Page:Popular Science Monthly Volume 61.djvu/194

188 the crater and set loose floods of lava. As the energy expends itself, the violence declines and disappears. The volcano then yields only hot springs and gaseous emissions called fumaroles, until it is stone-cold.

earth is easily the most interesting and the best known to us of the bodies of the universe which have been subjected to scientific investigation. Not only do we know more of the earth than of any other member of the solar system, but we know more of the earth than of any of the smaller bodies which have been studied minutely in laboratories. It is true, of course, that a few bodies, like standards of length and mass, have been determined with great precision with respect to one or two of their properties; but our knowledge of the earth includes many of its properties, and some of them are known with a precision only surpassed by that of the standards referred to.

The surface of the earth is closely that of an oblate spheroid whose axes are known within about the hundred thousandth part, a precision near the limit possible in laboratory measurements of such bodies as inch ball-bearing spheres now made with wonderful exactness for commercial purposes. The surface and volume of this spheroid are found to be in round numbers two hundred million square miles and two hundred and sixty thousand million cubic miles respectively; and these numbers are known with an accuracy far surpassing that of the measured areas, for example, of the most valuable city properties, which are relieved of the necessity for precise measurement by the legal phrase, 'be the contents of the same more or less.' The magnitude of the work which has led to these results may be appreciated to some extent if one considers seriously how one would measure an area of two hundred million square miles with an error not greater, say than one fifty-thousandth part.

Less definite but of a higher order of magnitude are the figures expressing the quantity of mass of the earth, or what is sometimes designated by the scientifically meaningless phrase 'the weight of the earth.' We all have a tolerably clear idea of the mass in a ton of coal, but few of us are fitted to realize the nearly equally definite quantity of the earth's mass, which is in round numbers six thousand six hundred million million million tons. Of this total mass, the atmosphere, whose lenticular-shaped envelope includes a volume about one hundred and fifty times that of the solid part of the earth, contributes somewhat more than one millionth part, a small fraction of the whole, but yet millions of millions of tons in amount.

The regularity of rotation of the earth, or its constancy as a timekeeper, is no less surprising when expressed in numbers. The variation from day to day in the time of rotation is probably less than the hundredth of a second, or no greater, say, than the ten-millionth part of a day. Our best clocks and chronometers, and they are marvels of mechanical perfection, fall far short of this degree of constancy. Equally remarkable for stately regularity are the precession and nutation of the earth, by reason of which its axis of rotation describes a slightly fluted cone in the heavens, making one complete revolution in the leisurely interval of about twenty-six thousand years and thus rendering it essential for us to change pole stars from time to time. And still more noteworthy are the lately discovered minute wabblings of the earth with respect to its axis of rotation, whereby the latitude of a place varies from month to month, running through a lesser cycle in about fourteen months and through a greater cycle in