter part closed. The bend, a b, is just filled with mercury, and b d contains air of the same density as that without the tube. If mercury is poured in until the height hc gives as much pressure as the atmosphere (about thirty inches of mercury), the air in c d is under double pressure and occupies but half its usual volume. The relation is true for any ordinary pressures, the volume is inversely as the pressure. To state this in reverse order, the less volume that a given amount of air occupies the greater is the pressure on the surrounding surfaces. This pressure, as we have seen is due to the rapid blows of the almost infinite number of particles. When we get about three hundred quentillions of these in a cubic inch, at ordinary temperature, these blows give a steady pressure of fifteen pounds to the square inch.

Some Florentine pump-makers, and doubtless others, found they could only raise water about thirty feet high, by "suction," in their pumps; and that if the pistons were pulled much higher the water would not follow. They appealed to the philosopher, Galileo, for an explanation. The easy philosophy of that day explained the rising of the water by saying nature abhors a vacuum and tries to fill it; whether nature abhorred a vacuum only to the height of a little more than thirty feet, or whether she got tired of filling it and stopped there, I believe was never decided. Galileo knew that liquids transmit pressure in all directions undiminished and so that equal areas of the surrounding surfaces sustain equal pressures; so that a pressure on the piston P, Fig. 4, would be transmitted down the tube, then horizontally, then upward, and every area of W equal to P would experience a pressure equal to that on P. He knew that air had weight, and must press on the water in a well, Fig. 4, which pressure would be transmitted downward and horizontally through the water, and so would force the water up into the tube of the pump from which the air had been removed, moreover, it should rise until the pressure produced at the bottom of the tube by the column of water in it equals the pressure of the air (and water) outside the tube at the bottom of it.

One of Galileo's pupils, Toricelli, said if this was the true explanation, then the pressure due to the weight of the air should sustain a column of mercury, 13.6 times as dense as water, only one 13.6 part as high as the column of water. He filled a tube, Fig. 5, a yard in length with mercury and inverted it with the open end in a bowl of mercury. The mercury in the tube settled to about thirty inches above the surface of that in the bowl. To complete the proof, he asked a relative living near a mountain to repeat the experiment on the mountain. Here, above a portion of the air, the mercury in the tube stood at but twenty-seven inches.

Thus the action of the common pump was fully explained and proved to be due to the pressure of the atmosphere arising from its weight; and that pressure was also measured, since a column of mercury of one square inch section and thirty inches high weighs fifteen pounds.

These experiments also introduced a most important instrument, the barometer. This instrument measures the varying pressure of the atmosphere. Its simpler forms are familiar to everybody. In the aneroid barometer, the pressure is measured by an index moved by the greater or less flattening of a curved cover of a tightly closed and exhausted metallic box. If a pencil is attached to this index and presses a sheet of paper drawn uniformly past it by clock work, it gives a continuous register of the varying atmospheric pressure. The mercurial barometer is made self-registering in a number of ways. One is suggested in Fig. 18, where a piston, P, floats on the mercury and works a lever, L, with a pencil marking on C, a cylinder driven by a spring clock and covered with a sheet of paper ruled horizontally for fractions of inches difference of level of mercury in tube and cistern, and ruled vertically for time.

The barometer has been of the greatest importance in the study of meteorology and the prediction of weather. A knowledge of the monthly and annual mean pressure of the atmosphere has greatly assisted to explain the more general and permanent or periodic phenomena of the winds of the earth, and the continual variations of pressure from day to day at different places are essential factors in weather forecasts.

Fig. 6 represents the mean annual pressure at different latitudes. It shows the greatest average pressure at about 35° N. latitude, with nearly as great a mean pressure about 30° S. latitude. Three zones of minimum average pressure are shown, the equatorial region, between 60° and 70° N. latitude, and about 70° S. latitude.

At any given place, the monthly means show maxima and minima, occurring pretty regularly from year to year. Fig. 7 shows the mean pressure of each month as found from the observations of several years at Pennsylvania State College. And the curve obtained from the observations of any one year, would not differ very greatly from the curve of Fig. 7.

And again, at any given place, the mean pressure varies with the time of day. Observations at Philadelphia, continued for a considerable period, show the mean pressures at the hours 0, 3, 6, 9, 12 (noon), 15, 18, 21, 24, as represented in Fig. 8.

Another regular variation of mean pressure, but of very little magnitude, is produced by the moon during each lunation.

Combined with these regular periodic changes of pressure are continually occurring irregular variations of pressure due to changes of temperature, humidity, and, perhaps electrical and other conditions of the air. So it is only by taking the means for a long time that the monthly, daily, or other regular periodic changes for any place can be found.

Solar Radiation.

Byron, though no scientist, has given a vivid picture, in his Dream of Darkness, of the conditions that would quickly follow the shutting off of the solar radiations from our atmosphere:

"The bright sun was extinguished, and the stars
Did wander darkling in the eternal space,

Rayless and pathless; and the icy earth

Swung blind and blackening in the moonless air.

We want to understand something about this wonderful process of solar radiation which causes nearly all the varied and never ending phenomena of the skies and is absolutely essential to all life, vegetable and animal on the earth. What are solar radiations, and their properties, and how do they interact with such an atmosphere as ours to produce the seasons and climates, the winds and rains, the pressures and temperatures, the vital conditions of this habitable and glorious earth?

We will take a ray, or beam, of solar radiations, R, Fig. 11, admitted to a darkened room, and passed through a glass prism, P. On a screen, ZS, we have a band, A Z, of brilliant colors, a spectrum, S. By experiments and reasonings and tests too lengthy to describe here, it is established that the original ray consisted of vibratory movements across its direction, in every plane, and of many millions dif ferent rates, or periods, of vibration and different wave-lengths, B, Fig. 11. These different vibrations are differently retarded, and therefore differently bent in passing through P, and so are separated into the band, or spectrum, S, which extends much farther at each end than the space covered by the colors seen by the eye.

Three methods are now possible to us for studying these radiations and learning more about their nature and properties. These are illustrated, typically, in Fig. 12, in which S is the spectrum from the prism. We may move any delicate thermometric instrument along S and observe its changes. We may note the difference to the eye along a portion of S, from R to V; the rest of it is not visible. We may put certain chemical compounds, as iodide or bromide of silver, in different parts of S and observe how they are affected.

Interesting as the study of color may be, and important as is the chemical action of the solar beam, both in the arts of life and in the processes of nature, we shall omit further notice of them, and consider only the thermic, or heat, effects of solar radiation.

Practically all the heat of the surface of the earth comes to it in solar radiations through the atmosphere. The amount of heat thus received annually is inconceivably great. To measure heat and say how much there is of it, we must take some quantity of heat as a unit of heat.

In the English system of measures, the unit of heat is the quantity of heat that will change the temperature of one pound of water one degree Fahrenheit. In Fig. 9 are two equal gas flames with two exactly similar vessels over them. In one vessel is a pound of water at 32° F., in the other a pound of mercury at 32° F., and in each a thermometer, t, t'. When the water has reached the temperature 35°, a rise of 3, the mercury has a temperature of about 130°, a rise of 98°, nearly thirty-three times as great a change of temperature as was produced in the water, and yet both received the same amount of heat. So, we see that temperature read from a thermometer does not measure quantity of heat. Therefore, the unit of heat is taken as defined above.

Now this amount of heat, unit of heat, can be produced by friction or blows, in which there is just the amount of work done that is done in lifting one pound 772 feet high, or 772 pounds one foot high; and

it is also found that for every unit of heat, calorie, used in the cylinder of a steam engine the piston does 772 foot-pounds of work. In general, a unit of heat is equivalent to, or will do, 772 foot-pounds of work. The ability of anything to do work is called energy. The energy of one unit of heat-772 foot-pounds.

"The observation of the amount of heat the sun sends the earth is among the most important and difficult in astronomical physics, and is also the fundamental problem in meteorology, nearly all whose phenomena would become predictable if we knew both the original quantity and kind of this heat; how it affects the constituents of our atmosphere on its passage earthward; how much of it reaches the soil; how, through the aid of the atmosphere, it maintains the surface temperature of the planet; and how, in diminished quantity and altered. kind, it is finally returned to outer space." [S. P. Langley, Researches on Solar Heat, p. 11.]

This observation has occupied much of the labors of Herschel, Pouillet, Erricson, Crova and others, in the last fifty or sixty years. The latest and best work in this research is that of Prof. Langley, of Allegheny Observatory. Fig. 10 gives a view (vertical section) of an actinometer, used by Crova and by Langley. S and S' are spherical shells between which water enters at P', and discharges at P. Two tubes are inserted in S, and two thermometers, t and t', pass through the tubular axes of support, one into S and the other into the water. The solar rays fall a noted time on t, and its change of temperature is noted. If, now, we know the amount of heat, in heat units, required to make any given change of temperature in the thermometer t, by finding the rate at which its temperature begins to change, we have the means of determining the amount of heat received per second on each square inch of surface perpendicular to the rays.

Langley gives the latest results of actinometric research. He says, "my conclusion is that we can adopt three calories as the most probable value of the solar constant, by which I mean that at the earth's mean distance, in the absence of its absorbing atmosphere, the solar rays would raise one gramme of water three degrees Centigrade for each square centimeter of surface perpendicular to them." [The calorie he uses is about 0.001 as much heat as that defined above.] "This would melt an ice shell 178.6 feet thick annually all over the earth. Somewhat less than two-thirds of this amount reaches us at the sea level ordinarily from a zenith sun," and increasingly less as the sun is nearer the horizon. "Yet the temperature of the earth's surface is not due principally to this direct radiation, but to the quality of selective absorption in our atmosphere, without which the temperature of the soil in the tropics under a vertical sun would probably not rise above —200° C.”

The surface temperature of the earth, and with it the existence of our race, and of all organized life, are dependent on this selective absorption, a physical process in the atmosphere due to the physical properties of air and of solar radiations.

Returning to Fig. 12, we have the solar radiations separated and spread along a band, or spectrum, s, arranged in the order of some of their physical properties, wave-length and period of vibration. At a and z, the longest and the shortest wave-lengths yet measured, they are 0.000118-inch and 0.000012-inch; at r and v, the extreme visible parts, or colors, of the spectrum, the wave-lengths are 0.000030-inch and 0.000015-inch. If we take a very narrow vessel of water con

taining a fine thermometer and expose it (Fig. 13) for the same length of time in different parts of the spectrum, we shall find the changes of temperature quite unequal, lessening notably toward the end z, and also very near the end a. If we weigh the water and take its changes of temperature in equal times along the spectrum, we can find the relative amount of, heat received at each place, and from this, as explained above, the relative energy or working ability of the rays of different wave-lengths. Instead of a vessel of water and thermometer, a very much more sensitive and accurate apparatus (a thermopile, T, and galvanometer, G. Fig. 13) is actually used in such work.

Professor Langley greatly improved the construction of this apparatus, in a new instrument called a bolometer. With this he has very carefully measured the energy at every part of the spectrum, and plotted it by setting up lines proportioned to the energy at each part and connecting the tops of these lines; this gives the curve I, Fig. 14. He then went to Mt. Whitney, in the Sierra Nevadas, over 14,000 feet high, leaving more than one-third of the atmosphere below him, and again measured the heat along the spectrum, and plotted the energy of the different radiations when they had passed through less than two-thirds of the earth's atmosphere. From this, he determines what the energy of each part of the spectrum would be outside the atmosphere. The mean of his results is represented in curve II, Fig. 14. It is notably different from I, not only in greater amount, but in the distribution of energy. This shows that some of the radiations are much more absorbed than others by the air. While the radiations of great wave-length have nearly the same energy in both curves (at mean sea level and outside the atmosphere), those of short wave-length have lost a great part of their energy at sea level; or rather, a great part of these, two-thirds of some of them, has been absorbed by the air. The greater the wave-length, then, the greater the percentage transmitted through the air. Now when these radiations fall upon the earth they produce heat and so maintain the necessary temperature of the globe. To do this it is necessary that they should not pass out again through the air as readily as they were first transmitted. The highest temperature of the earth's surface gives radiations of much greater wave-length than the longest measured in the spectrum, and to these the atmosphere must be highly impervious. Their absorption by the air near the earth's surface heats this lower air; and thus the solar radiations that have passed through to the earth, and undergone a change at its surface, are largely retained and furnish heat where it is needed, at the bottom of the atmosphere.

Curve I. Fig. 14, is much notched, showing excessive absorption of certain radiations. It is found that very small percentages, or even a fractional per cent. of certain gases mixed with air, have a wonderful absorptive power for heat and light radiations. Fig. 15, H, is a vessel of hot water and a thermometer, t. The radiations pass through the long tube with glass or rock salt covers at the ends and fall on the thermopile, P. By the small lateral tubes, the air may be removed or different gases admitted. Tyndall found that if the absorption of heat radiations by one atmosphere of air was called one, that by one atmosphere of carbon dioxide (carbonic acid) was 90, marsh gas 403, sulphurous acid gas 610, ammonia gas 1195, and water vapor 14,400. The small portions of these substances in the air (the water vapor