Familiar Talks on Science-World-Building and Life. Earth Part 7

It will be seen that a large part of the compressed air is wasted in cooling the remainder sufficiently to liquefy.

The use to which liquid air may be put, advantageously, is an unsolved problem; but no doubt it will have a place in time. All great discoveries do. Electricity had to wait a long time for recognition; but what a part it plays now in the everyday life of the whole civilized world!

Curious effects are produced by this intense cold. Meat may be frozen so hard that it will give off a musical tone when struck. Here is a pointer for the seeker of novelties in the line of musical instruments.

Liquid air furnishes a beautiful ill.u.s.tration of the fact that a burning gas jet is continually forming water as well as giving out heat and light. If we put liquid air into a tea kettle and hold it over a gas jet, ice will form on the bottom from the water created by the flame, and it will freeze so hard that the flame will make no impression upon it, other than to make the ice cake grow larger.

Although liquid air is not found in nature, and is therefore called an artificial product, it is produced by taking advantage of natural law.

Without the intellect of man it never would have been seen upon this earth; and the same may be said concerning many things in our world, both animate and inanimate. The genius of man is G.o.d-like. He lifts the veil that shrouds the mysteries of nature, and here he comes in very touch with the mind of the Infinite. Man interprets this thought through the medium of natural law, and lo, a new product!

How much life would have been robbed of its charm and interest if all these things had been worked out for us from the beginning! For there is no interest so absorbing and no pleasure so keen as that of pursuit when the pursuer is reaching out after the hidden things that are locked up in Nature's great storehouse. From time to time she yields up her secrets, little by little, to encourage those who love her and are willing to work, not only for the pleasure of the getting, but for the highest and best good of their fellows.

WATER.

CHAPTER XIX.

RIVERS AND FLOODS.

Water covers such a large proportion of the earth's surface and is such an important factor in the economy of nature that it becomes a matter of interest to study the process of its distribution. Water is to our globe what blood is to our bodies. A constant circulation must be kept up or all animal and vegetable life would suffer. Here, as in every other operation of nature, the sun is the great heart and motive power that is active in the distribution of moisture over the face of the globe.

The total annual rainfall on the whole surface of the earth amounts to about 28,000 cubic miles of water. Only about one-fourth of this amount ever reaches the ocean, but it is either absorbed for a time by animal and vegetable life or lifted through the process of evaporation into the air as invisible moisture, when it is carried over the region of rainfall and there condensed into water and falls back upon the earth--only to go through the same operation again. The whole surface of the earth is divided into drainage areas that lead either directly through rivulets and rivers to the ocean, or into some land-locked basin, where it either finds an outlet under ground or is kept within bounds through the process of evaporation, the same as is the case with our great oceans. In North America the amount of drainage area that has no outlet to the ocean amounts to about 3 per cent. of the whole surface. In other countries the percentage of inland drainage is much larger. The great Salt Lake in Utah is an instance where there is no outlet for the water except through the medium of evaporation. Inasmuch as all rivers and streams contain a certain proportion of salt,--especially in such strongly alkaline land regions as the Great Basin of the North American continent,--these inland lakes in time become saturated with this and other mineral substances.

Salt is constantly being carried into the lake by the water of the stream that feeds it, and the water is continually being evaporated, leaving the salt behind. This process has been going on in the valley of Utah for so long a period that 17 per cent. of the contents of the lake is salt. The Humboldt River in Nevada, which empties into a small lake of the same name, and lies at the foot of the Humboldt Mountains, is said to have an underground outlet. This must be the case, because the area of the lake is very small as compared with Salt Lake, while the river that feeds the latter is very small compared with the one that flows into the former. That is to say, in the one case a very small stream empties into a large lake, while in the other case a much larger stream feeds a very small lake. Besides, Humboldt Lake, unlike the Great Salt Lake, is said to be a fresh-water lake; if it had no outlet it would become in time saturated with salt. The largest body of water in the world having no outlet to the ocean is the Caspian Sea, on the border between Asia and Russia in Europe, it being 180,000 square miles in extent.

Where rivers empty into large bodies of water, such as the great chain of lakes on the northern border of the United States (and these lakes have an outlet connecting one with the other, and finally by a river to the ocean) a constant circulation is being kept up, and the water remains fresh. Owing to the fact, however, of the great evaporating surface that these lakes afford, there is a greater disproportion between the rainfall upon the drainage area tributary to these lakes, and the amount of discharge through the St. Lawrence River, than would be the case with a river that was not connected with a system of lakes.

The amount of rainfall upon the area drained by the Mississippi River during one year amounts to about 614 cubic miles of water, while the discharge at the mouth of the Mississippi River is only about 154 cubic miles. The difference between the two figures has been carried up by the process of evaporation or stored in vegetation. These figures vary considerably, however, with different years.

The proportion of rainfall to discharge will vary greatly in different rivers from other causes than having a large evaporating surface. This variation is due to the difference in the ability of the soil to retain water after a rainfall. In some drainage areas the ground is more or less impermeable to water, and in this case the water runs readily off, causing a sudden rise in the river; and as suddenly it reaches the low-water mark. In other drainage areas the ground is very permeable to water, so that the rain penetrates to a greater depth into the earth, where it is held, and by a slow process drains into the rivers, while much more of it is carried off by evaporation and into vegetation than is the case in the drainage district before mentioned.

The courses of rivers are determined by the topography of the country through which they flow. The sinuous windings, that are found to be a characteristic of nearly all rivers, are caused by the water, through the force of gravity, seeking the lowest level, and avoiding obstructions, which they can flow around more easily than remove.

Great rivers often change their courses, especially where they flow through a region of made earth, such as is the case with the lower Mississippi River, and in other great rivers of the world. The loose earth is continually shifted by the current, and where the current is not very strong it will often hold the water back to such an extent of acc.u.mulated weight that the flood will break over at some weak point on its banks and make a new course for itself.

One of the great rivers of China--the Hw.a.n.gho--often causes dire destruction to life and property owing to change in its bed from time to time. It is estimated that between the years of 1851-66 this river caused the loss of from 30,000,000 to 40,000,000 lives through drowning and famine by the destruction of crops.

Floods in rivers are occasioned from various causes. Of course the primary cause is the same in all cases, that is, from precipitation of moisture in the form of rain or snow. Some rivers are so related to the area of rainfall and to the permeability of the soil that there is but little variation in the amount of discharge throughout the year. The great river of South America, the Amazon, is an instance of a river of this cla.s.s. A certain number of the smaller rivers that feed it lie in the area of rainfall during the whole of the year; for instance, the streams of the upper Amazon are being fed by rains at one season of the year, when those feeding the river lower down are at the lowest stage.

When the rainy season prevails in the upper section of the river the dry season prevails farther down, while at another season of the year these conditions are reversed. Therefore, though the Amazon has a larger drainage basin than any other river in the world, and in some parts the yearly rainfall is 280 inches, there is no very great fluctuation in the stages of water. The Orinoco River, which flows through Venezuela, and whose drainage area is largely covered with mountains, has a greater fluctuation than any other river, the difference between high and low water amounting to seventy feet.

The River Nile has an annual rise of from fourteen to twenty-six feet.

This river is the sole dependence of the inhabitants of lower Egypt, and their sustenance depends upon the height to which the river rises; if it does not rise high enough the agricultural lands are not sufficiently irrigated, and if it rises too high their crops are destroyed by the floods. In this section they depend entirely upon the overflow of the Nile for irrigation, and not upon the rainfall. There is scarcely ever a rainfall in lower Egypt except about once a year on the coast of the Mediterranean. After ascending the river for a short distance we come into an area of no rain for a distance of 1500 miles along the river.

Egypt has a superficial area of about 115,200 square miles, and only about one-twelfth of this area is in a position to be cultivated.

As there is no rainfall in this region, the sole dependence for agricultural purposes is from the River Nile when it rises to a sufficient height to admit of irrigation. The river brings down quant.i.ties of rich earth which during the overflow is deposited, and thus the agricultural regions are refertilized annually.

The River Nile is what is called a tropical river and is fed by the rains in upper Egypt caused by the monsoon winds that prevail in that section of Africa during the summer season, as they do in India. As has been explained in a former chapter, the monsoon winds blow steadily for about six months from off the southern ocean. These winds are highly charged with moisture, which is not precipitated till it strikes the mountainous regions of the interior. Here the high mountains, which are often snow-capped, cause a profuse precipitation, which runs off into the various feeders of the Nile, causing a gradual rise in the river that reaches the highest point about September of each year. If the Nile should dry up, or if the annual floods should materially change in height, it would make a desert region of all that portion of Egypt now so productive.

The great rivers of China, the Yang-tse-Kiang and the Hw.a.n.gho, are also tropical rivers and have an annual flood. Sometimes the rise is as much as fifty-six feet. These annual floods are also caused by the monsoon winds that carry moisture from the ocean, which is condensed and precipitated in the mountains of central Asia. The conditions are substantially the same as those which prevail at the sources of the Nile in Africa.

Rivers are produced from all sorts of causes, some of them flowing only during the rainy season, while others are fed by melting snow from the higher mountains, and as the snow is rarely melted away entirely during the summer, in the high mountains, there is a continual flow from this source. The snow forms a system of storage, so that the water is held back and is gradually given up as it melts. If this were not true mountainous regions would be subjected to disastrous floods. If the precipitation were always in the form of rain it would immediately run off instead of being distributed over a whole season. The Platte is an instance of a river largely fed by the melting snows--of the Rocky Mountains.

In the region of glaciers in the mountains of Alaska and Switzerland rivers are fed by the melting ice. These rivers are usually of a milky color occasioned by the pulverization of rock caused by the grinding of the great glaciers as they flow down the gulches in the mountain side.

In some regions these glacial rivers have a diurnal variation. This is caused by the fact that the glacier is so situated that it freezes at night, which checks the flow, and thaws in the daytime, which increases it.

Rivers are to the globe what the veins are to the animal organization.

They pick up the surplus moisture not needed in the growth of vegetation and for the sustenance of animal life, and carry it on, together with the debris that it gathers in its course, to the great reservoirs, the seas and oceans, where it is redistilled and purified by the action of the sun's rays. From here it is carried back in the form of invisible moisture and again precipitated in the purified state, to help carry on the great operations of growth--animal and vegetable. The vaporized moisture that is carried back by the winds and redistributed corresponds to the blood, after it has been purified and is carried back through the arteries to the extremities and capillary vessels which feed and nourish the bodily organs.

CHAPTER XX.

TIDES.

Anyone who has spent a summer at the seash.o.r.e has observed that the water level of the ocean changes twice in about twenty-four hours, or perhaps it would be a better statement to say that it is continually changing and that twice in twenty-four hours there is a point when it reaches its highest level and another when it reaches its lowest. It swings back and forth like a pendulum, making a complete oscillation once in twelve hours. When we come to study this phenomenon closely we find that it varies each day, and that for a certain period of time the water will reach a higher level each succeeding day until it culminates in a maximum height, when it begins to gradually diminish from day to day until it has reached a minimum. Here it turns and goes over the same round again. It will be further observed that the time occupied between one high tide and the next one is a trifle over twelve hours. That is to say, the two ebbs and flows that occur each day require a little more than twenty-four hours, so that the tidal day is a little longer than the solar day. It corresponds to what we call the lunar day.

As all know, the moon goes through all its phases once in twenty-eight days. The tide considered in its simplest aspect is a struggle on the part of the water to follow the moon. There is a mutual attraction of gravitation between the earth and the moon. Because the water of the earth is mobile it tends to pile up at a point nearest the moon. But the earth as a whole also moves toward the moon, and more than the water does, keeping its round shape, while its movable water (practically enveloping it) is piled up before it toward the moon and left acc.u.mulated behind it away from the moon. So that in a rough way it is a solid sound earth, surrounded by an oval body of water: the long axis of the oval representing the high tides, which, as they follow the moon, slide completely around the earth once in every twenty-four hours. Thus, there are really two high tides and two low tides moving around the earth at the same time; and this accounts for the two daily tides.

We have accounted for the time when they occur in the fact that the water attempts to follow the moon, but this does not account for the gradual changes in the amount of fluctuation from day to day. The problem is complicated by the fact that the sun also has an attraction for the earth as well as the moon. But from the fact that the sun is something like 400 times further from the earth than the moon is, and also the fact that the attraction of one body for another varies inversely as the square of the distance, the moon has an immense advantage over the sun, although so much smaller. If the power of the moon were entirely suspended, or if the moon were blotted out of existence, there would still be a tide. The fluctuation between high and low tide would not be nearly so great as it is at present, but it would occur at the same time each day, because it would be wholly a product of the sun.

It will be easily seen that these two forces acting upon the water at the same time will cause a complicated condition in the movement of the waters of the ocean. There will come a time once in twenty-eight days when the sun and the moon will act conjointly, and both will pull in the same direction at the same time upon the water. This joint action of the sun and moon produces the highest tide, which is called the "spring"

tide. From this point, however, the tides will grow less each day, because the relation of the sun and moon is constantly changing, owing to the fact that it requires 365 days for the sun to complete his apparent revolution around the earth, while the moon does her actual course in twenty-eight days. When the sun and moon have changed their relative positions so that they are at right angles to each other with reference to the earth--at a quarter-circle apart--the sun and moon will be pulling against each other; at least this is the point where the moon is at the greatest disadvantage with reference to its ability to attract the water.

Because one-quarter around the earth the sun is creating his own tide, which to that extent counteracts the effect produced by the moon, the tide under the moon at this point is at its lowest point and is called the "neap" tide. When the moon has pa.s.sed on around the earth to a point where it is opposite to that of the sun--at a half-circle apart--there will be another spring tide, and then another neap tide when it is on the last quarter, and from that point the tide will increase daily until it reaches the point where the sun and moon are in exact line with reference to the earth's center, when another spring tide occurs. From this it will be seen that there are two spring tides and two neap tides in each twenty-eight days. This is the fundamental law governing tides.

There are many other conditions that modify tidal effects. Neither the sun nor the moon is always at the same distance from the earth. So that there will be a variation at times in high and low tides. For instance, it will happen sometimes that when both the sun and moon are acting conjointly they will both be at their nearest point to the earth, and when this is the case the spring tide will be much higher than usual.

For many years the writer has observed that artesian wells, made by deep borings of small diameter into the earth to a water supply, have a daily period of ebb and flow, as well as a neap and spring tide, the same as the tides of the ocean, except that the process is reversed. The time of greatest flow of an artesian well will occur at low tide in the ocean.

This might be accounted for from the fact that when the tide is at its height the moon is also pulling upon the crust of the earth, which would tend to take the pressure off the sand rock which lies one or two thousand feet below the surface and through which the flow of water comes, and thus slacken the flow. When the moon is in position for low tide, the crust of the earth would settle back and thus produce a greater pressure upon the water-bearing rock. This is the only theory that has suggested itself to the writer that would seem to account for these phenomena.

Looked at from one standpoint, it is easy to account for tidal action.

But when we attempt to make up a table giving the hour and minute as well as the height of the tide at that particular time we find that we have a very complicated mathematical problem. However, tables are made out so that we know at just what time in the day a tide will occur every day in the year.

CHAPTER XXI.

WHAT IS A SPONGE?