HOW THE GEOLOGIST ESTIMATES THE AGE OF THE EARTH. When the ocean condensed from the atmosphere, where it had existed in the form of a vapor until the crust became sufficiently cool to permit its existence as a liquid, the sedimentary rocks began forming. From that day to this they have constantly increased in thickness until at the present time they reach an aggregate, according to Houghton, of 177,200 feet of rock.

But how, you ask, is an estimate made as to the time required for the formation of a rock mass of this thickness? To do this we must know: (1) The area of the sea floor on which these rocks are deposited; (2) the area of the land. from which the sediments composing these rocks is derived; (3) the rate at which the material is carried from the land to the sea.

Now these points in order: (1) It is well known that the sediments carried by rivers to the sea are deposited along the shores, the finest material only being carried any distance from the mouths of rivers. It seems that on an average these sediments are not carried more than thirty miles from the shores, and if we take 100,000 miles as the total length of the coast lines of the world, then we have 3,000,000 square miles as the area on which sediments are now accumulating; (2) the area of the land from which the sediments covering this area are derived is

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about 52,745,000 square miles; (3) next the important factor, how fast are the land areas supplying the sea floor with sediments? By determining the material which a river carries on the average, and knowing the area of its basin, we can determine how fast land areas are being carried to the sea; or in other words, the rate at which the land is being lowered. urally the rate of reduction varies. It depends on the amount of rainfall, the slope of the land, the kinds. of rocks, frost, heat, etc. Thus the Po which receives an abundant rainfall and which has a high velocity, lowers its basin one foot in 760 years; the Ganges, lying at the base of the mighty Himalaya, has a high velocity and the great precipitation falling during onehalf of the year produces floods which lower the basin one foot in 1,751 years. The Mississippi, which in point of rainfall and slope is more normal, lowers its basin one foot in 4,640 years. This rate may be taken as the average rate of land erosion of the world, though of course it is not certain that this rate is the correct one. While many rivers lower their basin at a more rapid rate than this, it must be kept in mind that others erode at a much slower rate. Think of the barren tracts of western North America, the Sahara of Africa, the deserts of Asia, and the poorly watered districts of Australia! In all these the rate of land erosion must

be much less than that of the Mississippi basin.

Since the land area of the globe is 52,745,000 square miles, or more than seventeen times the area of the marginal sea belt on which the sediments are deposited, it follows that a reduction of one foot of the land areas of the world will make a deposit of 17 feet along the marginal sea floor. But if the land areas are reduced, as assumed, one foot in 4,640 years, a deposit of one foot on the marginal sea floor will require 273 years. Now if the total maximum thickness of the sedimentary rocks be 177,200 feet, as stated by Houghton, then to form these there would be required 177,200 X 273, or 48,375,600 years. In other words, on this basis, the time that has elapsed since the sedimentary rocks began to be formed, is 48,375,600 years.

However, in this estimate it has been assumed that there are no limestones in the sedimentary rocks, while in fact there are many thousands of feet of this formation. The rate of growth of limestone is not known. Professor Alexander Winchell has estimated that they form only one-fifth as fast as the other kinds of sedimentary rocks. Taking this point in consideration, it would greatly increase the time required for the deposition of the entire group of rocks.

To this should be added the time that elapsed between the formation.

of the earth's first crust and the

depositions of the earliest sediments. This period must have been very long, perhaps sufficient to bring geological time up to 100,000,000 years.

HOW THE PHYSICIST ESTIMATES
THE AGE OF THE EARTH.

The method of the physicist is far more abstruse than that of the geologist. It involves physical and mathematical calculations of the most difficult kind, and moreover does not have the definite data to work on that the geologist has.

The problem to the physicist, in its simplest terms, is this: If we know the temperature of the earth when the solid crust first began to form; the amount of heat lost since that time; and the rate at which it has cooled, we can calculate its age.

It will be apparent to the thoughtful reader that this method, while of great interest, may give results that are far from the truth.. How can we determine the temperature of the first crust? And how may we ascertain the present temperature of our planet; or in other words, the amount of heat given off during the past ages? Obviously an exact determination of these points cannot be made, and the results will of course vary as these assumptions vary. While we must admire the mathematical genius and ability of the men who attack a problem of this sort, yet in the language of Huxley we should remember that the mathematical mill, while a good one,

We have never had any experience with matter which was without weight. Whatever the cause of weight may be it is always present and it is impossible as yet to insulate matter from its influence. Consequently it is not strange that many get the idea that weight is essential to matter; that it is inherent in matter and one. of its properties. The error arises from a confusion of mass with weight. The mass is the amount of matter which a body contains, while the weight is simply a convenient means of comparing masses in any given locality. The mass is independent of the presence of other bodies, weight is wholly dependent upon them. If a body could. exist alone it could have no weight. In the same place, however, the

weight is exactly proportional to the quantity of matter and a certain amount of cork will weigh the same as an equal quantity of lead or gold.

Cannon-balls are used which weigh one ton; now if this ball should lose all weight so that I might toss it up as a toy balloon in my hand, still if it were fired from the cannon in the usual way it would be just as destructive as before, for its energy depends only. upon its mass and its velocity.

The difficulty of moving a heavy door on its hinges must not be ascribed to its weight, for without weight, the same mass would be as difficult to move, save a slight friction of the hinges.

A well known law states that weight varies directly as the pro

ducts of the masses concerned and inversely as the square of the distance they are apart. So that certain masses which we handle keep practically the same weight because the earth's mass remains constant and our operations are all at about the same distance from the earth's center; but if the amount of matter in our earth should suddenly change till it had decreased to that of the moon and it would yet retain its present volume we would find that a boy who weighed 81 lbs. would then weigh just one pound; and the applause which we now give to the athlete who beats the record in a high jump would then be accorded only to one who could leap over Washington's monument, 555 ft.

Again if the mass of the earth should change to that of the sun and its volume remain unchanged the change would be still more surprising, for the boy mentioned above would weigh now 13,365 tons; of course he could not jump from the ground but would be crushed to the earth by his enormous weight, and even if he could stand he would not be able with all his strength to lift a lady's thimble from the floor.

Two bodies are always concerned in what we call weight. One of these is always the earth. When the storekeeper weighs out two pounds of sugar he assumes that if he doubles the mass the earth-pull will be twice as great as for one pound. Workmen have

learned by long experience to judge the weight by the mass, and stockbuyers can tell quite accurately the weight of a steer or a hog by its appearance; so used have we become to the constancy of the conditions of our ordinary dealings. If two masses, one weighing one pound and the other four pounds be carried to the top of a building and let drop together, some would suspect that the heavy body would reach the ground first because the force upon it is four times as great, but the mass too is four times as great and so they will reach the bottom of their flight in exactly the same time.

While most of our experiences with weight arise from variations in mass, yet any considerable difference in our distance from the center of the earth, rapidly changes the weight. In the case of the boy mentioned above we assumed that his distance from the center of mass did not change while the mass changed to that of the sun or the moon. If this boy is transported to the surface of the moon it becomes a very different problem for he is then much nearer the center of mass and his weight would be reduced only five-sixths, and on the sun his distance from the center would be so vastly increased that his weight would be multiplied only 23 2/3 times.

We do not in our business transactions make any account of differences in weight which arise from differences in distance

less while he is standing than when sitting. The iron in the upper stories of a building does not weigh as much as it would have if placed in a lower story. There are a number of structures in the world which reach one-tenth of a mile in height. It is an easy calculation to show that at that height every ten tons of material as weighed at the bottom will weigh one pound less when elevated to that height. We have a number of mountain peaks five miles high and every Troy pound brought down from that altitude to sea level would weigh 14.37 grains more. So if there were a gold mine at this height the owner would get $620 more on every thousand pounds by first bringing it down to sea-level and then weighing it out, providing the weighing be done on spring balance.

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In buying and selling we are not particular as to whether the storekeeper places his scales on the counter or near the floor as the difference is slight and if the weighing is done on the beam balance no difference appears as the counterpoise is affected in a similar manner. All we contend for here is that a change in the distance

from the earth's center of even one inch affects the weight in a measureable degree.

The distance from the center of the earth to one of the poles is 131 miles less than from the center to the equator and so equal masses at these two points have weights. which are to each other as 568 to 567.

The closer we can get to the center of the earth without getting any of the earth's mass above us the more we will weigh. It is not a very hard problem in mathematics to show that if the earth were a hollow sphere, then an object placed anywhere within the shell would weigh nothing. So it is plain that a mass at the bottom of a shaft one mile deep will derive no weight from the crust of the earth one mile thick all over the surface of the globe and it will be only the remaining part of the earth which will give the mass weight. Consequently the weight of an object would decrease uniformly as we descend into the earth if the earth's mass were homogeneous, but since it is not homogeneous and no one has ever gone to the earth's center on a tour of observation, then no one can state any law for the rate of variation as we approach the center.

Above the earth we have clear sailing and can give a law without fear of contradiction, that the weight decreases as the square of the distance increases. An object. 4,000 miles above the earth's sur